occurrence and habitat of breeding least bitterns at st

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ONTARIO BIRDS DECEMBER 2008

Occurrence and habitat of breeding

Least Bitternsat St.Clair National

Wildlife AreaShawn W. Meyer and Christian A. Friis

Figure 1. Least Bittern nest sites in below: typical habitat and inset: marginal habitat at the St. Clair National Wildlife Area in Typha spp.Her Majesty the Queen in Right of Canada: Photos: Canadian Wildlife Service, 2008

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Introduction The Least Bittern (Petit Blongios) (Ixo-brychus exilis) is a small buffy-colouredheron (~ 30 cm long and 80g in weight)that breeds primarily in southern marsh-es in Manitoba, Ontario, Quebec, NewBrunswick, and potentially Nova Scotia.Its contrasting black head and backagainst its white underside and buffywing patches readily distinguishes thismarsh bird from all others. In Ont ario, itbreeds primarily in cattail (Typha sp.),but has also been obser ved nesting inbulrush (Scirpus sp.), grasses, horsetail(Equis etum sp.), sedges (Carex sp.), wil-lows (Salix sp.) and dogwood (Cornussp.). In Ont ario, Least Bitterns typicallyinitiate breeding in mid-May, with pairformation, territory defence and eventu-al egg laying (egg dates: 15 May to 2August) (Peck and James 1983). It is avery secretive marsh bird and, therefore,difficult to detect. Its soft, low-pitched“coo-coo-coo” is often unnoticed, and itsavoidance of call broadcasts (Tozer et al.2007) results in poor detection rates, andthus, an incomplete population estimatein Ontario (Woodliffe 2008). Habitatdeg ra dation and habitat loss continue tothreaten this and other marsh birdspecies, and have resulted in the LeastBittern being designated as a Threatenedspecies in Ontario and Canada.

Ontario’s population of Least Bitternis concentrated around Lake St. Clair,Long Point and areas south of the Cana-dian Shield, from approximately Peter-borough to Kingston, including the

marshes of eastern Lake Ontario (Wood -liffe 2008). In the Lake St. Clair vicinity,marshes associated with the shorelineprovide breeding habitat for many LeastBitterns. The St. Clair National WildlifeArea (SCNWA) is one such marsh. Itwas designated for inclusion on theRamsar List of Wetlands of Internation-al Importance in 1985, and it currentlyconsists of two units: (1) Main Unit(~244.0 ha) and (2) Bear Creek Unit(~89.5 ha). It is recognized for its highbiodiversity and nat ional significance towildlife, and has become a perennialbirding “hotspot” in southwesternOntario. Both historical and currentrecords from the SCNWA suggest thatnumerous Least Bitterns nest withinthese marshes. The purpose of this arti-cle is to document the occurrence andhabitat of breeding Least Bitterns inboth units of the SCNWA, with addi-tional reference to other southwesternOntario sites. This information will sup-port the identification of critical habitatfor this species at risk, as well as helpguide SCNWA management.

ObservationsData for breeding Least Bitterns werecollected between 9 May and 15 July2007 and 8 and 9 July 2008. They werecollected in 2007 while conductingmarsh bird and Least Bittern surveys inthe SCNWA and while collecting nest-ing habitat data in 2008 to help identifycritical habitat for Least Bitterns on theSCNWA. Nest record data were collected

following the methodology of theOntario Nest Records Scheme (Peck etal. 2001). In all instances, disturbance tothe adult birds and/or young was mini-mized. For example, data collectionaround the nest was limited to a fewminutes and was not collected duringinclement weather such as rain or heavywinds. Global Positioning System (GPS)coordinates were collected using a Trim-ble GeoXT (accuracy ± 1.0 m). Resultsare summarized into four sections: (1)

nesting, (2) semi-colonialism, (3) nest-site fidelity and (4) reproductive success.If data were collected over multiple visitsto a particular nest, either the highestrecorded breeding level (i.e., eggs <chicks < fledglings) or that of the lastrecorded visit was used in the summary.

NestingIn total, 14 and 27 active Least Bitternnests were found in 2007 and 2008,respectively (Table 1) (Figure 1a and 1b).

