thrush guide

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A Land Manager's Guide to

Improving Habitat forForest Thrushes


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Please cite this publication as Rosenberg, K.V., R.S. Hames, R.W. Rohrbaugh, Jr., S. Barker Swarthout, J.D. Lowe, and A.A. Dhondt. 2003. A land manager’s guide to improving habitat

for forest thrushes. The Cornell Lab of Ornithology.

© 2003 by the Cornell Lab of Ornithology

S p o t t e d T h r u s h e s b y J a m e s C o e

Acknowledgements Funding for the Birds in Forested Landscapes project and production of this publication was provided by the

National Fish and Wildlife Foundation, the USDA Forest Service, the Environmental Protection Agency,and voluntary contributions from many Birds in Forested Landscapes participants.

We would also like to thank our many colleagues at the Cornell Lab of Ornithology, Cornell University,

and federal and state wildlife agencies for discussions of forest-bird management and for helping us improve thisdocument. Special thanks to Cristina Faustino for her numerous contributions to the data analysis and Barb Kottfor her coordination of the project in conjunction with the Forest Service. Most importantly we appreciate thededication, time, and continued effort of thousands of Birds in Forested Landscapes participants who collected

the data on forest birds. Without their support and hard work in the field this publication would not be possible.


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A Land Manager’s Guide toImproving Habitat for Forest Thrushes

Kenneth V. Rosenberg, Ralph S. Hames, Ronald W. Rohrbaugh, Jr.,Sara Barker Swarthout, James D. Lowe, André A. Dhondt

Contents Preface ................................ 2Summary ............................. 2Introduction......................... 3Managing the Contemporary Forest ..................................................................................................................... 5

What Is Forest Fragmentation? ....................................................................................................................... 5Birds in Forested Landscapes: A scientific approach to managing forest birds ................................................ 6

General Guidelines: Key Forest Characteristics .................................................................................................... 7Forest Size and Isolation 7Edges ............................ 8Brood Parasitism and Nest Predation ............................................................................................................. 8Landscape Context ........ 9Forest Structure ........... 10

Regional Management Guidelines for Thrushes ................................................................................................. 11Using These Guidelines ............................................................................................................................... 11Using Minimum-Area Tables ....................................................................................................................... 11

East/Midwest Forest Region 12Management Summary ............................................................................................................................... 15

Northern Forest Region ...... 16Management Summary ............................................................................................................................... 18

Western Forest Region ....... 19Management Summary ............................................................................................................................... 21

Biographies of Forest Thrushes ........................................................................................................................... 22Veery .......................... 22Swainson's Thrush ....... 23Hermit Thrush ............. 24Wood Thrush .............. 25Varied Thrush .............. 26

Literature Cited .................. 27

V a r i e d T h r u s h b y J a m e s C o e


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Why thrushes? Nearly every forest in North America is inhabited

by one or more thrush species. These drab-colored

songbirds are not often seen, yet their beautiful, flute-like songs are one of the most characteristic sounds ofour forested landscapes in summer. Forest thrushes,like their more familiar cousin the American Robin,feed primarily on the ground and build their nestson or near the ground in low, dense vegetation. Eachspecies selects its own habitat, in terms of forest type,elevation, and moisture regime, and all are sensitive tothe structure, productivity, and configuration of the for-est. These birds are therefore good indicators of foresthealth and the ability of our forests to support healthybird populations. As several thrush species continue todecline, managing forest habitats for these and other

songbirds is one way that landowners and managerscan contribute to bird conservation efforts.

Preface

2

This is the second publication in a series of forest habitat management guidelines produced by the CornellLab of Ornithology as a tool to help those interested in managing and protecting habitat for birds. The firstpublication, A Land Manager’s Guide to Improving Habitat for Scarlet Tanagers and other Forest-interior Birds,was published in 1999 and has been used widely by agencies and private landowners. These guidelines offer aset of “management prescriptions”—descriptions of the kinds and amounts of habitat that are required to sustainhealthy bird populations. This guide is focused on improving habitat for five forest-dwelling thrushes, includingthe Veery (Catharus fuscescens), Swainson’s Thrush (Catharus ustulatus), Hermit Thrush (Catharus guttatus), WoodThrush (Hylocichla mustelina), and Varied Thrush (Ixoreus naevius). Providing well-managed habitat for thesewide-ranging species that use a variety of forest habitats also will benefit a host of other forest birds. We first offera series of general management guidelines that are applicable in both eastern and western landscapes. Theseinclude preventing the fragmentation of large forest tracts, minimizing the creation of edge habitats, establish-ing forested corridors to reduce isolation of small patches, and maintaining structural and plant-species diversitywithin existing forests.

Unlike tanagers, thrushes are birds of the forest understory, requiring dense shrub or sapling layers and awell-developed layer of leaf litter on the forest floor. These conditions exist in some mature and old-growth foresttypes, but most often are enhanced by small-scale disturbances within forested regions. Silvicultural practicesthat mimic natural disturbance and promote deciduous shrubs and saplings can benefit thrush populations, as

long as overall forest cover in a landscape is not permenantly reduced. In all forested regions, general guidelinesfor thrushes and other forest species include: (1) maintaining large and unfragmented forest blocks; (2) promotingunderstory growth through natural disturbance or management; and (3) limiting overbrowsing by deer, livestock,and other ungulates. In the western region, protection and enhancement of riparian forest are critical for thrushand other bird populations. In eastern and northern forests, patches of 150 acres (62 ha) or larger generally willprovide thrushes with high to moderate levels of habitat suitability.

Summary

Purpose and Use of this Guide This publication is a tool to help improve habitat

for populations of forest-interior birds. These guide-

lines are written for two types of land managers: thoseresponsible for large landscapes, such as public landsor entire states; and private landowners who man-age small blocks of forest. We first discuss conceptsassociated with forest fragmentation at both landscapeand smaller habitat scales and provide general man-agement suggestions that benefit many forest-interiorbirds. We then focus directly on the habitat-arearequirements of the Veery, Swainson’s Thrush, HermitThrush, Wood Thrush, and Varied Thrush. Specifically,we provide estimates of how much appropriate foresthabitat is necessary to sustain breeding populations ofeach of these species in three geographic regions of

the U.S. and Canada.


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Figure 1. Vast coniferous forests of western North America support diverse bird communities, including large populations of Hermit Thrush, Swainson's Thrush, and in some regions, Varied Thrush. Photo by Melissa S. Barker.

IntroductionFew choices that a bird makes during its life will

affect its survival and ability to reproduce as profound-ly as its choice of habitat. A bird’s habitat must containall the resources needed for that individual’s growth,survival, and reproduction, including food, water,cover, and roosting and nesting sites. The process canbe complicated, as each bird is faced with a mosaic ofacceptable habitats from which to choose, and eachchoice often occurs at different geographic scales(Hames et al. 2001). For example, a migratory WoodThrush returning to its breeding grounds must first se-lect an appropriate forest patch, then a territory withinthe patch, and finally a nest site within the territory(Figure 2). These choices are made based on habitattype and quality. Each bird species has a unique set of

habitat requirements based on its ecology and behav-ior. Some species, such as the Bicknell’s Thrush, whichonly nests in montane forest dominated by balsam fir,have very specific habitat requirements. Other speciesmay key in on a specific habitat type, such as for-est, but will tolerate a range of conditions within thathabitat type. Suites of species may respond similarlyto variation in basic forest characteristics such as size,shape, and surrounding land use. For this reason,targeted forest management for selected species willoften benefit a larger guild of forest-dwelling birds.

Sometimes two locations may contain all the ba-sic requisites for appropriate breeding habitat, but theymay differ in overall habitat quality. For example, onelocation may contain more food, better nest sites, orfewer brood parasites (i.e., Brown-headed Cowbirds)and nest predators. Such quality differences amonghabitats often are revealed by their rate of occupancyby breeding birds (Hames et al. 2001) and the nestingsuccess experienced by those occupants. The guide-lines contained in this publication were created bystatistically modeling the relationships between thrushoccupancy rates and numerous forest characteristicsthought to affect habitat quality.

