effects of prescribed burning on avian nest survival in the southern great plains

Habitat Relations

Effects of Prescribed Burning on Avian NestSurvival in the Southern Great Plains

ASHLEY M. LONG, Department of Biological Sciences, Emporia State University, Campus Box 4050, 1200 Commercial St. Emporia,KS 66801, USA

WILLIAM E. JENSEN,1 Department of Biological Sciences, Emporia State University, Campus Box 4050, 1200 Commercial St. Emporia,KS 66801, USA

RAYMOND S. MATLACK, Department of Life, Earth, and Environmental Sciences, West Texas A&M University, Canyon, TX 79015, USA

ABSTRACT Shrubs, such as mesquite (Prosopis spp.) and cholla (Opuntia spp.), now dominate fire-suppressed grasslands in southwestern North America. Responses of birds to prescribed burning of theshortgrass prairie in this region are poorly understood. We examined daily survival rates of mourning dove(Zenaida macroura) and lark sparrow (Chondestes grammacus) nests in an experimental landscape (4,811 ha) ofspatially replicated, inter-annual fire frequencies (burning every 2 yr, 4 yr, or 10 yr) near Amarillo, Texas.Herbaceous habitat structure was most developed in infrequently burned plots, but shrub densities were lessvariable among the burn treatments. We modeled daily nest survival (DSR) against burn frequency, shrubdensity at nest sites, and nest stage (incubation or nestling). Daily survival of mourning dove nests was notwell-related to any measured covariate, but lark sparrow DSR was negatively related to the duration of inter-annual burn frequency. In semiarid grasslands heavily inundated with shrubs, prescribed burning maypositively influence the nest success of some bird species. � 2011 The Wildlife Society.

KEY WORDS grassland birds, great plains, lark sparrow, mourning dove, nest survival, prescribed burning, shortgrassprairie, woody encroachment.

As found across the Great Plains, grassland bird species thatoccur in the southwestern United States, such as the northernbobwhite (Colinus virginianus), Cassin’s sparrow (Peucaeacassinii), lark sparrow (Chondestes grammacus), and westernmeadowlark (Sturnella neglecta), have exhibited range-widepopulation declines throughout the last century (Peterjohnand Sauer 1999, Butcher and Niven 2007). Habitat loss islikely the most influential ecological driver of these declines(Herkert et al. 1995, Vickery and Herkert 2001), but degra-dation of remnant grasslands (Brawn et al. 2001, With et al.2008) may also limit grassland bird populations. Fire sup-pression and overgrazing have driven habitat conversion ofsemiarid grasslands in the southwestern United States,resulting in the encroachment and dominance of woodyspecies, such as creosotebush (Larrea tridentata), cholla(Opuntia spp.), mesquite (Prosopis spp.), and salt cedar(Tamarix spp.; Archer 1989, Bahre 1991, Bahre andShelton 1993, Bahre 1995, Van Auken 2000). Honey mes-quite (P. glandulosa), the most prominently encroachingshrub species, is now estimated to cover more than 38 millionha of shortgrass prairie (Archer 1989, Van Auken 2000,Brockway et al. 2002).We have limited understanding of how grassland birds

respond to shrub encroachment and management used toreduce shrub density in the southwestern United States