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Figure 2. Least Bittern feeding platform in Typha angustifolia at the St. Clair National Wildlife Area. Her Majesty the Queen in Right of Canada: Photo: Canadian Wildlife Service, 2008

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2007 Typha angustifolia 7 Eggs = 3.57 ± 1.81 Well hidden (2) 52.14 ± 17.64 Narrow-leaved Cattail Partially hidden (5)

Typha glauca 3 Eggs = 4.00 ± 1.00 Well hidden (1) 49.00 ± 21.07 Common Cattail Young = 0.67 ± 1.15 Partially hidden (1)

Exposed (1)

Typha spp. 2 Eggs = 2.50 ± 3.54 Partially hidden (2) 39.00 ± 21.21 Cattail species Young = 1.00 ± 1.41

Scirpus validus 1 Eggs = 4 Exposed 22.00Softstem Bulrush

Sparganium 1 Eggs = 4 Exposed N/A*eurycarpumLarge-fruited Burreed

TOTAL 14† Eggs = 3.50 ± 1.70 — 45.78 ± 18.52 Young = 2.00 ± 0.00

2008 Typha angustifolia 12† Eggs = 2.50 ± 1.68 Well hidden (8) 66.82 ± 15.96 Narrow-leaved Cattail Young = 0.50 ± 1.24 Partially hidden (3)

Typha glauca 5 Eggs = 1.60 ± 1.95 Well hidden (3) 54.60 ± 7.89 Common Cattail Young = 1.40 ± 1.95 Partially hidden (1)

Exposed (1)

Typha spp. 2 Eggs = 4.50 ± 0.71 N/A* N/A*Cattail species

Sparganium 7† Eggs = 2.29 ± 1.70 Well hidden (4)eurycarpum Young = 1.29 ± 1.70 Partially hidden (2) 57.00 ± 21.55 Large-fruited Burreed

Acorus calamus 1 At least 1 egg* Partially hidden 75.00Sweetflag, Calamus

TOTAL 27 Eggs = 2.37 ± 1.74 — 61.96 ± 16.47 Young = 0.81 ± 1.47

* - data were not collected at nest to avoid disturbance. † - one nest was excluded from summary due to lack of data

Year Dominant Number Average number of Nest Exposure Average water depth Vegetation at Nest of Nests eggs or young ± SD (number of nests) at nest (cm) ± SD

Table 1. Habitat type, nest contents, locations and characteristics of Least Bittern nests recorded at the St. Clair National Wildlife Area in 2007 and 2008.

Only active nests (i.e., nests with eggsand/or young) were recorded becauseLeast Bitterns are known to build dum -my nests or platforms for feeding andbrood rearing (Gibbs et al. 1992) (Fig-ure 2). In 2007, 12 nests were built incattail with one in softstem bulrush andone in burreed (Sparganium sp.). Nestheights, to the bottom of the nest, var-ied between a low of 23 cm (bulrush)and a high of 115 cm (cattail) with amedian of 77 cm (mean = ~74 cm).Average cup depth was 5 cm in vegeta-tion that was growing in approximately46 cm of water (Table 1).

In 2008, 19 active nests were foundin cattail plus eight in non-persistentemergent vegetation (e.g., burreed andsweetflag [Acorus calamus]) (Figures 3a,3b, 3c and 3d). Nest heights, to the bot-tom of the nest, varied between 22 cm(burreed) and 120 cm (cattail) with amedian of 50 cm (mean = ~56 cm).Aver age cup depth was approximately 4 cm and nesting vegetation was onaverage in 62 cm of water (Table 1).

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84.29 ± 19.81 4.33 ± 0.82

65.00 ± 24.06 6.33 ± 3.21

95.00 ± 14.14 7.50 ± 3.54

23.00 2.00

N/A* N/A*

74.14 ± 26.80 5.08 ± 2.36

66.18 ± 22.57 3.38 ± 2.26

72.40 ± 25.67 3.75 ± 2.36

N/A* N/A*

29.67 ± 6.65 2.33 ± 1.63

30.00 20.00

56.43 ± 26.34 4.00 ± 4.36

Average height of nest Average cup depth above water (cm) ± SD of nest (cm) ± SD

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Figure 3a. Least Bittern nest inTypha x glauca at the St. Clair National WildlifeArea. Her Majesty the Queenin Right of Canada: Photos:Canadian Wildlife Service,2008

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top: Figure 3b. Least Bittern nest Sparganium eurycarpum (encircled),right: Figure 3c. Least Bittern nest Acorus calamus, above: Figure 3d. Least Bittern nest Typha angustifolia (encircled), at the St. Clair National Wildlife Area. Her Majesty the Queen in Right of Canada: Photos: Canadian Wildlife Service, 2008.

