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HABITAT RELATIONSHIPS OF BIRDS OVERWINTERING IN A MANAGEDCOASTAL PRAIRIEAuthor(s): HEATHER Q. BALDWIN, JAMES B. GRACE, WYLIE C. BARROW JR., and FRANK C.ROHWERSource: The Wilson Journal of Ornithology, 119(2):189-197. 2007.Published By: The Wilson Ornithological SocietyDOI: http://dx.doi.org/10.1676/05-129.1URL: http://www.bioone.org/doi/full/10.1676/05-129.1

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189

The Wilson Journal of Ornithology 119(2):189–197, 2007

HABITAT RELATIONSHIPS OF BIRDS OVERWINTERING IN AMANAGED COASTAL PRAIRIE

HEATHER Q. BALDWIN,1,4 JAMES B. GRACE,2 WYLIE C. BARROW, JR.,2 ANDFRANK C. ROHWER3

ABSTRACT.—Grassland birds are considered to be rapidly declining in North America. Management ap-proaches for grassland birds frequently rely on prescribed burning to maintain habitat in suitable condition. Weevaluated the relationships among years since burn, vegetation structure, and overwintering grassland bird abun-dance in coastal prairie. Le Conte’s Sparrows (Ammodramus leconteii) were most common in areas that had:(1) been burned within the previous 2 years, (2) medium density herbaceous vegetation, and (3) sparse shrubdensities. Savannah Sparrows (Passerculus sandwichensis) were associated with areas: (1) burned within 1 year,(2) with sparse herbaceous vegetation, and (3) with sparse shrub densities. Sedge Wrens (Cistothorus platensis)were most common in areas that had: (1) burned greater than 2 years prior and (2) dense herbaceous vegetation.Swamp Sparrows (Melospiza georgiana): (1) were most common in areas of dense shrubs, (2) not related totime since burnings, and (3) demonstrated no relationship to herbaceous vegetation densities. The relationshipsto fire histories for all four bird species could be explained by the associated vegetation characteristics indicatingthe need for a mosaic of burn rotations and modest levels of woody vegetation. Received 19 October 2005.Accepted 3 September 2006.

Population declines of grassland birds havebeen more severe than for any other group ofbirds in North America (Peterjohn and Sauer1999, Sauer et al. 2004). Declines have beenattributed to degradation and loss of habitat inboth breeding and wintering areas (Knopf1994, Arey et al. 1998, Peterjohn and Sauer1999). Prescribed fire is commonly used tomanage habitat conditions for birds in somegrassland types (Van’t Hul et al. 1997, Mad-den et al. 1999, Tucker and Robinson 2003),particularly in the southeastern United States.The effects of different burn rotations are gen-erally not well defined, in part, because factorsnot necessarily related to fire are also impor-tant for grassland bird management. For ex-ample, vegetation structure has been associ-ated with grassland bird abundance (Wiensand Rotenberry 1981, Cully and Michaels2000), but the responses of vegetation follow-ing fire can be influenced by factors such asrainfall, soil type, and season of burning.Since fire is a primary means of managing

1 IAP World Services at USGS National WetlandsResearch Center, 700 Cajundome Blvd., Lafayette, LA70506, USA.

2 USGS National Wetlands Research Center, 700Cajundome Blvd., Lafayette, LA 70506, USA.

3 Louisiana State University, 227 Renewable NaturalResources, Baton Rouge, LA 70803, USA.

4 Corresponding author; e-mail:heather�baldwin@usgs.gov

grasslands and grassland birds, it is imperativethat we understand bird species use patternsof burned habitat and the role of habitat struc-ture.