(Ford and McPherson 1996, Brawn et al. 2001, Bock andBlock 2005). This is not the case for tallgrass prairie, wherethe biological consequences of fire suppression, subsequentwoody encroachment, and avian responses to grassland res-toration techniques have been well-studied (Zimmerman1992; Herkert 1994a,b; Reinking 2005; Powell 2006;Rahmig et al. 2009). Depredation and brood parasitism ofgrassland bird nests are positively related to nest proximity towoody vegetation in tallgrass prairie (Johnson and Temple1990, Winter et al. 2000, Jensen and Finck 2004, Jensen andCully 2005), thus prescribed burning that reduces overallshrub density should favor grassland-nesting species.Following fire, vegetation composition in shortgrass prairiemay also shift to favor greater abundances of grassland birds,suggesting that the reintroduction of fire as a managementtechnique may be positive for these species (Bock and Bock1992a,b; Kirkpatrick et al. 2002). However, prescribed burn-ing in this region is limited by land manager uncertaintiesregarding seasonal and historic fire frequencies, nutrientturnover rates, slow re-growth of vegetation following fireevents, and resistance to burning by private landowners(Wright and Bailey 1982, Brockway et al. 2002). Thus,such management is rarely attempted and, as a consequence,responses of grassland-dependent wildlife to prescribedburning in grasslands of the southwestern Great Plains arerelatively unknown (Ford andMcPherson 1996, Brawn et al.2001, Bock and Block 2005).Past research regarding fire, woody encroachment, and

birds in shortgrass prairie has lacked the use of spatially

Received: 24 January 2011; Accepted: 24 October 2011

1E-mail: [email protected]

The Journal of Wildlife Management 9999:1–8; 2011; DOI: 10.1002/jwmg.328

Long et al. � Nest Survival in Burned Shortgrass Prairie 1

replicated experimental units (Bock and Bock 1988) or hasbeen conducted in relation to single wildfire events (Bockand Bock 1992a, Reynolds and Krausman 1998, Kirkpatricket al. 2002). Also lacking have been comprehensive exami-nations of avian demographic consequences of fire suppres-sion and woody encroachment in shortgrass prairie (Bockand Block 2005). We examined daily survival of bird nests inrelation to experimental burning of a shortgrass prairie heavi-ly encroached by honey mesquite and cholla in the southernGreat Plains. Historically, the avian assemblage in the regionwas likely dominated by grassland species common to thesouthwestern United States, such as Cassin’s sparrows andwestern meadowlarks (Seyffert 2001); however, the currentavian community is more characteristic of shrub-steppevegetation (Seyffert 2001, Long 2010). We, therefore,limited our investigation to 2 species that are facultativeground nesters in grasslands (Drobney et al. 1998, Martinand Parrish 2000): mourning doves (Zenaida macroura) andlark sparrows. These species provided relatively large samplesizes of nests (n ¼ 56 and 318, respectively) and werethought to best represent the regional grassland bird com-munity on our site. We predicted vegetation structure, in-cluding shrub density, to generally diminish in response toprescribed burning, and, based on patterns of nest successand woody vegetation observed elsewhere (Johnson andTemple 1990, With 1994, Klug et al. 2010), daily nestsurvival (DSR) was predicted to be highest in areas thatwere more frequently burned.

STUDY AREA

We conducted our research from May to July of 2008 and2009 at the 4,811-ha Cross Bar Cooperative ManagementArea (CCMA) located approximately 20 km northwest ofAmarillo, Texas (358N, 1018W). The CCMA was histori-cally semiarid shortgrass prairie composed of graminoidspecies, such as blue grama (Bouteloua gracilis) and buffalograss (Buchloe dactyloides) and forb species, such as smallsoapweed yucca (Yucca glauca; Tanner 2004). In the past,honey mesquite and cholla were found intermittently at theCCMA; however, these 2 woody species now dominate thelandscape (Tanner 2004). Average monthly temperaturesand monthly precipitation from May to July, 1971–2000,ranged from 18.08 C to 26.08 C and 6.4–8.3 cm (NationalWeather Service 2009). Average monthly temperatures andprecipitation ranged from 19.18 C to 25.28 C and 5.3–12.6 cm during data collection in 2008 (National WeatherService 2009). During the 2009 field season, average month-ly temperatures and precipitation ranged from 17.78 C to25.68 C and 1.1–9.6 cm (National Weather Service 2009).The CCMA was originally purchased for use as a helium

storage facility by the United States Bureau of Mines in 1931(Tanner 2004). While being used for underground storage,the land was heavily grazed for >70 years (Tanner 2004).Grazing was discontinued on a majority of the property in1993 and was completely eliminated in 1998 after the landwas acquired by the United States Bureau of LandManagement (BLM; Tanner 2004). Similar to other south-western grasslands, years of overgrazing and fire suppression