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Semi-colonialismAlthough the term “semi-colonialism” isnot defined clearly in the literature, ourresults confirm anecdotal evidence thatsome Least Bitterns at the SCNWA tendto nest in small colonies (13 of 41 nests).Least Bitterns have been documented assemi-colonial nesters in southern Florida,where the range from nearest neighbourwas 1.0 to 6.1 m (Kushlan 1973).

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Figure 4a above and 4b right:Locations of selected active Least Bittern nests atthe St. Clair National Wildlife Area showing proxim-ity to nearest neighbouring nest (inner ring = 6 m /outer ring = 12 m). Maps produced by CWS-ON – Background Data:True Colour Ortho Photos, 2006 © First Base Solutions, 2006.

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To illustrate this range with nests fromSCNWA, all active Least Bittern nestswere mapped using ArcGIS 9.2 softwarefrom GPS coordinates collected in thefield. A concentric ring corresponding toa 6-m radius was then overlayed on activenest locations in close proximity to oneanother to show potential semi-colonial-ism. A 12-m radius was also overlayed forcomparative purposes, and to investigate

possible ranges to nearest neighbour atthis latitude (Kushlan 1973). In 2007and 2008, evidence of semi-colonialismexisted in one and three distinct areas ofSCNWA, respectively (Figures 4a and4b). In these four areas groups were offour, four, two, and three nests, with arange from closest to furthest distance of4.5-64, 20-108, 35 and 46-135 metres,respectively.

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Figure 5. Selected Least Bitternnest locations between 2007(blue) and 2008 (yellow) at St. Clair National Wildlife Area,illustrating proximity betweennesting seasons (inner ring = 1 m / outer ring = 5 m). Maps produced by CWS-ON –Background Data: True ColourOrtho Photos, 2006 © FirstBase Solutions, 2006.

Nest-site Fidelity No record of Least Bitternbanding at SCNWA or inthe surrounding vicinity isknown. Therefore, deter -mining nest-site fidelity ofindividual birds is notpos sible. The results, how -ever, show that some LeastBitterns are nesting withina few metres of wherebirds nested the previousyear (Figure 5). This sug-gests re-occupancy of thesame territory in subse-quent years by returningpairs.

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Reproductive SuccessTargeted Least Bittern surveys were com-pleted at SCNWA in 2007 as well as atLong Point and Big Creek NationalWildlife Areas. Data from these surveysshow nesting at all sites and in relativelyhigh numbers. Although reproductivesuccess was not completely documented(all sites combined: 13 of 21 nests hadeggs or young on the last visit; 4 of 21failed at egg stage; 4 of 21 had adults onthe nest when visited), it is probable thatmany of these birds, and those not detec -ted, successfully reproduced in 2007.Similarly at the SCNWA in both years,reproductive success was not document-ed (23 out of 27 nests had eggs in July2008; 5 of 14 in July 2007).

DiscussionOur results show thatLeast Bitterns use floodedemergent vegetation fornesting, will nest in closeproximity to one another(i.e.,semi-colonially), andwill use similar locationsin the marsh each year ifhabitat conditions do notchange. Results from thisstudy also quan tify theextent to which Least Bit-terns nest in non-persist-ent emergent vegetation(e.g., burreed) in SCN -WA, and show that LeastBitterns nest in somenumbers where habitatconditions are appropri-

ate. Future research will be required tohelp understand what “ideal” habitatconditions are, as well as more details ofthe breeding biology of Least Bittern inOntario, to help guide its recovery andassist SCNWA management.

NestingTo our knowledge, this is the first studyin Ontario that quantifies the use ofemergent vegetation by nesting Least Bit-terns at a wetland. Numerous studieshave documented the use of emergentvegetation (e.g., cattail, sedges, bulrush,etc.) by breeding Least Bitterns (Weller1961, Manci and Rusch 1988, Gibbs etal. 1992, Bogner and Baldassarre 2002,

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Winstead and King 2006a, Winstead andKing 2006b, Rehm and Baldassarre2007) and more specifically, nest habitatcharacteristics of Least Bittern in Ont ario(Peck and James 1983). Our results cor-roborate ranges of nest habitat character-istics from Ontario data but differ fromranges in other North American studies(e.g., nests from this study were generallyplaced higher above water and in deeperwater) (see above references). Futurestudies should examine microhabitatcharacteristics (e.g., plant stem densityand residual vegetation cover) aroundnests to help refine critical habitat fornesting Least Bittern.