The coastal prairie is the southernmost unitof the tallgrass prairie biome and lies alongthe northwestern coast of the Gulf of Mexico.This ecosystem is listed as ‘‘imperiled glob-ally’’ by The Nature Conservancy and theTexas Natural Heritage Program lists it as‘‘critically imperiled’’ (Grace 2000). Coastalprairie once spread over 2.4 million ha fromsouthwest Louisiana to the lower Texas coast(Arey et al. 1998). Less than 1% of the his-toric coastal prairie ecosystem now remains inrelatively pristine condition (Diamond andSmeins 1984, Arey et al. 1998). Specific ob-jectives of fire management for coastal prairieinclude frequent burn rotations (generally ev-ery 3–5 years), to suppress growth of woodyplants, primarily eastern baccharis (Baccharishalimifolia) and Chinese tallow tree (Triadicasebifera), to maintain open grassland habitat(Grace 1998, Allain and Grace 2001).

We investigated the relative abundance ofLe Conte’s Sparrow (Ammodramus leconteii),Sedge Wrens (Cistothorus platensis), Savan-nah Sparrow (Passerculus sandwichensis),and Swamp Sparrow (Melospiza georgiana)in areas burned within the previous 1, 2 or 3years to examine the effect of fire on herba-ceous and woody vegetation, and habitat as-sociations of individual bird species.

190 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007

METHODS

Study Area.—Our study was conducted dur-ing winter 2002–2003 at the Hoskins Mound(upland coastal prairie) portion of the BrazoriaNational Wildlife Refuge along the upper Tex-as coast of the Gulf of Mexico. Historically,the land was used for cattle and rice produc-tion, resulting in the dissection of the refugeinto numerous well-defined units surroundedby fire breaks such as levees, roads, and ditch-es. The units studied have not been grazed orfarmed since at least 1990 (J. B. Grace and L.K. Allain, pers. comm.). Dominant plant spe-cies in the units studied included little blue-stem (Schizachyrium scoparium), broomsedge (Andropogon virginicus), saltmeadowcordgrass (Spartina patens), switchgrass(Panicum virgatum), cutover muhly (Muhl-enbergia capillaris var. trichopodes), narrow-leaf marshelder (Iva angustifolia), longspiketridens (Tridens strictus), rosette grass (Di-chanthelium spp.), and sedges (Cyperus spp.).Eastern baccharis is a native and invasiveshrub common in coastal areas. This speciesoccurred in scattered clumps throughout muchof the refuge and dominated portions of manyunits. Other woody trees and vines on studysites included blackberry (Rubus spp.), Ma-cartney rose (Rosa bracteata), wax myrtle(Morella cerifera), and Chinese tallow tree.

Approximately 3,200 ha of upland prairieon the refuge are actively managed with pre-scribed burning. The burning program was es-tablished in 1997 with emphasis on growingseason fires. Mowing and haying have alsobeen used for management of some areas. Themanagement plan seeks to achieve an averageburn rotation of 3–5 years, depending on siteconditions and successional status.

Field Methods.—We selected 15 burn unitsfor study with five replicates for each of threeburn histories (burned within the past 1, 2, or3 years). We did not sample areas burned be-yond 3 years because we did not expect LeConte’s Sparrow in sufficiently comparablenumbers. Units ranged in size from 40 to 520ha. Twelve 100-m transects were randomly lo-cated in each unit using a randomization pro-cedure in ARC-GIS with the restriction theyhad to be a minimum of 50 m apart and havea 50-m buffer zone from edges such as roadsand firebreaks. All transects were surveyed

once for birds and vegetation. Three transectsfrom each burn treatment were surveyed eachday (n � 9, 100-m transects/day) and unitswere selected in random order. Thus, 180 linetransects were surveyed.

Protocols for line transect surveys were de-signed to survey grassland birds (Shackelfordet al. 2001, Carrie et al. 2002). Transect di-rection was northeast to southwest becausebirds were more easily identified when the sunwas behind the observer. A team of three peo-ple, consisting of one observer and two ‘‘beat-ers’’ who disturbed vegetation with 3-m poles,walked each transect. Individual birds wereidentified in flight because most are difficultto observe in grasslands and usually do notperch, sing, or call in winter. We recorded spe-cies and distance from the center line in 1-mincrements from where the bird was first ob-served. We conducted surveys when weatherconditions were not expected to impede ourability to detect birds (i.e., rain, fog, or windspeed �20 km/hr; Gabrey et al. 1999).