resulted in high levels of woody encroachment (Tanner2004). In 2002, the BLM, in cooperation with WestTexas A&MUniversity (WTAMU), established and burned9 experimental units (hereafter plots; 120–220 ha) on theCCMA to examine the effect of prescribed burning on shrubdensity. Each plot was assigned a burn treatment (i.e., fireevery 2 yr, 4 yr, or 10 yr) in a stratified, random design, with3 replicates of each burn treatment. Most prescribed burns atthe CCMA were conducted in January and February.

METHODS

Habitat MeasuresWe measured habitat structure to 1) characterize habitatvariation among the burn units, and 2) provide covariatesthat might explain variation in nest success. Wemodified ourhabitat measurements from the Breeding Biology ResearchandMonitoring (BBIRD) Grassland Field Protocol (Martinet al. 1997). We made measurements across each plot(22 Jun–31 Jul 2008; 22 Jun–31 Jul 2009) from the centerof 13, 5-m radius circles located within 100-m radius circles.These 100-m radius circles were initially established system-atically across each plot to examine the effects of prescribedburning on avian abundance and community structure (seeLong 2010 for specific methodology and estimates of abun-dance, species richness, and diversity obtained using point-count sampling). The number of circles established per plot(range: 4–11) was determined by the total area of the plot,resulting in 30 circles located within plots burned every2 years, 34 circles located within plots burned every 4 years,and 44 circles located within plots burned every 10 years. Thevegetation sampling points were located at the center of eachcircle and at 30 m, 60 m, and 90 m from the center of thecircle in each cardinal direction. We also made the samehabitat measurements at each nest site upon the completionof each nesting attempt, where sampling points were cen-tered on nest locations.Wemeasured herbaceous plant height (cm) from the center

of each 5-m radius circle at 1 m, 3 m, and 5 m in eachcardinal direction using a tape measure. We made a modifi-cation to the Robel method (Robel et al. 1970), otherwisesuggested by BBIRD protocol, because of our limited reso-lution in estimating visual obstruction in the short-staturedvegetation of recently burned plots. We used median heightat each vegetation sampling point to reduce the effect ofextreme values in summarizing vertical vegetation structure.We visually estimated percent horizontal coverage of grasses,forbs, and bare ground within 4 quadrants of each 5-mvegetation sampling point. We then used mean percentcoverages to summarize horizontal cover for each cover type.We measured the density of shrubs to examine potential

influences of shrub structure on nest success. We used thepoint-quarter method (Cottam and Curtis 1956) to estimatedensities (stems/ha) of shrub species categories across theplots. We used a rangefinder to measure the distance fromvegetation sampling point centers to the closest shrub, perspecies category, in each quadrant per habitat samplingpoint. Shrub species categories included the following: large

2 The Journal of Wildlife Management � 9999

(�1.5 m) and small (<1.5 m) mesquite and cholla; we fur-ther categorized mesquite size classes as live and top-killedgrowth forms (i.e., basal re-growth following fire). We lim-ited our measurements of shrub density to these speciesbecause of their dominance on the landscape of theCCMA. Because of the varied sizes and growth forms ofthese shrub species, we estimated densities of several shrubcategories, including all shrubs (across species and size clas-ses), large shrubs (both species), small shrubs (both species),all mesquite (both size classes and growth forms), and allcholla (both size classes).We recorded additional habitat data specific to nest sites,

including nesting plant species, nest location (on-ground orin-shrub), nest height (cm from ground to the lowest portionof the nest), and distance from nearest grazed land.