Semi-colonialismAlthough most members of the Ardeidfamily nest colonially, Least Bitterns arepredominantly solitary nesters, and areknown to nest in loose colonies (Kushlan1973, Gibbs et al. 1992). This study pres-ents the first documented evidence ofsemi-colonial nesting of Least Bitterns atSCNWA and confirms previous anecdot-al evidence (John Haggeman, pers.comm.). More locations of semi-colo-nialism were evident in 2008 (n = 3) thanin 2007 (n = 1) suggesting that changesmay have occurred in either populationdemographics (ratio of adults to sub-adults) or resource availability. Thereturn of many sub-dominant birds mayexplain semi-colonial nesting of LeastBitterns at SCNWA presumably becausesub-dominant birds are less aggressiveand/or experienced when defending ter-ritories. Banding with further observa-

tion would corroborate possible changesin population demographics as well asconfirm natal philopatry.

Changes in resources, specificallynesting habitat and prey availability, mayalso explain the high concentration,including semi-colonialism, of breedingLeast Bitterns at SCNWA. The germina-tion and growth of wetland plants isdetermined by many factors (e.g., turbid-ity, soil type, available nutrients) but ulti-mately water levels drive the distributionand extent of wetland plants such as cat-tail and burreed (Keddy and Reznicek1986, Mitsch and Gosselink 2000). Lowwater levels in 2007 resulted in the ger-mination of some non-persistent emer-gent vegetation, but without a completedrawdown (i.e., where water levels aresignificantly lowered in a managedmarsh), cattail cover was generally notaffected. Consequently, in 2008, morenon-persistent emergent vegetation, inconjunction with higher interspersion,likely provided more high quality breed-ing habitat for Least Bitterns comparedto 2007. This, in turn, affected otherbiotic communities (e.g., fish, amphib-ians and aquatic invertebrates) whichhave been shown to depend on wetlandplant species (Turner and McCarty 1998,Angradi et al. 2001) as well as wetlandhabitat quality (Burton et al. 2008).Many of these faunal species are prey forLeast Bittern (Gibbs et al. 1992) and thuslikely affected food availability, andpotentially allowed them to nest in highdensities (i.e., semi-colonially).

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Although most polygynous species inNorth America are found in wetlandsand grasslands (Cech et al. 2001), polyg-yny has not been documented in LeastBittern (Gibbs et al. 1992). Resourceavailability (specifically food) has beenshown to affect rates of polygyny in otherbirds, such as the Marsh Wren (Cist o - thorus palustris), Savannah Sparrow(Passerculus sandwichensis) and Red-winged Blackbird (Agelaius phoeniceus)(Kroodsma and Verner 1997, Wheel-right and Rising 1993, Yasukawa andSearcy 1995). Thus, further studiesshould examine the dynamics amongwetland vegetation, prey availability andbreeding behaviour of Least Bittern (pos-sibly polygyny) particularly in relation towetland water-level management.

Nest-site FidelityAlthough the results of this study do notconfirm nest-site fidelity in Least Bit-terns, the data show that in someinstances the same nesting areas (within5 m) were used between years. Birds weregenerally evenly distributedacross the SCNWA, andwhether birds returned tothe same area because thehabitat was of high quality,or they simply used an arearepeatedly, remains unclear.There is undoubtedly highquality breeding habitat (i.e.,nesting and prey resources)at these sites, and futurestudies are required to help

determine the extent to which Least Bit-terns use a marsh complex as a whole andat finer scales. Studies that identify themicrohabitat around nests that are usedsuccessively over time will help to eluci-date answers to this question. In addi-tion, reproductive success of these nestsas well as others will need to be docu-mented in order to determine whatresources (i.e., habitat, prey) are impor-tant for breeding Least Bitterns.

Reproductive SuccessThe annual breeding phenology of manybirds is determined by environmentalcues, such as the weather (Reed andElphick 2001). Colder temperatures andmore days with winds greater than 50km/h in 2008 may explain the high pro-portion of nests detected with eggs inearly July of this year (Table 2).