Vegetation was sampled after completingbird surveys each day. Vegetation character-istics were measured at three random pointswithin 10 m along each transect line (n � 540points). Selection of habitat variables wasbased on previous studies that examined hab-itat associations with birds in similar environ-ments (Pulliam and Mills 1977, Wiens andRotenberry 1981, Skinner 1982, Thatcher etal. 2006). Height of herbaceous vegetationwas measured by recording the tallest herba-ceous vegetation within 1 m of each pointfrom four cardinal directions. Herbaceousdensity was measured using a variation of thepole method (Plentovich et al. 1999) modifiedfrom Mills et al. (1989). We recorded thenumber of herbaceous contacts on a pole (�5cm in diameter) in 5-cm increments up to 20cm (sum of 0–20 cm increments) to measureground density, or leaf litter. We counted thenumber of contacts every 10 cm up to 2 m(sum of 0–200 cm increments) to measureoverall herbaceous density (VEGDEN). Themaximum number of contacts counted foreach increment (every 5 or 10 cm) was 10(Plentovich et al. 1999). Community type wasrecorded for each transect based on the dom-inant plants (little bluestem, saltmeadow cord-grass, or a mixture of these two species).

Woody vegetation density, primarily com-

191Baldwin et al. • GRASSLAND BIRDS IN COASTAL PRAIRIE

posed of eastern baccharis, was measured us-ing the point-centered quarter (PCQ) method(Cottam et al. 1953). We estimated woodyplant density by measuring the distance to thenearest woody plant in each of four quadrants(northwest, northeast, southwest, southeast) ateach sample point along transects. Eachwoody plant was placed into a size class byheight (e.g., 0–1.0, 1.1–2.0, 2.1–3.0 m, etc.).Density was estimated for all woody plants(ALLSHR), live eastern baccharis (LBACC),and dead eastern baccharis (DBACC). Pres-ence or absence of all shrub/tree species wasrecorded [Macartney rose, wax myrtle, Chi-nese tallow tree, elm (Ulmus spp.), and holly(Ilex spp.)].

Data Analyses.—Program DISTANCE wasused to evaluate which detection variablemight bias estimates of population densities toenhance overall density estimates of the fourmost common bird species (Buckland et al.2001, 2004). It was necessary to convert thebird survey data to presence/absence (cate-gorical) responses to analyze habitat associa-tions. Vegetation variables were categorizedinto low, medium, and high categories bynumber of total observations.

We used structural equation modeling(SEM) (Bollen 1989, McCune and Grace2002, Pugesek et al. 2004, Grace 2006) to ex-amine multivariate relations between individ-ual bird species and habitat conditions. SEMis a tool for comparing observed patterns indata to those predicted from hypothesizedpath models, typically relying on maximumlikelihood procedures for solution. The result-ing estimates for path coefficients in structuralequation models represent the implied sensi-tivities of response variables to variations inindividual predictors (Grace and Bollen2005).

The goals of the SEM analyses were to: (1)examine the extent to which the association ofbirds with different post-burn conditions couldbe explained by related habitat features, and(2) estimate the relative sensitivities of birdsto different vegetation conditions using Mplus(Muthen and Muthen 2004). We performedSEM in a categorical modeling mode becausethe response variable, presence or absence ofbird species in a transect, is binary.

We examined the data for forms of bivariaterelationships between predictor and response

variables using both graphical techniques andexploratory logistic regression. The results in-dicated nonlinear relationships (e.g., the prob-ability of observing a bird of a particular spe-cies was highest at some intermediate level ofvegetation density) for all of the predictors.Attempts to model these results using poly-nomial models were inefficient (large, system-atic deviations). We tried an alternative ap-proach to modeling nonlinear relationshipsthat involved categorizing continuous re-sponse variables and treating the categories asfixed-state dummy variables (Grace 2006).This approach to modeling nonlinear relation-ships was consistently more efficient.