Nest MonitoringIn 2008 (18 May–31 Jul) and 2009 (17 May–31 Jul), wesearched for nests on each plot every 3–4 days for approxi-mately 3–4 hours/plot. We used a combination of visual andauditory cues to locate each nest (Martin and Geupel 1993).Because of coverage constraints related to plot size, werotated the location from which searching began to ensureuniform search effort within plots. We marked most nestlocations by placing flagging approximately 5–10 m fromeach nest and relocated these nests using GlobalPositioning System (GPS) units. We did not mark somenest locations with flagging, but rather marked them usingonly GPS coordinates for an ancillary study on investigatoreffects on nest success. However, monitoring technique didnot affect daily nest mortality (Jacobson et al. 2011). Fieldpersonnel trained in proper monitoring techniques moni-tored most nests every 3–4 days (Martin and Geupel 1993)and rarely every 8–13 days for nests included in Jacobsonet al. (2011). We recorded nest contents until nestcompletion.We determined nest fate using evidence at the nest site.We

considered nests unsuccessful if found empty before estimat-ed fledging dates or if abandoned (i.e.,>3 visits to nests witheggs where incubation was not evident).We considered nestsfound empty on monitoring visits proximate to predictedfledging dates successful if fledglings were observed aroundthe nest, if fecal sacs were present, or if parental behaviorindicated the presence of unseen fledglings (Martin andGeupel 1993).

Statistical AnalysisWe averaged herbaceous plant height, percent ground cover(grasses, forbs, and bare ground), and shrub density (allcategories) across point-count circles per plot replicate.We then used 2-way analyses of variance (ANOVA) andFisher’s least significant difference follow-up tests in ProcGLM in SAS (SAS 9.2; SAS Institute, Cary, NC) tocompare habitat variable means among the burn treatmentsand to test for burn � year interactions. When the interac-tion between burn treatment and year was not significant, weused 1-way ANOVA with the single burn effect to assessvariation in habitat structure among the burn treatments.When burn � year was significant (P < 0.10), 1-way

ANOVAs were used to test the burn effect separately for2008 and 2009. We considered differences statisticallysignificant when P < 0.05 and marginally significant (i.e.,potentially biologically significant) when 0.05 < P < 0.10,as alpha is arbitrary (Johnson 1999).We used the logistic exposure method of Shaffer (2004),

using Proc GENMOD in SAS, to model DSR in relation toenvironmental covariates. For overall period survival esti-mates, from incubation through fledging of young, we as-sumed 29-day and 23-day nest periods for mourning dovesand lark sparrows, respectively (Baicich and Harrison 1997).We used an information–theoretic model selection proce-dure to determine the predictive ability for DSR of inter-annual burn frequency and the density of all shrub speciesaround nests. We modeled inter-annual burn frequency as acategorical covariate. We included the density of all shrubs atnest sites in models to generally represent woody encroach-ment. Other shrub species, and their growth forms, werecorrelated with the density of all shrubs and thus not includ-ed in candidate models. Additionally, we included develop-mental stage of offspring (nest stage; i.e., incubation ornestling) to models with burn treatment or shrub densityto test for the improvement of this covariate upon model fit.We did not model nest stage separately from burn treatmentor shrub density because we were interested in testing theparticular effect of the latter variables on DSR, not exploringbest predictors of DSR. Only those variables that were notcorrelated (Pearson’s correlation, P < 0.05) among nests ofparticular species were included in the same model. Weconsidered other covariates for DSR (e.g., year, date ofseason, and nest height) but these potential covariateswere correlated with both burn frequency and shrub densityand thus not included in candidate model sets. The candidatesets of models per species also included a constant (null)model of DSR from which to compare support for modelscontaining covariates. All other models containing covariatescontained at least the covariates for burn frequency or shrubdensity.We ranked model fit among the candidate set per species