In our first forest management publication, ALand Manager’s Guide to Improving Habitat for Scarlet

Tanagers and other Forest-interior Birds, we focusedon eastern forests, as this was the extent of the Scar-let Tanager’s breeding distribution (Rosenberg et al.1999b). In this publication, the second in our manage-ment series, we expand our taxonomic and geographicscope by providing management guidelines for theWood Thrush, Hermit Thrush, Swainson’s Thrush, Vee-ry, and Varied Thrush. Like the tanager managementguidelines, the thrush guidelines will also benefit manyother forest-dwelling birds. The five forest thrushesconsidered in this publication have a combined geo-


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Figure 2. Wood Thrush

Habitat Selection: Formany species, includingthe Wood Thrush, select-ing a place to nest involvesmaking choices at several

geographic scales. A migrat-ing Wood Thrush returningto its natal breeding area inan agricultural setting is firstfaced with locating a wood-lot suitable for establishinga breeding territory. The

woodlot must be of appropriate size, age, shape, anddistance to key landscapefeatures, such as water. Af-ter selecting a woodlot, thebird must then stake out aterritory of about five acres(2 ha), usually containinga mix of understory andcanopy trees. Finally, thebird must select an ap-

propriate place within theterritory to build its nest,

which is placed on a forkedbranch of an understorytree or shrub, such as flow-ering dogwood. From theLab’s Home Study Coursein Bird Biology (Podulka etal. 2001). Illustration by N.

John Schmitt.

graphic distribution that includes much of the forestedhabitat in North America, spanning eastern decidu-ous forest, boreal forest, and western montane forest.As a group, the forest thrushes require a set of similarhabitat conditions. Each species, however, occupies aunique ecological niche and thus requires a specificset of forest conditions to breed successfully and sus-tain viable populations. Furthermore, individuals of the

same species breeding in geographically disjunct pop-ulations, may have somewhat different habitat needs,thus requiring slightly different management actions.For example, appropriate forest patch size is importantfor Hermit Thrushes nesting in both Vermont and Idaho(Hames et al. 2002); however, because of differencesin forest type and the amount of forest available in thelandscape, management actions designed to achieve

the same population goals for this species may bevery different in the two states. The degree to whichindividual characteristics, such as forest size or shape,play a role in determining habitat quality is highlyvariable from place to place (Rosenberg et al. 1999b),as geographic location can have profound effects onfactors like climate and presence of competitors andpredators. To measure geographic variation in species-

specific habitat requirements, we studied each thrushspecies throughout its range (see sidebar, page 6). Thegoal of this publication is to provide forest managerswith regionally specific, science-based guidelines forassessing habitat quality, making important forest plan-ning and mitigation decisions, and improving habitatconditions for thrushes and other forest birds.

Which Woodlot?

Which Territory?

Which Nest Site?


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Figure 3. Forests fragmented by agricultural develop-ment persist as isolated patches that may vary in size,shape, and degree of isolation. Photo by Kenneth V.Rosenberg.

Managing the Contemporary Forest Throughout North American history, forests have

received intense pressure because of their ability toprovide renewable resources and sites for agricultureand development. The amount and quality of for-est habitat available for breeding birds is constantlychanging because of a cycle of forest clearing followedby regrowth. In the 1800s, millions of acres of forest inthe East and Midwest were cleared for agriculture andthe wood products they provided. Much of the farm-land created during this era of agriculture has beensubsequently abandoned and replaced by forest, caus-ing a net increase in forest area in the rural Northeastduring the past 100 years. Conversely, residential andcommercial development is outpacing forest regrowthin the suburban Southeast and Midwest, causing a netforest decline in those regions. The amount of forestin the mountainous West has remained roughly stable

during the past few decades because of vast acreagesowned and protected by the federal government. Theend result is a dynamic situation where the amount offorest is constantly changing within and among majorgeographical regions.

In addition to forest loss, there are a variety ofthreats that degrade habitat quality for forest birds. Anumber of forest health issues, such as hemlock woolyadelgid, beech bark scale, viburnum leaf beetle, acidrain deposition, and overbrowsing by white-taileddeer, threaten eastern forests. In western coniferousforests, fire suppression has altered the natural foreststructure and composition. Furthermore, increased fuel

loads in fire-suppressed areas lead to wildfires that aredevastating as opposed to rejuvenating. Finally, per-vasive forest fragmentation reduces habitat quality forbreeding birds in both eastern and western landscapes(Hames et al. 2002).

Effective forest management for bird populationsrelies on long-term planning, periodic needs assess-ment to ensure adequate amounts of high-quality for-est habitat in all regions, and science-based guidelinesthat consider forest bird ecology and contemporarythreats. The responsibility for long-term planning atecoregional scales falls mostly to state and federalagencies and some large conservation landowners

such as The Nature Conservancy. Yet, the contribu-tions of private landowners, individual natural resourcemanagers, and commercial forest owners is paramountto providing adequate amounts of quality breedinghabitat for forest birds.

Some key characteristics that determine a forest’squality as breeding bird habitat include its size andshape, how isolated it is from other forests, the land-use surrounding it, and its age and structural develop-ment. Most of these characteristics can be manipulatedby forest managers to benefit birds. Furthermore, these

characteristics are interrelated and influenced by theamount of forest fragmentation in the surroundinglandscape, permitting managers to focus on one or twokey characteristics that can potentially improve theoverall quality of the habitat. Management activitiesthat benefit forest birds will often involve the replica-tion of small-scale natural disturbances that wouldnaturally influence ecological succession. For instance,removal of selected overstory trees can create gaps inthe forest canopy, thus simulating damage caused bystorms. These gaps stimulate understory growth, whichultimately provides food and nest sites for forest birds.

What Is Forest Fragmentation? Forest fragmentation occurs when large, contigu-

ous forests are divided into smaller patches by residen-tial and commercial development, roads, agriculture,and, in some cases, timber harvesting (Figure 3). Clear-cutting can temporarily fragment mature, contiguousforest until the clear-cut area has regenerated to a suc-cessional stage in which it no longer poses an ecologi-cal barrier to forest-interior birds. As mature forests be-come fragmented, less habitat is available for breeding


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birds, and a variety of factors, such as increased broodparasitism and nest predation, result in lower repro-ductive success in the habitat that remains (Brittingham& Temple 1983; Wilcove 1985; Martin 1988; Robinsonet al. 1995). Thus, forest fragmentation not only causesa net loss of habitat, it can also reduce the suitability ofremaining habitat in a region.

It is important to distinguish between a forest that

is fragmented by agricultural or urban developmentand a forested landscape composed of a mosaic ofmature and regenerating stands that result from timberharvesting. The first situation typically is more damag-ing to forest bird populations and may represent per-manent habitat loss, whereas the latter situation mayonly cause a temporary reduction in habitat for certainforest-interior species that rely on mature forests. Fur-thermore, early successional forests do provide habitatfor many bird species, including some Neotropicalmigrants that are declining. Forest fragmentation playsan important role in determining the characteristicsdescribed below.

Birds in Forested Landscapes: A scientific approach tomanaging forest birds

The Birds in Forested Landscapes (BFL) projectbegan in 1997, evolving from the highly success-ful Project Tanager. Together, these projects amassedperhaps the largest dataset ever collected on forest

fragmentation and birds. Project Tanager and BFL haveproved that it is possible for birders and amateur sci-entists alike to conduct continent-wide field research,helping to answer important scientific questions thatcan only be addressed through the collective efforts ofthousands of volunteer citizen-scientists.

BFL, with support from the National Fish andWildlife Foundation, the USDA Forest Service, and theEnvironmental Protection Agency has focused its effortson forest hawks, thrushes, and, since 2001, additionalspecies identified as high conservation priorities. Over1,000 trained volunteers surveyed more than 3,000study sites throughout North America, helping to deter-mine the habitat and landscape requirements of forest-dwelling birds.

BFL participants follow a simple yet rigorously tested protocol that includes selecting suitable studysites, visiting these sites at least twice during the breeding season to search for thrushes and hawks andlook for evidence of breeding, measure a suite of habitat variables, and then return data to Cornell foranalysis. Many land managers contributed to the project as active participants and site coordinators, help-ing volunteers with landscape measurements, obtaining maps, or gaining access to study sites. The man-agement guidelines for thrushes, resulting directly from this important study, are the second in a series ofconservation plans developed to help maintain and enhance populations of forest birds into the future.

P h o t o b y K e n n e t h V . R o s e n b e r g

Figure 4. The Varied Thrush breeds primarily inthe wet coniferous forests of the Pacific Northwest.

Photo by Donald Waite/CLO.


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General Guidelines: Key Forest Characteristics breeding individuals. Similarly, a forest in the shapeof a long corridor or peninsula does not offer as muchinterior habitat as a circular or square forest.