Bivariate associations between individualpredictors and bird responses were calculatedfor descriptive purposes. The presentation ofthese results was for comparisons betweenraw, bivariate/univariate relationships. Thepredictors used in the SEM analysis were allcategorical (years since burn, vegetation den-sity categories, and vegetative communitytypes) and we used categorical modelingmethods (PROC FREQ; SAS Institute, Inc.1999) to describe associations and narrow thevariables used.

Nesting of transects within burn units wasaccommodated in the SEM using the ‘‘clus-ter’’ option in Mplus. This option accounts fornon-independencies in the data by calculatingadjusted standard errors and alternative testsof model fit that are robust to hierarchicalsampling (Muthen and Satorra 1995). Modelfit for all analyses (one for each of the fourmost common bird species) was assessed us-ing Chi-squares calculated from a negativelog-likelihood fitting function. T-values andtheir associated P-values for individual pa-rameters were used in interpreting results. Thealpha level used for model evaluations was0.05. All results presented are based on mod-els having good measures of fit.

We included composite variables (Heise1972, Bollen and Lennox 1991) in modelswhen the net effect of two related and stronglyintercorrelated variables was of interest. Com-posite variables represent the net strengths ofmultiple pathways in a single path, permittingthe overall sensitivity of birds to a categoryof influences such as time since burn or veg-etative community types to be represented.

192 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007

TABLE 1. Percent of transects in each category of total pole contacts or herbaceous density (VEGDEN)and shrub density (SHRUBDEN) across burn treatments. Both variables vary between burn years (P � 0.05).

Burn 1

Low Medium High

Burn 2

Low Medium High

Burn 3

Low Medium High

VEGDEN 76.6 16.7 6.7 20.6 42.9 36.5 5.0 30.0 65.0SHRUBDEN 53.3 11.7 35.0 33.3 38.1 28.6 13.3 50.0 36.7

RESULTS

Little bluestem was the most common com-munity type (dominating 44% of all sites),saltmeadow cordgrass was second (dominat-ing 29%), while 27% of the sites were dom-inated by a mixture of both species. We ob-served 314 birds representing 18 species dur-ing transect surveys. We observed a mean of1.1 � 0.08 (SE) bird species per transect con-ducted per site per day and a mean of 5.3 �0.43 species per burn unit. Species richnessper burn unit was not different across burnyears (df � 8; 1 vs. 2, t � �0.17, P � 0.87;1 vs. 3, t � 0.30, P � 0.77; 2 vs. 3, t � 0.74,P � 0.48). The less common GrasshopperSparrow (Ammodramus savannarum) (n � 2),Sprague’s Pipit (Anthus spragueii) (n � 2),and Sandhill Crane (Grus canadensis) (n � 7)were only observed in first year burns.

Bird Abundance.—Histograms (programDISTANCE) revealed that detection probabil-ity decreased as distance from the center lineincreased for all species except SavannahSparrow. The pattern for Savannah Sparrowsuggested the birds moved away from the ob-server prior to detection. Thus, for this spe-cies, the detection curve was adjusted usingstandard DISTANCE methods, permitting anestimate of abundance.

Abundance of Le Conte’s Sparrows was es-timated to be 4.91 � 1.49 (SE) birds/ha witha 0.54 � 0.13 probability of detecting all in-dividuals within the survey area. Estimatedstandard half-width (ESW) was 2.17 � 0.51m with truncation at 4 m. Estimated abun-dance of Savannah Sparrows was 2.87 � 0.90birds/ha with a 0.78 � 0.11 probability of de-tecting all individuals within the survey area.ESW was 4.66 � 0.66 m with truncation at 6m. Sedge Wrens had an estimated abundanceof 5.55 � 1.97 birds/ha with a 0.45 � 0.14probability of detecting all individuals withinthe survey area. Raw observations were trun-cated at 5 m and analyses produced an ESW

of 2.27 � 0.69 m. Estimated abundance ofSwamp Sparrows was 4.40 � 1.48 birds/hawith a 0.63 � 0.09 probability of detecting allindividuals within the survey area. ESW was3.16 � 0.44 m with truncation at 5 m.