using Akaike’s Information Criterion (AICc ; Burnham andAnderson 2002) corrected for small sample size (AICc ;Sugiura 1978). We then used differences in AICc (DAICc)and Akaike weights ðwiÞ to determine model support(Burnham and Anderson 2002). We considered modelswith DAICc � 2 competitive models. We calculated oddsratios (for categorical burn frequency ¼ ebi/ebj for burnintervals i and j) and covariate coefficients (for continuouscovariates), and their 95% confidence intervals, to determinethe effect of each covariate on DSR included best candidatemodels. Where we found multiple plausible models(DAICc � 2.0), we calculated model-averaged estimates ofcovariate coefficients across these top models. We estimatedperiod survival (Shaffer and Thompson 2007) using model-averaged slope estimates 1) overall per species, using meanlevels of continuous covariates and observed proportions ofobservations per categorical burn variable (if included intop models), and 2) per burn frequency, where burnwas determined to have a significant effect on DSR, while


holding any other covariates in those models at their meanvalues. An index of overdispersion ðcÞ was calculated bydividing model deviance by model degrees of freedom.

RESULTS

Habitat MeasuresMean herbaceous plant height, mean percent grass cover, andmean percent bare ground were the only habitat variablesthat showed statistically significant interactions betweenburn treatment and year (Table 1; i.e., the differences amongthe burn treatments depended upon the year of study). In2008, mean herbaceous plant height was marginally differentacross the 3 burn treatments, with mean herbaceous plantheight higher in plots burned every 10 years than in plotsburned every 2 and 4 years (Table 1). In 2009, mean herba-ceous plant height was significantly higher in plots burnedevery 4 and 10 years than in plots burned every 2 years(Table 1). Mean percent grass cover was significantly greaterin plots burned every 10 years than in plots burned every2 years and 4 years in 2008 (Table 1). In 2009, mean percentgrass cover was significantly greater in plots burned every4 years and 10 years than in plots burned every 2 years(Table 1). Mean percent bare ground was significantly great-er in plots burned every 2 years and 4 years than in plotsburned every 10 years in 2008 (Table 1). In 2009, meanpercent bare ground was significantly greater in plots burnedevery 2 years than in plots burned every 4 years and 10 years(Table 1). We found marginally significant differences in thedensities of large shrubs and large top-killed mesquite amongthe burn treatments with means of both being highest inplots burned every 2 years and 10 years and lowest in plots

burned every 4 years. The means of both of these shrubdensity categories were greatest, though not significantly so,in plots burned every 10 years (Table 1). We found noadditional differences in any other shrub density categoryamong burn frequencies (Table 1).

Nest SurvivalNest sample sizes per burn treatment for mourning dovestotaled 22 nests in 2-year burns, 20 nests in 4-year burns, and14 nests in 10-year burns. Lark sparrow nests were distrib-uted among burn treatments as follows: 150 nests in 2-yearburns, 102 nests in 4-year burns, and 66 nests in 10-yearburns. Ten percent of lark sparrow nests were parasitized bybrown-headed cowbirds (Molothrus ater), with only 1 nestcontaining >1 cowbird egg.Effects of overall shrub density and nest stage were includ-

ed in best fit models (DAICc � 2.0) of mourning dove DSR(Table 2). Both of these top models were reasonably wellfit ðc¼ 1:15-1:16Þ. The model-averaged estimate ofoverall DSR from these best models was 0.921 (95% CI:0.813–0.969) yielding an overall period survival (incubationto fledging) estimate of 9.3% (95% CI: 0.2–40.5%).Confidence limits of coefficients for both shrub density(model-averaged b < 0.001, 95% CI: �0.0001 to 0.0002)and nest stage (b < 0.001, 95% CI: �0.242 to 0.322) in-cluded 0, indicating no substantial effect of these covariates.A single model including the covariates burn and all-shrub

density was superior (DAICc � 2.0, wi ¼ 0:734) among thecandidate set in explaining variation in DSR of lark sparrows(Table 2; model fit was reasonable: c¼ 1:26). The estimate ofoverall DSR from the top model was 0.890 (95% CI: 0.799–0.943) yielding an overall period survival estimate of 6.9%

Table 1. Comparison of mean habitat variables (mean � SE) across 3 prescribed burn treatments (fire every 2 yr, 4 yr, and 10 yr) at the Cross Bar CooperativeManagement located near Amarillo, Texas, during 2008–2009. Means are listed separately per year where interactions between burn treatment and year werestatistically significant.