While Least Bittern nesting dates atSCNWA fell within the range of expect-ed egg dates (Peck and James 1983), onaverage, most of these birds would beexpected to initiate nesting closer to the

Year Mean Number of Days temperature (oC) with winds >50 km/hr

(Maximum wind speed)

May June May June

2007 13.93 19.16 3 (69) 6 (67)

2008 11.24 19.72 10 (72) 7 (85)

Table 2. Mean temperature and days with a wind speedgreater than or equal to 50 km/hr in the months of Mayand June of 2007 and 2008 (Environment Canada 2008).

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peak nesting period. Harsh weather in2008, however, may have delayed nestingor resulted in a high proportion of birdsre-nesting due to nest loss. High windscan result in nest failure due to eggsrolling out of nests (SWM and CAF pers.obs.) and may explain the increased use ofnon-persistent emergent vegetation in2008, as compared to 2007, as a result ofbirds attempting to re-nest. Conversely, itis also possible that many of the nestobservations in 2008 were birds attempt-ing to double brood; Least Bittern fledg-lings, many greater than two weeks old,were observed at this time. Bogner andBaldassarre (2002) documented doublebrooding of Least Bitterns in New YorkState. SCNWA is at a similar latitude asnorthern New York State. Therefore, it ispossible that Least Bitterns in Ontariomay double brood and this behaviourshould be looked for in Ontario.

Although natal philopatry has notbeen confirmed in Least Bitterns (Gibbset al. 1992), it is possible that high repro-ductive success in 2007 resulted in a highreturn of sub-adults to the SCNWA in2008. This, then, may have resulted inmany inexperienced and sub-dominantbirds returning in 2008 and nesting inless-preferred vegetation (e.g., burreedand sweetflag) because dominant birdshad already established breeding territo-ries in the marsh. This may also explainthe location of some Least Bittern nests inmarginal habitat, such as fringing cattailseparated from main stands (Figure 1b).

ConclusionResults from these observations showthat, at a minimum, 27 pairs of Least Bit-terns nested within both units of theSCNWA in 2008. In 2007, there were 14nesting pairs within both units. This sug-gests that Least Bitterns will quicklyrespond to high quality habitat condi-tions by breeding in high densities andsemi-colonially. Observations from thisstudy, however, also indicate that furtherresearch is required in order to helpunderstand the breeding biology of LeastBittern in Ontario, as well as continue toadaptively manage the SCNWA for thefuture benefit of all wildlife.

AcknowledgementsWe wish to thank Graham Bryan, GlennCoady, Lesley Dunn, John Haggeman, RossJames, Nancy Patterson and Chip Weseloh forcomments on this article.

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Bogner, H.E. and G.A. Baldassarre. 2002.Home range, movement, and nesting of Least Bitterns in western New York. Wilson Bulletin 114:297-308.

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Burton, T.M., J.C. Brazner, J.J.H. Ciborowk-si, G.P. Grabas, J. Hummer, J. Schneider andD.G. Uzarski (Eds.). 2008. Great Lakes CoastalWetlands Consortium Monitoring Plan. GreatLakes Coastal Wetlands Consortium. UnitedStates Environmental Protection Agency –Great Lakes National Program Office.http://www.glc.org/ wetlands/final-report.html

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Manci, K.M. and D.H. Rusch. 1988. Indicesto distribution and abundance of some incon-spicuous waterbirds on Horicon Marsh. Journalof Field Ornithology 59(1):67-75.

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Tozer, D.C., K.F. Abraham and E. Nol. 2007.Short call-broadcasts fail to detect nesting Least Bitterns (Ixobrychus exilis). Northeastern Naturalist 14:637-642.

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Shawn Meyer, Environment Canada, Canadian Wildlife Service – Ontario, 4905 Dufferin St. Toronto, ON M3H [email protected].

Christian Friis, Environment Canada, Canadian Wildlife Service – Ontario, 905 Dufferin St. Toronto, ON M3H [email protected].

Erratum: Great Egret sighted in the AzoresThe caption for Figure 1 on page 136should read: This map shows the Not-tawasaga Island banding location (star) ofa Great Egret and the Azores Islands wherethe bird was resighted during November2005-January 2006. Map courtesy ofAndrew Jano.

occurrence and habitat of breeding least bitterns at st

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