The degree of isolation, or the distance betweena patch and the surrounding forest, may also be impor-tant in determining the long-term persistence of birdpopulations (Hames et al. 2001). For nonmigratoryspecies, such as woodpeckers and owls, the ability ofyoung birds to disperse and establish new territoriesis greatly reduced when the habitat is isolated (Figure5). Forested corridors connecting isolated patches maybe critical for these populations. Also, if fragments areseparated by agriculture or regenerating forest, thepossibility for dispersal may be greater than betweenfragments that are separated by residential or commer-cial development where movement is limited.

Management Tips

• Maintaining Large Forest Tracts: Whenever pos-sible, avoid fragmenting large contiguous forest

Figure 5. Dispersal of Fledg- ling Northern Spotted Owl:When young birds disperse fromtheir natal areas, their survivaloften depends on their abilityto find suitable habitat nearby.Dwindling habitat in the PacificNorthwest makes this especially

true for the Northern SpottedOwl: loss and fragmentation ofthe old-growth coniferous forestswhere the subspecies makes itshome leave young owls with fewhabitat choices and expose themto many risks. Studies indicatethat if immature Northern Spot-ted Owls cannot find suitablehabitat within approximately 12miles (about 20 km) of their natalterritory soon after dispersing,they often succumb to starvation

or predation by Great HornedOwls. Some old-growth forest

patches fail to meet the bird'sneeds because they are too faraway or are too small in area.Other unsuitable forests are tooyoung, or contain a high propor-tion of deciduous trees. From theLab's Home Study Course in BirdBiology (Podulka et al. 2001). Il-lustration by N. John Schmitt.

Natal Territory Suitable Habitat, Acceptable Distancefrom Natal Territory

Habitat AreaToo Small

Habitat Too Far fromNatal Territory

Habitat Too Young or TooMany Deciduous Trees

Forest Size and IsolationEffects

The number of bird species breeding in a forest

patch is directly related to the size of the patch (Free-mark and Collins 1992). As the size of the forest in-creases, the number of breeding bird species increases.Bird species whose occurrence or reproductive successis reduced in small habitat patches are referred to asarea sensitive (Robbins et al. 1989). For example, somespecies will only establish breeding territories in theinterior of large forest tracts, far from an edge; theseare known as forest-interior specialists. Area-sensitivespecies may attempt to nest in small forest patches butare often unable to raise young successfully becauseof high rates of nest predation or brood parasitism.For area-sensitive species, the size and shape of for-

est tracts are crucial factors for successful breeding(Whitcomb et al. 1981). Forest patches that are toosmall may not offer enough interior habitat to sustain


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tracts; these areas have the ability to support thelargest number of forest-interior birds and will alsobe more likely to provide habitat for area-sensitivespecies. Also, it is much easier to protect existingforest than to “create” new forested areas. The totalamount of contiguous forest required to supporthealthy populations depends on a given species’area sensitivity. Some area-sensitive species such as

the Northern Goshawk require hundreds of acres tobreed successfully, while other less sensitive speciesmay require only 25 acres (10 ha). In addition, it’simportant to note that the landscape context (seepage 9) plays a critical role in determining a spe-cies’ habitat-area requirements.

• Minimizing Isolation: Minimize isolation of forestpatches by promoting reforestation of gaps betweendisconnected forest tracts. Forest birds generallyhave higher reproductive success in forest that iseither connected to or in close proximity to otherforest patches. This also facilitates dispersal and ter-

ritory establishment.

Edges Effects

The plight of many forest-nesting songbirds hasbrought into question the benefits of certain traditionalwildlife management techniques. For example, histori-cally land managers were trained to “develop as much‘edge’ habitat as possible because wildlife is a productof the places where two habitats meet” (Giles 1971).Creating edges increases local diversity by attracting

game species such as rabbits and deer, as well as avariety of nongame bird species such as Song Sparrowsand Northern Cardinals. We now know, however, thatforest-interior species may disappear from areas thatcontain extensive edge habitat. Gates and Gysel (1978)proposed the idea that edges may serve as “ecologicaltraps” for some breeding birds by providing a variety ofattractive habitat characteristics, while at the same timesubjecting the birds to higher rates of nest predationand parasitism. Evidence from several studies indicatesthat the detrimental effects of an edge can extend from150–300 feet (45–90 m) into the forest interior.

Management Tips

• Managing Forest Shape: Attempt to manage for for-est shapes that create the least amount of edge. Cir-cular plots are the best at maximizing forest interior,whereas square or rectangular plots are better thanlong, narrow strips (Figure 6). Circular forest plotsoffer less access to the interior for predators andnest parasites while maximizing habitat for area-sensitive species. Research shows that area-sensitivespecies tend to use forested areas that are at least330 feet (100 m) from an edge.

Brood Parasitism andNest Predation

Among the primary causes of loweredreproductive success of forest-interior birds infragmented habitats are increased rates of both

brood parasitism and nest predation. Broodparasites, notably Brown-headed Cowbirds(Molothrus ater ), are birds that do not build nestsor raise young on their own; instead, they layeggs in a host bird’s nest. The cowbird nestlinggenerally hatches first, outcompetes the host’snestlings for food, and receives the majority ofthe parental care. As cowbirds expanded theirhistoric range from midwestern prairies intonewly created agricultural land and fragmentedforests in the East, they encountered many newpotential host species (Robinson et al. 1995).Forest birds that have had no long-term exposure

to cowbirds often do not have mechanisms forrejecting cowbird eggs and are particularly vul-nerable to parasitism. Researchers believe thatparasitism by Brown-headed Cowbirds is a sig-nificant factor contributing to declining numbersof many songbirds in North America (Robinsonet al. 1995).

In addition to parasitic cowbirds, nestpredators such as jays, crows, raccoons, and do-mestic and feral cats that are not usually foundin extensive forests gain access to the interior offorest patches via roads, power-line cuts, andother openings. Studies consistently show thatnest predators have a greater affect on the repro-ductive success of forest birds in forest fragmentsthan in contiguous forest (Wilcove 1985, Smalland Hunter 1988).

Brown-headed Cowbird nestlings grow rapidly, fre-quently outcompeting the host’s nestlings for foodand parental care. This adult Common Yellow-throat is feeding a cowbird fledgling that’s morethan twice its size. Photo by John Gavin/CLO.


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Landscape Context Effects

The amount of forest in the landscape plays acritical role in determining the suitability of forestpatches, especially regarding patch size. In general,as the amount of forest in the surrounding landscapeincreases, the minimum required forest patch size formany species decreases. In the Midwest, for example,landscapes that remain at least 70% forested tend tominimize adverse effects of fragmentation on ScarletTanagers (Rosenberg et al. 1999b; Figure 7).

Management Tips

• Developing a Broad-scale Plan: Develop a long-range forest management plan at as large a scale aspossible. Plan to designate tracts that will be matureat each stage in the management plan and maintainconnections (corridors) between regenerating forestsand mature tracts to facilitate repopulation by birds.

In regions where public lands contain the majorityof contiguous or large forest tracts, the most benefi-cial management strategies may involve protectionor minimal disturbance to compensate for habitatloss and fragmentation on adjacent private lands.

• Promoting Reforestation: Promote reforestation ofartificial forest openings, areas surrounding forestpeninsulas, gaps between isolated forest tracts, andriparian corridors to create more forest interior forarea-sensitive species. Reforestation can be achievedby succession, essentially leaving the area un-touched for a number of years, or by planting nativetrees. Avoid planting monocultures, especially of

exotic species; these are less attractive to most forestbirds. Monitor forested plots for nonnative, invasiveplants, and devise a removal plan if necessary.

• Working Within Your Community: Work with yourneighbors and adjoining landowners to conserveadjoining forest to maintain contiguously forestedareas. Consider creating a landowners association,or work with a land trust or forest manager to pro-duce guidelines for people within your communitywho wish to maintain the commercial value andecological integrity of their forests. Take part in yourstate or regional Partners in Flight Working Group.

Figure 6. The shape of a forest patch affects the amountof edge and forest-interior habitat that is present. Roundand square patches have less edge and more forestinterior than long narrow-shaped patches. From theLab’s Home Study Course in Bird Biology (Podulka etal. 2001). Illustration by Keila Sydenstricker.

Figure 7. Minimum area requirements for area-sensitive species may depend on the amount of forest remainingin the surrounding landscape. In this example from the midwestern US, Scarlet Tanagers in a landscape that is70% forested require only a 66-acre (27 ha) forest patch for breeding. If the landscape is reduced to 40% forest,however, the minimum area required by tanagers is 605 acres (245 ha).

2,500 Acres with 40%Forest Cover

Minimum Patch Size forHighly Suitable Habitat is

605 acres.