Habitat variableaBurn treatmenta

2 4 10

Herbaceous plant height2008�� 11.0A (1.00) 11.3A (2.84) 18.7B (2.73)2009� 9.2A (0.73) 32.7B (4.81) 34.3B (6.57)

Percent grass cover�

2008� 64.8A (1.48) 63.0A (3.13) 74.4B (1.62)2009� 61.6A (0.19) 76.8B (0.80) 77.4B (3.25)

Percent forb cover 10.5 (1.60) 12.3 (0.63) 16.2 (0.74)Percent bare ground2008� 35.2A (1.48) 37.0A (1.67) 25.6B (0.52)2009� 38.3A (0.21) 22.9B (0.84) 22.1B (3.35)

All shrub density 683.3 (123.90) 532.4 (105.13) 627.8 (159.87)Large shrub density�� 95.4AB (9.76) 65.7A (9.86) 118.2B (21.62)Small shrub density 490.6 (87.09) 385.1 (71.79) 355.2 (63.02)Mesquite density 313.8 (31.73) 256.0 (28.16) 316.0 (66.11)Large mesquite density 8.0 (1.98) 8.5 (3.68) 17.0 (4.58)Small mesquite density 43.8 (6.31) 52.3 (4.79) 60.0 (11.14)Large top-killed mesquite density�� 48.8AB (6.26) 29.8A (4.79) 57.4B (11.89)Small top-killed mesquite density 91.3 (14.75) 86.3 (16.27) 62.7 (5.80)Cholla density 138.2 (31.83) 100.7 (36.89) 111.5 (26.02)Large cholla density 18.5 (5.83) 14.3 (1.65) 21.0 (2.85)Small cholla density 93.8 (21.92) 81.0 (31.36) 73.3 (12.29)

a Significantly different means (�) are indicated with different letters when overall P-values indicate a statistically significant difference (P < 0.05) across theburn treatments. Marginally significant means (��) are also indicated with different letters when 0.05 � P � 0.10.


(95% CI: 0.6–26.1%). The model indicated that lark sparrowDSR was positively related to burn frequency. Odds ratiosindicated that daily survival was 1.55 times (95% CI: 1.10–2.18) more likely in 2-year than in 10-year burn plots and1.40 times (95% CI: 0.97–2.01) more likely in 4-year than in10-year burn plots. The odds ratio between 2-year and 4-yearburn treatments was 1.11 (95% CI: 0.83–1.49), indicating asmaller difference in lark sparrow DSR between theseburn treatments. Period survival estimates of lark sparrownests among the 3 burn frequencies were 9.0% (95% CI: 0.7–31.5%) in 2-year burns, 7.0% (95% CI: 0.4–28.6%) in 4-yearburns, and 2.7% (95% CI: 0.5–8.6%) in 10-year burns. Wedid not find a substantial relationship between DSR andshrub density at lark sparrow nest sites (0 > b > �0.001,95% CI: �0.0001 to 0.0001).

DISCUSSION

As predicted, lark sparrow DSR was lower in less frequentlyburned plots. However, we are not clear if this pattern wasdue to predator responses to variation in shrub densityamong the burn treatments. Because shrub density wassimilar in plots burned every 2 years and 10 years, greaterlark sparrow DSR in frequently burned (biennial and qua-drennial) plots may instead reflect reductions in herbaceouscover for predators in those areas (e.g., snakes; Cavitt 2000).Also, direct mortality of predators is possibly more commonin frequently burned plots. For our other species of interest,the mourning dove, burn treatment did not explain variationin DSR. The limited differences in shrub density across theburn treatments may have contributed to this pattern. Ourability to detect a significant pattern in DSR across the burntreatments for mourning doves may have also been limited bynest sample sizes. Otherwise, we are unclear why burn