Vegetation Associations with Fire.—Burnyear was significantly associated with everyherbaceous and shrub variable except com-munity type. Both vegetation measures (VEG-DEN, SHRUBDEN) increased from sparse todense as time after burn increased (Table 1).Community type had no association with timesince burn, consistent with a priori expecta-tions.

Bird Associations with Fire.—A significantrelationship was found between burn year andthe occurrence of three of the four most com-mon bird species (Fig. 1A). Le Conte’s Spar-rows were more common in 2- than 3-yearburn sites (df � 2, P � 0.016), SavannahSparrows were more common in 1- than 2-(df � 2, P � 0.001) and 3-year burn sites (df� 2, P � 0.001), and Sedge Wrens were morecommon in 2- (df � 2, P � 0.015) and 3-yearburn sites (df � 2, P � 0.001) than in 1-yearburn sites. There was no significant differencebetween 2- and 3-year burn sites nor were sig-nificant relationships found between burn yearand the occurrence of Swamp Sparrows.

Bird Associations with Vegetation.—LeConte’s Sparrows were more commonly ob-served in areas of medium vegetation density(df � 2, P � 0.015) and sparse shrubs (df �2, P � 0.018) (Fig. 1). Le Conte’s Sparrowswere less common where Macartney rose (df� 1, P � 0.003) was present, and more com-mon where wax myrtle (df � 1, P � 0.005)was present. Le Conte’s Sparrows were alsomore common in mixed versus saltmeadowcordgrass (df � 2, P � 0.047). SavannahSparrows were more common in areas withsparse herbaceous density (df � 2, P � 0.001)and sparse shrub density (df � 2, P � 0.001)(Fig. 1). Savannah Sparrows were more com-

193Baldwin et al. • GRASSLAND BIRDS IN COASTAL PRAIRIE

FIG. 1. Ratio (observed/expected) of observations for (A) burn treatment, and (D) community type (domi-nants). Percent transects birds were observed in high, medium, and low categories of (B) herbaceous density,and (C) woody plant density. Asterisks (*) indicate differences (P � 0.05) among categories in a category.LCSP—Le Conte’s Sparrow, SAVS—Savannah Sparrow, SEWR—Sedge Wren, and SWSP—Swamp Sparrow.

monly observed in areas with high densitiesof eastern baccharis (df � 2, P � 0.001) andwhere Chinese tallow tree (df � 1, P �0.001), Macartney rose (df � 1, P � 0.001),and other woody species (df � 1, P � 0.012),including Ulmus spp., Ilex spp., and other un-identifiable woody plants were absent. Theywere also more common in mixed versus littlebluestem and saltmeadow cordgrass (df � 2,P � 0.003). Sedge Wrens were more commonin areas of dense herbaceous vegetation (df �2, P � 0.011) with no significant relationshipsto total shrub densities (Fig. 1). They weremore common in areas where eastern bac-charis (df � 1, P � 0.006) and Chinese tallowtree (df � 1, P � 0.009) were present. SedgeWrens were more common in little bluestem(df � 2, P � 0.049) or saltmeadow cordgrass(df � 2, P � 0.019) compared to mixed stands(Fig. 1D). Swamp Sparrows were not associ-ated with a particular level of vegetation den-sity, but were more common in areas of denseshrubs (df � 2, P � 0.035) (Fig. 1C). They

were also more common in areas where Chi-nese tallow tree (df � 1, P � 0.042) and otherwoody species (df � 1, P � 0.001) were pre-sent.