Forested LandUnforested LandHighly Suitable Habitat

2,500 Acres with 70%Forest Cover

Minimum Patch Size forHighly Suitable Habitat is

66 acres.

Forested LandUnforested LandHighly Suitable Habitat


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Forest Structure Effects

When all the landscape level characteristics,such as forest size, degree of isolation, and landscapecontext are acceptable, forest structure and age be-come the key characteristics that determine a habitat’ssuitability for a particular bird species. A healthy forestaging under the natural forces of ecological successiontypically has the most diverse vegetation structure. Animportant aspect of forest structure is the amount ofvertical layering. Early and mid-successional forestsoften have two layers—an understory of shrubs andyoung trees and an overstory (canopy) of interlockingbranches created by mature trees. Late-successional(mature) forests usually provide the greatest amountof vertical layering. It’s possible to have several layers,including herbaceous ground cover, an understory ofshrubs and saplings, an intermediate layer of smalltrees, a lower canopy of medium sized trees, and a

primary canopy of mature trees (Figure 8). The diversehabitat associated with this vertical layering supportsa wide variety of birds. For example, you may haveWood and Hermit thrushes in the understory, Red-eyedVireos and Rose-breasted Grosbeaks in the intermedi-

Figure 9. Heavy browsing by white-tailed deer some-times results in complete loss of understory shrubs andsaplings, reducing the suitability of forests to thrushesand many other birds. Photo by Kenneth V. Rosenberg.

ate layers, and Scarlet Tanagers and Black-throatedGreen Warblers in the canopy. Many forest birds,including the thrushes, depend on the cover, food, andnest sites provided by a diverse and well-developedunderstory.

Management Tips

• Creating a Broken Canopy: Permitting sunlight to

reach the forest floor by creating gaps in the forestcanopy will help to stimulate a poorly developedunderstory in mid-successional and mature forests.When understory vegetation is very deficient orabsent, planting appropriate native, shade-tolerantshrubs, in addition to maintaining a broken canopy,may be required to establish an understory layer.

• Employing Silviculture: Silvicultural practices thatcreate uneven-aged forests with well-developed, butbroken, canopies usually provide the best habitat forforest birds. Avoid loss or change in tree-species di-versity and fitness by minimizing “high-grading” (the

removal of only the most valuable species and themost structurally superior trees). Where possible, pre-serve existing old-growth stands and provide for thedevelopment of future old growth by leaving areasundisturbed or unharvested for 150 years or more.

• Controlling Ungulates: Given the tremendousincrease in white-tailed deer densities during thepast three decades, it may be necessary to controldeer numbers or forest access to preserve a well-de-veloped understory (Figure 9). Where appropriate,hunting can help to control deer numbers. Whenhunting isn’t feasible, deer exclosures (fences) can

help to protect understory vegetation. In addition,grazing by livestock should be monitored carefullyto detect signs of damage to the understory layer.

Figure 8. Late-successional forests often provide a highdegree of vertical layering, including a dense shrubbyunderstory important for thrushes and other birds.Photo by Kenneth V. Rosenberg


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Regional Management Guidelines for Thrushes Using These Guidelines

The following sections provide specific informa-tion about the habitat requirements of forest thrushesin three forested regions—Eastern/Midwestern Forests,

Western Forests, and Northern Forests. These guide-lines are based primarily on the results of the Birdsin Forested Landscapes project and focus on mitigat-ing the effects of forest fragmentation on breedingthrushes. They describe the minimum area required toprovide thrushes with high, moderate, or low levels ofhabitat suitability. In addition, for some species theydescribe the importance of selected patch-level char-acteristics, such as forest structure and amount of corehabitat area.

Results from Birds in Forested Landscapes suggestthat each species of forest thrush responds differentlyto forest fragmentation, and the degree of area sensitiv-ity expressed varies by geographic region. We presentspecific guidelines for each of the three forest regionsto assess more accurately the effects of forest fragmen-tation, and provide guidelines that are relevant withinvarious portions of each thrush species’ breedingrange. For each forest region, we provide informationdesigned to help land managers evaluate and improvehabitat for thrushes in their area. Each regional profileincludes:

• a map and description of available forest types

• literature review of habitat relationships for thrushesin that region

• estimates of minimum habitat areas required to sup-port each thrush species

• the most common trees at sites containing thrushes

• results of forest structure analyses for each thrushspecies

• regional summaries that briefly present the mostimportant management strategies for that region.

Using Minimum-Area Tables Within each region, the likelihood that a for-

est patch of a given size will attract a given species ofthrush varies based on the amount of forest remainingin the surrounding landscape and how close the patchis to the nearest large forest (more than 500 acres or200 ha). We define a landscape as a 2,500-acre (1,000-ha) block surrounding each forest patch. Because theprobability of finding thrushes in a forest patch may bedependent on the amount of forest in the surrounding

landscape, we offer a range of minimum-area estimatesfor landscapes with differing proportions of forest. Theminimum-area estimates are arranged in MinimumArea Tables that provide a range of patch sizes required

to achieve varying levels of habitat suitability.The Minimum-Area Tables can be used to helpimprove habitat for breeding thrushes. For instance,the tables can be used:

• In conjunction with topographical maps, aerialphotos, and Geographic Information Systems (GIS),to remotely identify forest patches with a high prob-ability of supporting breeding thrushes. Identifyingforest patches remotely allows managers to maxi-mize their inventory and management efficiency byfirst focusing their efforts on sites with the greatestneed for protection and management.

• To predict the impacts of proposed managementactions or land-use changes on the suitability ofhabitat for breeding thrushes.

• To determine the degree of habitat enhancement(reforestation) needed to increase the probability ofattracting thrushes to a given forest patch.

• To develop long-term management plans at local,state, and regional scales.

• To make planning and mitigation decisions whenhabitat loss and fragmentation are unavoidable.

In the Minimum-Area Tables provided with each

regional profile, acreage estimates are listed for threelevels of habitat suitability. These are defined as follows:

• High Suitability: Habitats of this size have the sameprobability of supporting breeding thrushes as alarge patch of unfragmented forest.

• Moderate Suitability: Habitats of this size are 25%less likely to support breeding thrushes relative tounfragmented forest.

• Low Suitability: Habitats of this size are 50% lesslikely to support breeding thrushes relative to un-fragmented forest.

To use the minimum-area tables, first estimate

the percentage of forested land occurring in a roughly2,500-acre (1,000-ha) block surrounding your area ofinterest. Then, read across the row to determine theminimum area required to achieve desired habitatsuitability for thrushes. You can also use the tables todetermine the suitability of your particular forest patchbased on its size.


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East/Midwest Forest RegionDescription

We define the East/Midwest Forest Re-gion as the vast area east of the Great Plainsdominated by deciduous forest. Although

highly variable in tree composition and foreststructure, this large region tends to supporta rather uniform bird community, includingmany widespread forest-breeding speciessuch as the Wood Thrush.

Because the Great Plains experiencesless precipitation than areas to the east, thebroadleaf deciduous forests of this area tend toinclude more of the drought-tolerant oak-hick-ory association (Bailey 1995). Forests in thenorthern and eastern parts of the Midwest con-tain more maples, American beech, and bass-wood, whereas the western part becomes savanna-likeas it gradually gives way to prairie. Oaks and hickoriesdominate upland forests in the prairie region, whereasfarther west, floodplain forests contain cottonwood,willows, and elms (Bailey 1995). The more mountain-ous areas of Arkansas support oak-hickory forests thatinclude coniferous trees, especially pines. Forests of theMidwest are typically highly fragmented by agriculturaland, more recently, by urban development.

The Appalachian Mountains area is more heav-ily forested, with tree species composition changingwith elevation. Vast forests are dominated by oaks,yet include some of the most diverse forests in North

America. Lower valleys support mixed oak-pineforests, which are largely cleared, while at higherelevations the composition includes birches, Americanbeech, maples, elms, oaks, and basswood, with someeastern hemlock and white pine. Finally, spruce-fir for-ests and meadows can be found on the highest peaksin some areas.

Along the Atlantic Coast, relatively small for-ested areas are typically found within a largely urbanand suburban landscape. This is especially true in thecorridor between Boston and Washington, D.C. Someextensively forested areas occur in central Massachu-setts, southern New Jersey, and Virginia. The forests of

the northern part of this area are mostly deciduous,especially the Appalachian oak association. Pine-oakforests (pine barrens) are also found in dry sandy loca-tions that are frequently exposed to naturally occurringfires (Bailey 1995). Oaks, hickories, sweetgum, black-gum, red maple, and winged elm are also common.