treatment explained variation in lark sparrow—but notmourning dove—nest survival.We found few patterns between DSR and shrub density at

nest sites. However, such patterns might exist in landscapesexhibiting more pronounced variation in shrub density. Nestpredation tends to be greater near woody vegetation in tall-(Johnson and Temple 1990, Winter et al. 2000, Klug et al.2010) and short-grass prairies (With 1994). Shrubland birdspecies, such as the greater roadrunner (Geococcyx california-nus; Hughs 1996) and northern mockingbird (Mimus poly-glottos; Derrickson and Breitwisch 1992) are potential nestpredators in our study region. Small mammals also depredatebird nests (Nour et al. 1993, With 1994) and recent researchconducted at our study site (R. Matlack, WTAMU, unpub-lished data) indicated that small mammal abundance andshrub density are positively correlated. Nest predation andsnake occurrence coincided with elevated shrub cover intallgrass prairie (Klug et al. 2010). In semiarid grasslands,Mendelson and Jennings (1992) similarly found that theabundance of some snakes (i.e., potential nest predators)was correlated with shrub density. Perhaps rodents, in addi-tion to being predators, might also attract snakes near shrubs.Similar to other studies conducted in shortgrass prairie

(Howard et al. 2001, Skagen et al. 2005, Yackel Adamset al. 2007), nest period survival estimates at our studysite were low (<20%). Additionally, estimated nest successrates for 6 other species studied on the site were estimated tobe less than 10% (Long 2010). If low nest success is a generalcondition for grassland birds in the region, this is of specialconcern as many species of this disturbance-dependent eco-system have experienced significant widespread populationdeclines in the recent past (Samson and Knopf, 1994,Peterjohn and Sauer 1999, Askins et al. 2007, Butcherand Niven 2007). Low nest success facilitated by changesin land use patterns, including increased shrub density, maybe driving these declines. Nest monitoring by biologists mayalso affect nest success if observers leave visual or olfactorycues for predators (Westmoreland and Best 1985, Whelanet al. 1994). We discount this possibility for our study. In2009, we conducted an ancillary investigation of observereffects on nest success on our site and found that nest survivalrates were not significantly affected by the presence of markerflags or varying visitation frequency by human observers(Jacobson et al. 2011). This suggests that some environmen-tal—rather than methodological—feature is responsible forlow nest success at our study site.After 7 years of prescribed burning at the CCMA, herba-

ceous cover responded negatively to prescribed burningthough prescribed burning had few, clear effects on thedensity of particular shrub species. Increased shrub densityin semiarid grasslands is globally wide-spread (Adamoli et al.1990, Burrows et al. 1990, Archer et al. 1995, Moreira 2000,Roques et al. 2001) and many ecological (Allen 1995,Samson et al. 2004, Brown et al. 2008) and social limitations(Samson and Knopf 2001, Yoder et al. 2003, Samson et al.2004) restrict the successful restoration and rehabilitation ofnative vegetation in grassland ecosystems. Previous researchin semiarid grasslands has shown that prescribed burning can

Table 2. Models of daily nest survival for mourning doves and lark sparrowsat the Cross Bar Cooperative Management Area, located near Amarillo,Texas, 2008–2009. Shown are the numbers of parameters per model (K),Akaike’s Information Criterion, corrected for small sample size (AICc),differences in AICc values (DAICc), and model weights ðwiÞ. Constantindicates a model without explanatory variables. Explanatory variables incandidate models are categorical burn frequency of habitat, density of allshrubs (stems/ha) at nest sites, and nest stage (developmental stage ofoffspring: eggs or nestlings).