Structural Equation Model.—SEM resultsfor Le Conte’s Sparrows (Fig. 2) indicated theprobability of bird occurrence was positivelyand directly related to a sparse density ofshrubs and a medium density of herbaceousvegetation. Le Conte’s Sparrows were corre-lated with recently burned sites (burn years 1and 2) and this indirect relationship can beexplained by the scarcity of shrubs in suchsites. Le Conte’s Sparrows were more likelyto be in areas dominated by little bluestem ora mixture of saltmeadow cordgrass and littlebluestem than in areas that were saltmeadowcordgrass. Le Conte’s Sparrows (Fig. 2) weremost strongly associated with open areas withsparse shrubs (shrublow) (standardized pathcoefficient � 0.47) and less related to mediumherbaceous densities (herbmed) and the two

194 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007

FIG. 2. Structural equation model for Le Conte’s Sparrow (LCSP), Savannah Sparrow (SAVS), Sedge Wren(SEWR), and Swamp Sparrow (SWSP). Rectangles representing measured variables (e.g., brnyr1) shown insideconstructs (dotted boxes) were significant contributors to prediction. Constructs that possessed no significantpredictors of bird observations are labeled ns. Abbreviations represent variables; ‘‘brnyr’’ refers to burn year,‘‘shrublow’’ refers to low densities of shrubs, ‘‘herblow’’ represents a low density of herbaceous vegetation, and‘‘SCSC’’ represents composite effects of the Schizachyrium scoparium community type and ‘‘SPSP’’ representsSpartina spartinae. Parameter values presented are standardized path coefficients representing the predictedsensitivities of response variables. Dashed lines indicate paths not detectably different from zero.

community types containing little bluestem(both with coefficients of 0.27).

Savannah Sparrows were more commonlyobserved in areas with sparse herbaceous veg-etation, regardless of shrub densities (Fig. 2).These areas were tightly associated with burnswithin the previous year, although this specieswas sometimes also in areas burned within 2years. Savannah Sparrows were directly as-sociated with sparse vegetation, somewhatmore common in the mixed-grass communi-ties, and only indirectly related to burning re-gime through associated effects on vegetation.

Sedge Wrens (Fig. 2) avoided areas havingsparse and medium herbaceous density.Sparse herbaceous vegetation was moststrongly associated with areas burned withinthe previous year. Medium levels of herba-

ceous vegetation occurred in areas with anyburn history. The occurrence of Sedge Wrenswas independent of shrub densities and types,and independent of community type, beingprimarily in areas with dense herbaceous veg-etation where fire was absent during the pre-vious 2 years.

The Swamp Sparrow (Fig. 2) was the onlyspecies unrelated to year since burn, either di-rectly or indirectly. This species was associ-ated with dense shrubs, but not with herba-ceous plant density. Swamp Sparrows wereuncommon where the two dominant grasseswere mixed, but were positively associatedwith both little bluestem and saltmeadowcordgrass dominated communities. SwampSparrows were most weakly associated withthe habitat parameters measured (R2 � 0.18).

195Baldwin et al. • GRASSLAND BIRDS IN COASTAL PRAIRIE

DISCUSSION

Fire management of coastal prairie servesmany purposes, among them, the creation ofconditions that provide habitat for grassland-dependent birds. What is generally not knownfor the coastal prairie ecosystem is the extentto which bird species discriminate among hab-itat conditions created by burning regimes, therole of specific vegetation conditions, andwhich species overlap in their habitat prefer-ences.

Relative abundance estimates were similaramong the four most common species. Detec-tion probabilities ranged from 0.78 and 0.63for Savannah Sparrows and Swamp Sparrowsto 0.54 and 0.45 for Le Conte’s Sparrows andSedge Wrens reflecting the elusive behaviorexpected in the wintering areas (Lowther1996, Herkert et al. 2001).