Farther south along the coastal plain, evergreenoaks, laurels, and magnolias are common, while gumand cypress dominate coastal swamps. Historically,savannas of pine forests (longleaf, slash, and loblolly)with an understory of grasses and sedges dominate

most of the uplands in this area. The piedmont formsa transition between the southeastern coastal plainand the Appalachian Mountains. It is dominated byboth southern pines (such as loblolly and shortleaf)and mixed southern hardwoods (such as oak, hickory,sweetgum, blackgum, red maple, and winged elm).The Lower Mississippi River Valley was covered byhuge tracts of bottomland-hardwood forest; however,most of the forests have now been cleared for agricul-ture. Other large and small river valleys still supportimportant tracts of bottomland hardwoods.

Habitat Guidelines for Thrushes Wood Thrush

The Wood Thrush is the most common thrushspecies throughout most of the East/Midwest ForestRegion, occurring in many forest types from south-ern Canada to the Gulf Coast and the Great Plains.Although often considered a bird of mature, intactforests, this species shows much variability in habitatpreferences across its range, and it certainly inhabitssmall woodlots in highly fragmented landscapes. Anumber of studies have shown, however, that habi-tats used by Wood Thrushes do not always providethe requisites for positive reproductive success, and

that populations in highly fragmented forests maynot be sustainable, perhaps contributing to regionalpopulation declines. Therefore, our guidelines belowregarding minimum areas for Wood Thrush occupancymust be considered in combination with other habitatfeatures that may be necessary for successful breeding.

In the Birds in Forested Landscapes study, WoodThrushes were detected at 848 (54%) of 1,568 sitessurveyed in the East/Midwest Region. Fifty-threepercent of sites with Wood Thrushes were located indeciduous forest, 45% in mixed forest, and 1% in co-

0-33% forest 34-66% forest 67-100% forest

Pixel size = 1 km2


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0 200 400 600 800 1000

Forest Patch Size (ac)

P r e d i c t e d

P r o b a b i l i t y

Effects of Patch Size on Probability of Wood Thrush Presence

0.40

0.45

0.50

0.55

0.60

0.65

niferous forest. The most common treeson sites occupied by Wood Thrush wereoaks (65%), maples (62%), pines (25%),American beech (22%), and hickories(19%). Results from this study indicatethat Wood Thrushes choose habitatsbased more on the structure of the forestthan on the degree of forest fragmenta-

tion in the landscape. Specifically, WoodThrushes selected areas at lower eleva-tions with a higher density of saplings inthe understory and a taller canopy. Theyavoided areas with a high proportion ofconiferous trees. In addition, the WoodThrush proved to be area sensitive,although the size of patches requireddid not appear to change with amountof forest fragmentation in the landscape.High suitability forest patches are atleast 200 acres (80 ha), and suitabilitydeclines rapidly in patches less than 100 acres (40 ha;Figure 10). Small patches offer moderate suitability interms of occurrence during the breeding season, butother research has shown that Wood Thrushes in smallpatches suffer decreased reproductive success.

In addition to landscape level considerations,most studies, including Birds in Forested Landscapes,indicate that a dense understory of saplings and shrubsis a key habitat requirement. In fragmented landscapes,an appropriate combination of habitat characteristics,such as forest size, amount of core area, amount ofedge, and vegetation structure must be met to ensureadequate levels of reproduction that offset adult and

fledgling mortality (Hoover 1992, Robinson and Wil-cove 1994, Driscoll et al. in press). Driscoll et al. (inpress) further documented that Wood Thrush popula-tions in fragmented habitat with much edge sufferednegative population growth rates, whereas populationsin forest interiors or fragmented forests with less edgeexperienced positive population growth.

In a study of two riparian areas in central Ohio,Groom and Grubb (2002) found that the width offorested riparian corridors was less important than theamount of forest in the landscape for predicting WoodThrush presence. Hoover (1992) found that nestingsuccess was significantly higher in large (1,200 acre

or 480 ha) forest patches versus small patches (23 acreor 9.2 ha) in Pennsylvania. The author also indicatedthat nesting survival was positively correlated withpercent forest within a 1.2-mile (2-km) radius and theamount of forest core area. High rates of nest preda-tion in small forest tracts (less than 200 acres or 80 ha)explained the lower nesting success in highly frag-mented landscapes (Hoover et al. 1995). Numerousauthors have found Wood Thrushes to be more abun-dant and successful in large forest patches, especiallyin highly fragmented landscapes such as the Midwest

(Whitcomb et al. 1981, Galli et al. 1976, Lynch 1987).In general, forest patches exceeding 250 acres (100ha) foster adequate rates of nesting success by mitigat-ing the effects of predation and sometimes cowbirdparasitism.

Robinson and Wilcove (1994) suggest that the ef-fects of silvicultural practices depend on the landscapecontext and that low-volume selective logging is pref-erable to clear-cutting. Crawford et al. (1981) foundthat Wood Thrush tolerate uneven-age forest manage-ment and selective removal of mature trees scatteredthroughout a stand creates favorable conditions for thespecies.

In the Piedmont National Wildlife Refuge in

central Georgia, which is actively managed for Red-cockaded Woodpeckers, Lang et al. (2002) found thatsmall-scale thinning and burning had little effect onWood Thrush populations. In southeastern Missouri,Wood Thrushes were most abundant in mature foreststands when compared with forest stands harvested byclearcut, shelterwood, group selection, and single-treeselection forest regeneration methods (Annand andThompson 1997).

Veery The Veery is a common thrush in the north-

ern portions of the East/Midwest Forest Region and

throughout the higher Appalachian Mountains. Theytend to occur at higher elevations than the WoodThrush and are associated more with northern-hard-wood forests than with oak-hickory or bottomlands.The Veery has a strong affinity for wet sites, andalthough they may be common in disturbed or regen-erating forests, they are highly area sensitive and nottolerant of high levels of forest fragmentation.

In the Birds and Forested Landscapes study,Veeries were detected at 302 (31%) of 971 sites sur-veyed in the East/Midwest Region. Fifty-five percent of

Figure 10. The probability of finding breeding Wood Thrushes in theEast decreases sharply in forest patches less than 200 acres (80 ha).


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sites with Veeries were located in mixed forest, 44%in deciduous forest, and 1% in coniferous forest. Themost common trees on sites occupied by Veeries weremaples (73%), oaks (60%), pines (28%), Americanbeech (21%), and hemlock (20%).

Results from this study indicate that Veeries aremost affected by the degree of fragmentation in thelandscape and by the size of forest patches, especially

the amount of “core forest” away from edges. The mostsuitable sites are also those with a denser deciduousshrub understory. Highest-suitability habitat only oc-curs in landscapes with at least 70% forest cover, andin these landscapes, patches larger than 1,000 acres(400 ha) are required (Table 1). Even in landscapeswith 80% forest, patches of roughly 400 acres (165 ha)are required; minimum area is reduced to 127 acres(51 ha) if forest cover increases to 90%. Smaller forestpatches in more fragmented landscapes do providemoderate habitat suitability, however, patches greaterthan 300 acres (120 ha) are still required in landscapeswith 40% forest.

Other studies suggest that Veeries use a variety ofhabitats in different parts of their range. In New YorkState, Veeries select early successional forest for breed-ing (Kendeigh 1945, 1946). In highland areas of NorthCarolina, it’s a bird of mature oak forest (Odum 1950)and on Apostle Island in Wisconsin, it’s found primar-ily in aspen and red oak forests where the understory ismost dense (Beals 1960). In New Jersey, Veeries prefermoist forests that are 40–60 years old with a denseunderstory (Suthers 1987–1988).

Unlike the other forest thrushes, Veeries show astrong tolerance for disturbed forest with a well-de-

veloped understory (Dilger 1956). Noon et al. (1976)found that Veeries bred in 77% of available disturbedand successional forests, but only 18% of mature

undisturbed habitat. Bertin (1977) suggested that inmature woodlands, moisture regime was the primaryfactor driving habitat selection, with the dampest areasoccupied first and pointed to available shrub coveras the most critical habitat component. Veeries maybenefit from selective harvesting that opens up thecanopy and allows the understory to develop (Webbet al. 1977, Thompson and Capen 1988). Derleth et al.

(1989) found that Veeries tolerated small clearcuts (2–20 acres or 1–8 ha), but Morgan and Freedman (1986)noted that Veeries were absent from large clearcuts forsix years following the harvest.