Common name andcandidate models K AICc DAICc wi

Mourning DoveAll shrub density 2 172.82 0.00 0.710All shrub density þ Nest stage 3 174.70 1.88 0.277Constant 1 181.44 8.63 0.009Burn 3 183.81 11.00 0.003Burn þ Nest stage 4 185.22 12.40 0.001

Lark Sparrowa

Burn þ All shrub density 4 988.36 0.00 0.734All shrub density 2 990.39 2.03 0.266Burn þ Nest stage 4 1033.72 45.36 <0.001Burn 3 1042.56 54.21 <0.001Constant 1 1044.73 56.38 <0.001

a The candidate set for lark sparrow differs from that of mourning dovebecause of differences in correlations between prospective covariates (andthus allowable, tandem covariates) for each bird species.


reduce the density of shrubs (Cable 1967, Britton andWright 1971, Heirman and Wright 1973, Williams et al.1999, Roques et al. 2001) and increase overall grass produc-tion (Heirman and Wright 1973, Medina and Silva 1990).However, management to successfully reduce shrub densitydepends upon several factors including extent of shrub inva-sion, fire frequency (Humphrey 1958, Heirman and Wright1973, Williams et al. 1999, Roques et al. 2001), herbaceousfuel load (i.e., fire intensity; Cable 1967, Britton andWright1971, Heirman and Wright 1973, Wright et al. 1976), andshrub age (i.e., stem diameter; Glandening and Paulson,1955, Dwyer and Pieper 1967, Wright et al. 1976).Substantial effects of prescribed burning on shrub densitymay be delayed in grasslands with advanced levels of shrubencroachment (Allen 1995, Van Auken 2000), conditionsfound consistently across shortgrass prairies of the south-western Great Plains.

MANAGEMENT IMPLICATIONS

Intact, shortgrass prairie landscapes of the southwesternGreat Plains offer tremendous conservation value for grass-land wildlife (Samson et al. 2004). Prescribed burning doesappear to have a positive influence on the nesting success of atleast some bird species in shrub-encroached, shortgrass prai-rie. We expect nest success of other grassland birds in thesouthern Great Plains to respond positively to prescribedburns of ungrazed habitat during the dormant season.However, the reduction in herbaceous cover from frequentfires (e.g., biennial burns) might not accommodate habitatrequirements of all grassland-dependent species (lark spar-row commonly nested above the ground in cholla cactus andits occurrence might be less affected by reductions in herba-ceous cover). Further improvements in nesting success mightbe achieved if other management techniques induce morepronounced reductions in shrub density than we observedresulting from dormant-season fires. Such reductions couldbe accomplished by means of chemical or mechanical shrubremoval. Burning in conditions that enhance fire intensity(e.g., high biomass, drought), or burns during the growingseason, may also increase shrub mortality, but should be donejudiciously. Conversely, burning under lower fuel loads(e.g., heavily grazed pastures), may be ineffectual in reducingshrub density. Large-scale adoption of prescribed burningof shortgrass prairie rangelands may be socially resisted.However, conservation officials in the southern GreatPlains should continue to promote the benefits of prescribedburning and other forms of brush removal, where appropri-ate. Coordination of such management efforts with scientif-ically rigorous research on wildlife responses would helpguide the restoration of southern Great Plains rangelands.

ACKNOWLEDGMENTS

We thank F. Thompson,W. Block, L. Brennan, B. Koerner,B. Thomas, and 2 anonymous reviewers for helpful com-ments on earlier drafts of this study. Financial support forthis project was provided by Emporia State University’s(ESU) Research and Creativity Grant awarded to W.Jensen, ESU’s Graduate Student Research Grant awarded

to A. Long, and the Kansas Ornithological Society’s StudentResearch Award awarded to A. Long. Land access, housing,transportation costs, and living expenses were provided bythe US BLM (P. Tanner). We give special thanks to L.Eaton, S. Epting, M. Jacobson, and E. Tsakiris who assistedwith data collection as part of the National ScienceFoundation Research Experience for Undergraduates pro-gram through WTAMU. We are grateful to T. Shaffer forconsultation on logistic-exposure modeling. We also thankall volunteers who contributed their time and resourcestowards the success of this project.

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Associate Editor: Leonard Brennan.


effects of prescribed burning on avian nest survival in the southern great plains

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