All species, except Sedge Wrens, were as-sociated with community type. Le Conte’sSparrows were more common in areas domi-nated by little bluestem and areas with a mixof the two dominant grasses suggesting theyprefer areas that are characteristic of more up-land prairie. Savannah Sparrows were asso-ciated with a mix of dominant grasses, char-acteristic of transition zones. This phenome-non may be conducive to producing areas withsparse vegetation where one dominant grass isdying while the other is becoming established.Swamp Sparrows were associated with areasdominated by either one grass or another; thisassociation remains unclear.

Species-specific habitat preferences are ex-pected for an avian assemblage occupying ahabitat (Pulliam and Mills 1977). These pref-erences are generally associated with vegeta-tion structure. Hypotheses explaining these re-lationships are primarily related to food abun-dance (Tucker and Robinson 2003) and pred-ator avoidance strategies (Pulliam and Mills1977). Grzybowski (1983) consistently foundthat solitary grassland birds did not occur insparse vegetation in mixed-grass prairie. LeConte’s Sparrows and Sedge Wrens in ourstudy were usually solitary when flushed andwere less common in areas with sparse her-baceous vegetation. Both are known to be rel-atively weak flyers (Shackleford et al. 2001)and may rely on dense cover for predator de-fense. Differential use of sites with medium

versus high VEGDEN is unclear, but may beinfluenced by foraging strategies and/or die-tary differences (Howell 1932, Easterla 1962,Imhof 1976).

Time-since-burn was not found to have adirect relationship with any of the species inthis study but strongly influenced vegetationcharacteristics preferred or avoided by threeof the four species. The Le Conte’s Sparrowwas the only species to have an indirect re-sponse to time-since-burn (associated withlow shrub densities), suggesting avoidance ofareas with shrubs and a preference for struc-tural homogeneity. Sedge Wrens and Savan-nah Sparrows were strongly associated withherbaceous density which was greatly influ-enced by time-since-burn. Herbaceous densityand shrub density tended to increase as time-since-burn increased. There was no relation-ship to shrub density for Sedge Wrens and Sa-vannah Sparrows, suggesting herbaceous den-sity was the primary habitat variable measuredin this study to predict their occurrence.

The Swamp Sparrow was the only speciesnot associated with burning either directly orindirectly. Swamp Sparrows are not obligateprairie birds and reportedly overwinter in avariety of habitat types, such as swamps andmarshes, along lakes and reservoirs, and incoastal marshes (Mowbray 1997). The areawith the highest concentration of SwampSparrows in this study had numerous waterbounded areas within the site and serving asborders. This burn unit also had the highestconcentration of shrubs and small trees.

Le Conte’s Sparrow, Savannah Sparrow,and Sedge Wren were strongly associated withparticular vegetation conditions and, indirect-ly, responded in different ways to time-since-burn. Swamp Sparrows, demonstrated a weakrelationship to habitat variables, probably be-cause of its broad association with a wide va-riety of habitats aside from coastal prairie. Ourresults and observations indicate that speciesincluded in this study are responsive to veg-etation succession management using fire. Theoccurrence of grassland sparrows generallydeclined in areas with dense woody plants.Our work suggests that a mosaic of coastalprairie in 2–3 year burn rotations providessuitable habitat conditions for overwinteringgrassland birds.

196 THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 119, No. 2, June 2007

ACKNOWLEDGMENTS

Funding was provided by the U.S. Geological Sur-vey in a cooperative effort with Regions 2 and 4 ofthe U.S. Fish and Wildlife Service. We thank the full-time field crew: M. A. Powell, S. W. Stuart, J. A. Um-mel, and M. L. Keprta. We especially thank employeesand volunteers associated with the Texas Mid-CoastNational Wildlife Refuge Complex, Gulf Coast BirdObservatory, USGS National Wetlands Research Cen-ter, and Texas Parks and Wildlife Department who de-voted their time to this project. We thank J. A. Sedg-wick, M. J. Baldwin, D. M. Soileau, Jr., D. J. Johnson,J. A. Davis, M. S. Woodrey, and two anonymous ref-erees for editorial comments and advice.

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