Habitat selection may vary when other thrushesare present. In many portions of their range, Veerybreed primarily in deciduous forest, even when conif-erous forest is available. In a study conducted in Ten-nessee, in areas where their breeding range overlappedwith Swainson’s Thrush, the Veery used deciduous,coniferous, and mixed forests (Noon 1977). Althoughit was once absent from higher elevation spruce forestsin the Appalachian Mountains, Veerys have expandedinto the southern Appalachian spruce forests whencongeners are not present (Noon 1981).

Hermit Thrush In the East/Midwest Forest Region, Hermit

Thrushes are restricted largely to the northernmostportions and also to higher elevations through theAppalachians where coniferous forests are dominant.Small populations occur in other coniferous habitats atlower elevations, including cedar swamps, hemlock ra-vines, conifer plantations, and pine barrens. Althoughthis thrush is common and apparently increasing, itis nonetheless shown to be highly area sensitive andintolerant of forest fragmentation within its range.

In the Birds in Forested Landscapes study, Hermit

Table 2. Minimum area required to provide high,moderate, or low habitat suitability for HermitThrush based on analysis of 347 study sites in theEast/Midwest region (see page 11 for definitionsof habitat suitability).

Percentageof forest in2,500-acreblock

Minimum area (acres)required for

High Moderate Low

90 32 5 Any size80 56 9 170 88 14 160 132 22 250 196 32 340 289 47 430 435 71 620 NAa 111 1010 NA 193 18aNot Available—acreage values exceed the amount of forest inthe 2,500-acre block.

Table 1. Minimum area required to provide high,moderate, or low habitat suitability for Veerybased on analysis of 685 study sites in the East/ Midwest region (see page 11 for definitions ofhabitat suitability).



High Moderate Low

90 127 3 Any size80 396 10 Any size70 1004 26 Any size60 NAa 61 150 NA 136 240 NA 305 430 NA 708 920 NA NA 2410 NA NA 74aNot Available—acreage values exceed the amount of forest inthe 2,500-acre block.


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Thrushes were detected at 142 (26%) of 553 sitessurveyed in the East/Midwest region. Seventy-sevenpercent of sites with Hermit Thrushes were located inmixed forest, 20% in deciduous forest, and 2% in co-niferous forest. The most common trees present at sitesoccupied by Hermit Thrush were maples (75%), oaks(53%), pines (41%), hemlock (32%), and beech (27%).

Results from this study indicate that Hermit

Thrushes are most attracted to forests with a high conif-erous component, and they avoid sites with a largelydeciduous shrub understory. They are also area sensi-tive, although less so than the Veery or Wood Thrush.In highly fragmented landscapes, single large forestpatches may still offer highly suitable habitat for HermitThrush (Table 2), and even in more intact (50% forest)landscapes, patches roughly 200 acres (80 ha) in sizeare required. Smaller forest patches do offer moderatelysuitable habitat, especially if they contain the conifers.

In New York, Hermit Thrushes selected sitesin dry coniferous, mixed, or deciduous forests andwere sometimes associated with internal forest edges(Andrle and Carroll 1988, Dilger 1956). In forest com-munities in northwestern Wisconsin the abundanceof Hermit Thrushes was related to the amount of edgeand variables relating to patch shape (Hawrot andNiemi 1996).

Researchers compared two harvest techniques inriparian areas of northern Wisconsin (full tree harvestand cut-to-length (CTL)). Bird surveys one year prior toharvest and three years after harvest showed that birdcommunities were affected by both harvest types. Her-mit Thrush, along with Ovenbird and Black-throatedGreen Warbler showed significant declines. Withinthree years of harvest no individuals of these specieswere observed (Hanowski et al. 2003).

Management Summary for the East/Midwest Forest RegionLandscape Level

In this region, the primary consideration for all thrush species is the maintenance of large contiguousforest patches that promote reproductive rates that compensate for adult and nestling mortality. Thrushes inthis region will tolerate and may even benefit from patch-level disturbances such as light selective harvest-ing that does not contribute to forest fragmentation. The Veery and Wood Thrush are the most area sensitiveof the three species in this region and thus set the bar for minimum forest size. The required forest patchsize needed for consistent successful reproduction is highly related to surrounding land use (see Tables 1and 2); in general however, forest patches of 150 acres (62 ha) or larger will be adequate to provide moder-ate levels of habitat suitability, except in landscapes with little forest. In landscapes with greater than 50%forest, smaller woodlots can provide suitable habitat. In highly fragmented forests or landscapes with verylittle forest cover, minimizing edge and increasing connectivity among forest patches may increase habitatquality in small patches. For Veeries, maximize core forest by reducing the amount of edge habitat associ-ated with forest fragments. This can be done by managing patch shape, creating transitional, as opposed toabrupt edges, and reforesting areas between disconnected patches. In lowlands, especially in damp sites,management should focus on Wood Thrush; at higher elevations and in moist, mixed forests managementshould focus on both Veeries and Wood Thrushes; and in mostly coniferous forests and on drier sites, man-agement should focus on Hermit Thrushes.

Site Level For Wood Thrush and Veery, create and maintain forest habitat with a dense understory of decidu-

ous saplings and shrubs. Because of its area sensitivity, focus site-level management for Wood Thrushes onpatches greater than 200 acres (80 ha). This can be accomplished by planting appropriate native trees andshrubs, creating gaps in the canopy that permit sunlight to reach the forest floor, and controlling browsingby white-tailed deer and livestock. Moderate selective harvesting of timber is often tolerated by thrushes,and serves to increase the shrub layer. Clear cutting and heavy selective cutting are not recommended forthrush management. Hermit Thrush prefer a strong coniferous component and may benefit from coniferplantings or silvicultural practices that favor existing conifers, especially pines and eastern hemlock. Pro-tecting stands of hemlock and cedar swamps within hardwood forests, as well as protecting large areas ofpine-barrens habitat will also benefit Hermit Thrushes.


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Northern Forest RegionDescription

We define the Northern Forest Regionas a wide band of coniferous and northernhardwood forest from the western Great

Lakes to the Adirondacks of New York,northern New England, and the CanadianMaritime Provinces. This region corre-sponds to the Boreal-Hardwood Transitionand Atlantic Northern Forest Bird Conser-vation Regions (U.S. NABCI Committee2000).

The region is heavily forestedthroughout. The northern portion is borealspruce-fir forest with deciduous species becomingmore common further south. The area surrounding theGreat Lakes and along the Atlantic coast is transitionalbetween southern deciduous forests and the north-ern boreal spruce-fir forest. Deciduous forests tendto occur on favorable sites with good soils whereasconiferous forests are found on less-favorable siteswith poor soils. Common tree species are yellow birch,sugar maple, American beech, white pine, and easternhemlock (Bailey 1995). The mountainous areas of NewYork and New England have a vertical zonation of treespecies. The valleys have deciduous forests of sugarmaple, yellow birch, beech, and some hemlock. Thelower slopes have mixed forests of spruce, fir, maple,beech, and birch. Finally, pure stands of balsam fir andred spruce can be found above the mixed-forest zone

(Bailey 1995).

Habitat Guidelines for Thrushes Wood Thrush

Within the Northern Forest Region, WoodThrushes are common only in the transitional mixed-hardwood forests around the Great Lakes, southeasternCanada, and the Adirondack Mountains. They arestrongly tied to the deciduous component of theseforests and occur primarily at lower elevations. WoodThrushes are a declining species in this region, andmanagement to enhance populations is a high conser-

vation priority.In the Birds in Forested Landscapes study, Wood

Thrushes were detected at 71 (30%) of 236 sitessurveyed in the Northern Forest Region. Sixty-ninepercent of sites with Wood Thrushes were located inmixed forest, 14% in deciduous forest, and 15% inconiferous forest. The most common trees on sitesoccupied by Wood Thrush were maples (70%), pines(51%), hemlock (38%), birch (38%), and beech (30%).

The relatively small sample of Wood Thrushesfrom this region indicates a high degree of area sen-

sitivity that increases with degree of fragmentation inthe landscape. For example, in landscapes with 70%forest cover, 200-acre (80-ha) patches are required toachieve high suitability for the Wood Thrush, and asforest cover drops below 40%, high habitat suitability

cannot be achieved (Table 3). For moderate suitability,smaller patches may suffice; for example, a 50-acre(20-ha) patch in a 50% forested landscape (Table 3).

In Quebec, preferred habitat for this species isdescribed as mature, undisturbed forest, with dampground, dense shrubby undergrowth, and a thick layerof ground litter (Gauthier and Aubry 1996). Thus,Wood Thrush may be more of a habitat specialist in thenorthern portion of its range than further south.

Veery The Veery is common throughout the southern

and eastern portions of the Northern Forest Region,

from Manitoba to the Maritime Provinces, northernNew England, and the Adirondack Mountains. They

Table 3. Minimum area required to provide high,moderate, or low habitat suitability for WoodThrush based on analysis of 108 study sites in theNorthern region (see page 11 for definitions ofhabitat suitability).



High Moderate Low

90 69 8 180 125 14 170 201 22 260 310 35 250 470 52 440 712 79 630 NAa 123 920 NA 198 1410 NA NA 26aNot Available—acreage values exceed the amount of forest inthe 2,500-acre block.


Pixel size = 1 km2


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are most abundant in northern hardwood and transi-tional forests with a strong deciduous component atlower elevations. In Ontario, the range extends northwhere deciduous growth occurs in wetlands, afterforest fires, or following timber harvest (Cadman et al.1987).

In the Birds and Forested Landscapes study, Vee-ries were detected at 124 (49%) of 253 sites surveyed

in the Northern Forest Region. Seventy-four percent ofsites with Veeries were located in mixed forest, 17% indeciduous, and 8% in coniferous forest. The most com-mon trees present on sites occupied by Veeries weremaples (67%), birches (38%), pines (37%), hemlock(35%), and ash (21%).

Results of this study indicate that Veeries are lessarea sensitive than they are further south, with virtu-ally any size forest patch providing suitable habitatin landscapes that are at least 50% forested. They aremost strongly associated with dense deciduous shrubsin the forest understory.

In Maine, where Veery and Wood Thrush overlap,successional stage was found to be the most importantdifference in their habitat. Veeries inhabit youngerforests and wetter areas than the Wood Thrush, whileHermit Thrushes and Veeries are sometimes foundtogether in mixed coniferous-deciduous and/or mixedalder and maple forests (Morse 1971). Hagan andGrove (1999) found Veery to be most abundant in par-tially-cut forest in northern Maine. In Quebec, Veeriesprefer an open canopy and dense understory (Gauthierand Aubry 1996), and they were predicted to benefitfrom tree removal in mature forests, resulting in greatershrub cover and understory development (Clark et al.

1983). In the White Mountains of New Hampshire,Welsh and Healy (1993) found Veeries significantlymore abundant in managed rather than unmanagedstands.

Swainson’s Thrush The range of the Swainson’s Thrush overlaps the

entire northern forest region and extends northwardinto the boreal forest. In the southern portions, theyoccur generally at higher elevations than the Veery orWood Thrush and are associated more with coniferousforests, especially spruce and fir.

In the Birds in Forested Landscapes study, Swain-

son’s Thrushes were detected at 37 (25%) of 146 sitessurveyed in the Northern Forest Region. Seventy-threepercent of sites with Swainson’s Thrushes were locatedin mixed forest, 22% in coniferous forest, and 5% indeciduous forest. The most common trees present onsites occupied by Swainson’s Thrushes were birches(41%), maples (41%), firs (35%), spruces (35%), andaspens (32%). The small sample in this study did notindicate an affect of forest fragmentation on the Swain-son’s Thrush or allow the calculation of minimum areaof forest required for high habitat suitability.

This species has been described as a bird of theextensive spruce-fir forests of Quebec, in areas of rela-tively mature forest with “not very heavy understory”(Gauthier and Aubry 1996). In Maine, Hagan andGrove (1999) found Swainson’s Thrush to be the onlythrush showing a significant association with matureforest, and to be more abundant in landscapes man-aged with partial-cut, rather than clearcut techniques.

In a comparison of forest management techniquesin Labrador that attempted to emulate natural distur-bance, researchers compared bird abundances amongburned and clear cut former black spruce sites after 5,14, and 27 years of succession. Hermit Thrushes andSwainson’s Thrushes were lower in clear-cut sites andpeaked in the 14-year old burns where abundanceexceeded that of mature forests. Swainson’s Thrush waspositively correlated with deciduous tree cover andnegatively correlated with conifer cover. Logged areasdid support some of the same bird species but didnot mimic fire. This study suggests that forest manag-ers may want to allow some forests to burn naturally(Simons et al. 2002).

Hermit Thrush The Hermit Thrush occupies the entire northern

forest region and extends northward through the borealforest. It is common in both northern-hardwood andconiferous forests and is more strongly associated withconifers than the other thrush species. It also occurs ondrier sites, such as jack pine and deciduous forests onsandy soils.

In the Birds in Forested Landscapes study, HermitThrushes were detected at 143 (54%) of 265 sitessurveyed in the Northern Forest Region. Eighty percentof sites with Hermit Thrushes were located in mixedforest, 10% in deciduous forest, and 10% in coniferous

Table 4. Minimum area required to provide high,moderate, or low habitat suitability for HermitThrush based on analysis of 111 study sites in theNorthern region (see page 11 for definitions ofhabitat suitability).



High Moderate Low

90 25 2 Any size80 71 7 Any size70 166 16 160 359 34 250 750 71 440 NAa 148 930 NA 321 1920 NA NA 4510 NA NA 127aNot Available—acreage values exceed the amount of forest inthe 2,500-acre block.


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forest. The most common trees present on sites occu-pied by Hermit Thrushes were maples (66%), birches(48%), pines (39%), hemlock (30%), and oaks (23%).

Results of this study indicate that Hermit Thrushesare adversely affected by forest fragmentation and arehighly area sensitive in this region. In landscapes thatare 70% forested, highly suitable habitat consists offorest patches at least 166 acres (67 ha). In 50% for-

ested landscapes, the minimum area required increas-es to 750 acres (304 ha) (Table 4). Smaller patchesprovide moderately suitable habitat, however, exceptin highly fragmented landscapes with less than 20%forest cover. Sites with a high proportion of coniferoustrees in the canopy are strongly favored.

The Hermit Thrush is often described as the mostversatile of the thrushes in terms of habitat use, oc-curring in conifer or mixed forests on damp or drysites (Gauthier and Aubry 1996). Morse (1971) foundthat Hermit Thrushes bred in dry mixed forests, while

Veery and Wood Thrush were most commonly associ-ated with wetter sites. In New Hampshire, Holmes andRobinson (1988) reported similar results with HermitThrush using dry forests away from streams with rela-tively little understory. They are also tolerant of clear-ings, burns, and regenerating areas within the forestedmatrix, as well as conifer plantations. Freedman et al.(1981) noted that in hardwood forests of Nova Scotia,

Hermit Thrushes were present in “strip cuts,” but not inclear cuts or thinned forests. In a comparison of burnedand clear cut spruce forests 5, 14, and 27 years aftersuccession in Labrador, Hermit Thrush density peakedin 14-year-old burns and was positively correlatedwith snag density (Simons et al. 2002). They were alsomost abundant in partially cut forest in northern Maine(Hagan and Grove 1999). Thus, this species appears tol-erant of, or even to benefit from disturbance, includingactive management, within large expanses of forest, butnot tolerant of high levels of forest fragmentation.

Management Summary for the Northern Forest RegionLandscape Level

In general, the Northern Forest Region is more heavily forested than the East/Midwest and WesternForest Regions. For this reason, habitat protection through long-term planning and enhancement of foreststructure are the most important management considerations. A shifting mosaic of forest habitats at varioussuccessional stages will likely benefit the most species as long as disturbance and silvicultural practicesdon’t lead to forest loss or fragmentation over the long-term. Small-scale timber harvesting may be compat-ible with forest bird management (including all thrushes) as long as forests don’t become fragmented. In theNorthern Forest Region, the most area sensitive species are the Wood Thrush and Hermit Thrush. Generally,

forests of 150 acres (62 ha) or larger will provide these two species with high to moderate levels of habitatsuitability. In highly fragmented landscapes with less than 40% forest, patches of 350 acres (140 ha) orlarger are needed to supply high to moderate habitat suitability for Wood and Hermit thrushes. Enhance-ment of forest structure in small forest patches may be beneficial for the less area sensitive Swainson’sThrush and Veery.

Site Level In southern portions of the region with more deciduous trees, management should focus on Wood

Thrush and Veery by promoting and retaining deciduous understory and maintaining deciduous riparianbuffers. In coniferous forest areas, management should focus on Swainson’s Thrush and Hermit Thrush—Swainson’s Thrush seems to prefer mature, undisturbed spruce-fir forests with a sparser understory, whereasHermit Thrushes are more tolerant of disturbance, but not forest loss or fragmentation. Create and main-tain forest habitat with a dense understory of saplings and deciduous shrubs. This can be accomplished

by planting appropriate native trees and shrubs, creating gaps in the canopy that permit sunlight to reachthe forest floor, and controlling browsing by white-tailed deer and livestock. Moderate selective harvest-ing of timber is often tolerated by thrushes, and serves to increase the shrub layer. Clear cutting and heavyselective cutting are not recommended for thrush management. Hermit Thrush prefer a strong coniferouscomponent and may benefit from conifer plantings or silvicultural practices that favor existing conifers,especially pines and eastern hemlock.


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Western Forest Regionin the Western Forest Region.This small sample did not allowanalysis of habitat preference oraffects of forest fragmentation.

Tewksbury et al. (2002)found that in riparian areas of theWest (Montana, Idaho, Nevada,and Oregon), Veeries were lesscommon in grazed sites, inheavily settled or agriculturallandscapes, or in areas with littledeciduous riparian habitat. Theyrecommended that “managementshould focus on (1) preservingand enlarging deciduous habi-tats, (2) reducing cattle grazing indeciduous habitats, and (3) pro-

tecting the few relatively pristinelandscapes surrounding large deciduous riparian areasin the West.”

Swainson’s Thrush The Swainson’s Thrush is widespread in the

Western Forest Region, with two distinct populationsshowing rather different habitat associations. In theinterior portions, this species is found in montaneconiferous forests, but is often associated with decidu-ous or riparian vegetation at lower elevations. On thepacific slope, Swainson’s Thrush is found to sea level,primarily in riparian forests.

In the Birds in Forested Landscapes study,Swainson’s Thrushes were detected at 156 (43%) of365 sites surveyed in the Western Region. Habitatsoccupied by Swainson’s Thrushes differed between thePacific and interior portions of the region; in the Pa-cific, 62% of occupied sites were in mixed forest, 20%in coniferous, and 16% in deciduous forest, whereasin the interior, 49% of sites were in mixed forest, 48%in coniferous, and only 2% in deciduous forest. Themost common trees on sites occupied by Swainson’sThrushes were Douglas-fir (56%), pines (33%), alder(22%), cedar (21%), and spruce (21%). Results of this

study indicate that Swainson’s Thrush is highly associ-ated with deciduous shrubs in the forest understory. Inthe Pacific portion of the region, this species was notadversely affected by reduction in forest area or patchsize, but in the interior portion, they were strongly areasensitive and found highly suitable habitat only in themost extensively forested landscapes (Figure 11).

Although characterized as an area-sensitivespecies, results from several western studies reportconflicting results. Swainson’s Thrushes were found tobe area sensitive in cedar-hemlock forest in Idaho and

DescriptionWe define the Western

Forest Region as all areas west ofthe Great Plains, including vast,

primarily coniferous forests fromthe Rocky Mountains west to thePacific Ocean.

Along the Pacific coast,forests are dominated by cedar,hemlock, and sitka spruce to thenorth, and Douglas-fir, westernred cedar, hemlock, and red-woods further south. Willow,cottonwood, alder, and birchcan be found at lower elevationsand along rivers (Bailey 1995).The coniferous forests of theSierra Nevada Mountains aredominated by ponderosa pine on western slopes withlodgepole pine and Jeffrey pine replacing the pon-derosa pine on the eastern slopes. Mountain hemlock,fir, spruce, and various pines can be found at higherelevations. In coastal California, the sclerophyll forestcontains species such as live oak, laurel, madrone, andbayberry that have thick, leathery leaves that can toler-ate dryness.

The remainder of the intermountain west consistsprimarily of coniferous forests at higher elevations,interspersed with grasslands, sagebrush, and shrubs in

the lowlands. Some areas of broadleaf forest, especial-ly aspen, are also found here. Tree species compositionis strongly dependent on elevation and slope exposure.Throughout the Rocky Mountains, Engelmann spruceand subalpine fir dominate most of the subalpineareas. Douglas-fir, western redcedar, western hemlock,western white pine, western larch, grand fir, ponderosapine, and lodgepole pine are common forest types be-low the subalpine zone (Bailey 1995). In the southernfoothills and lower plateaus, scrub oaks are commonalong with pinyon-juniper woodlands. Mountains ofArizona, New Mexico, south through northern Mex-ico support diverse pine-oak forests, with important

riparian woodlands occurring in canyons and alonglowland river valleys.

Habitat Guidelines for Thrushes Veery

The Veery is an uncommon species in westernforests, where it is largely restricted to riparian andother deciduous vegetation at middle and lower eleva-tions. In the Birds in Forested Landscapes study, Veerieswere detected at only 15 (11%) of 138 sites surveyed


Pixel size = 1 km2


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riparian areas in Nevada (Freemark et al. 1995), butnot in Douglas-fir forests of the Pacific Coast (Lehm-kuhl et al. 1991). Evans 1995 found that abundancewas not significantly correlated with patch size inmature forest or edge in mixed coniferous forest of thenorthern Rockies. Stands in more fragmented land-scapes; however, had fewer nests and lower nest suc-cess than stands in more contiguous landscapes in thesame study area (Evans et al. 1998). Hejl et al. (2002)reported that Swainson’s Thrushes in the Central RockyMountains were positively associated with amount offorest and patch size, and negatively associated withedge in “all coniferous forest,” but found no relation-ship among these habitat variables in “mixed conifer,”“cedar/hemlock,” or “ponderosa pine forest.”

In coniferous forests of the northern Rockies,there is a generally consistent response to silviculturalpractices, with the highest abundances occurring inunlogged old-growth stands (Johnston 1949, Hejl et al.1995). Along the Oregon coast, immediate and signifi-cant declines were noted after substantial reductions inforest cover that resulted from silvicultural practices.

In British Columbia, Swainson’s Thrushes are sen-sitive to the silvicultural practice of removing decidu-

ous vegetation from regenerating coniferous forests.In a comparison of 11–22 year old conifer plantations(54–116 acres or 22–47 ha), nest sites were positivelycorrelated with remaining deciduous vegetation. Thisrelationship was strongest in areas with the fewestdeciduous trees. Nest success was positively correlatedwith the amount of willow associated with the nest site(Easton and Martin 2002). In northwestern California,Swainson’s Thrush occurred in highest densities alongedges, but edges also experienced higher rates of nestpredation, suggesting that these habitats may repre-

sent ecological traps in this region (Brand andGeorge 2000).

Hermit Thrush The Hermit Thrush is widespread

throughout the Western Forest Region, inconiferous forests of the interior mountains, aswell as on the Pacific Coast. In the Birds in For-

ested Landscapes study, Hermit Thrushes weredetected at 142 (30%) of 470 sites surveyedin the Western Region. Fifty-seven percent ofsites with Hermit Thrushes were located inconiferous forest, 42% in mixed forest, and 1%in deciduous forest. The most common treespresent on sites occupied by Hermit Thrusheswere pines (56%), Douglas-fir (44%), fir (30%),spruce (27%), and aspen (23%). Comparedwith other thrushes, Hermits occupied driersites, with fewer than expected sites beingclose to streams or other surface water.

Results of this study indicate that Hermit

Thrushes are highly area sensitive and as-sociated with the most extensive forests. In

landscapes with 70% forest cover, this species requirespatches greater than 160 acres (65 ha) for high suitabil-ity habitat (Table 5). If forest cover is reduced to 50%,over 800 acres (320 ha) is required, and landscapeswith less than 40% forest cover do not provide anyhighly suitable habitat. Smaller patches may providemoderately suitable habitat, however. Hermit Thrushesin the West are not as strictly associated with conifer-ous forest cover as this species is in the East.

A study in southeastern Wyoming of the effectsof timber harvest that, at least initially, interruptedlarge expanses of old-growth forest with small strip orpatch clearcuts compared avian species composition

Table 5. Minimum area required to provide high,moderate, or low habitat suitability for HermitThrush based on analysis of 302 study sites inthe Western region (see page 11 for definitionsof habitat suitability).



High Moderate Low

90 20 1 Any size80 64 2 Any size70 163 5 Any size60 385 12 Any size50 873 26 Any size40 NAa 60 130 NA 141 120 NA 361 410 NA NA 12aNot Available—acreage values exceed the amount of forest inthe 2,500-acre block.

0

0.2

0.4

0.6

0 20 40 60 80 100

Forest Cover (%)

P r e d i c �

thrush guide

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