Distribution and reproductive success of native grassland birds in response to burning and field size at Fort Campbell Military Reservation : special focus on Henslow\u27s and grasshopper sparrows

Grassland birds have declined sharply over the last 30 years, based on Breeding Bird Survey (BBS) data. Development, agricultural practices, and fire suppression have reduced native warm season grass fields, which provide habitat for nesting grassland birds. Fort Campbell Military Reservation, located on the Kentucky-Tennessee border, has maintained a significant amount of native grass fields for training exercises primarily through prescribed burning. Research on the effects of different bum rotations on eastem grassland avifauna productivity is not well documented. Grassland bird nests were located and monitored at Fort Campbell during the breeding seasons of 1999 and 2000, focusing on Henslow\u27s sparrow (Ammodramus henslowii) and grasshopper sparrow (Ammodramus savannarum). Nesting success was calculated using the Mayfield method (1975). Vegetation characteristics were collected at Henslow\u27s and grasshopper sparrow nests, male perch sites, and random points to identify key parameters related to nesting and territory habitat, and nesting success. Thirty native grassland fields were selected for avian censusing using standard 10-minute point counts to document distribution related to field size and bum history. Forty-nine Henslow\u27s sparrow nests were located and monitored in 1999 and 2000. Mayfield nest success estimate for Henslow\u27s sparrow was 18.7% in 2000. Mean clutch size for Henslow\u27s sparrow was 4.2. Henslow\u27s sparrows fledged an average of 1.5 young per nest and 3.8 per successful nest. Brown-headed cowbird (Molothrus ater) parasitism rate was 2.0%. Fifty-four grasshopper sparrow nests were located and monitored during both field seasons. Mayfield nest success estimates were 41.8% and 38.2% for 1999 and 2000, respectively. Mean clutch size was 4.4. Grasshopper sparrows fledged an average of 2.9 young per nest and 4.0 per successful nest. Grasshopper sparrows had 1.9% of their nests parasitized by brown-headed cowbirds. Predation was the primary reason for nest failure for both sparrows and primary nest predators appeared to be snakes. A total of 512 grassland bird nests overall, representing 30 species, were located and monitored in 1999 and 2000. Mayfield nest success estimates of other priority concern species in 2000 included dickcissel (Spiza americana)( n=42; 26.5%), eastern meadowlark (Sturnella magna)( n=17; 12.1%), field sparrow (Spizella pusilla)( n=87; 15.4%), indigo bunting (Passerina cyanea)( n=32; 28.3%), prairie warbler (Dendroica discolor)( n=31; 36.5%), and yellow-breasted chat (Icteria virens)( n=30; 24.4%). Cool season grass coverage (P = 0.0377) and native warm season grass coverage (P = 0.0485) were greater at Henslow\u27s sparrow occupied sites than unoccupied sites in 1999. Field size was larger (P \u3c 0.0001), native warm season grass coverage was greater (P = 0.0007), and grass height was taller (P = 0.0196) at occupied sites than unoccupied sites in 2000. Field size was larger (P \u3c 0.0001) and woody vegetation was greater (P \u3c 0.0001) at male Henslow\u27s sparrow perch sites than random points in 1999. Woody vegetation was greater (P \u3c 0.0001), field size was larger (P \u3c 0.0001), and legume coverage was less at perch sites in 2000. Time since field was last burned was longer (P \u3c 0.0001), native warm season grass coverage was greater (P = 0.0003), field size was larger (P = 0.0011), and litter depth was less (P = 0.0012) at Henslow\u27s sparrow nest sites than random locations, in 2000. Percent native warm season grass (P = 0.0086) and litter depth (P = 0.0236) were significantly greater at successful Henslow\u27s sparrow nests than unsuccessful nests. Grasshopper sparrow perch sites were located in larger fields (P \u3c 0.0001) with lower grass height than random sites, in 1999 and 2000. Field size was larger (P = 0.0006), forb coverage was less (P = 0.0233), and legume coverage was greater (P = 0.0467) at nest sites than random sites in 1999. Field size was larger (P \u3c 0.0001), grass height was lower (P = 0.0008), and forb coverage was lower (P = 0.0185) at grasshopper sparrow nest sites in 2000. Logistic regression analysis detected no differences in habitat characteristics between successful and unsuccessful grasshopper sparrow nests. A total of 57 and 87 species, respectively, were counted within 50 m and from unlimited distance from 714 fixed radius plots during both years. One hundred twenty four bird species (breeding and non-breeding) were detected over the entire reservation during 1999 and 2000. Indigo bunting, common yellowthroat (Geothlypis trichas), field sparrow, and yellow-breasted chat were the most abundant birds in native grass fields. Other species of concern were documented including Bachman\u27s sparrow (Aimophila aestivalis). Bell\u27s vireo (Vireo bellii), black-billed cuckoo (Coccyzus erythropthalmus), bobolink (Dolichonyx oryzivorus), and lark sparrow (Chondestes grammacus). Mean avian richness, abundance, and diversity were all lower in 1999 than in 2000. Avian richness was marginally greater in the smallest and largest size-class fields burned one year previously. In 1999, richness was less in later counts (Jime 16 - July 15) whereas, in 2000, richness was greater during this same count period. Mean abundance did not differ among bum class or field size individually although there was a significant interaction between these two parameters. Abundance appeared to be greatest in the largest fields burned one year ago. Diversity was greater in the smaller fields early in the season and diversity was greater in the medium and larger fields later in the season. Twenty of the most abundant species on the point counts were used for analysis of field size and field bum class. Dickcissel, Henslow\u27s sparrow, and northern bobwhite (Colinus virginianus) were all positively associated with larger fields (\u3e50 ha) whereas indigo bunting, yellow-breasted chat, and prairie warbler were more abundant in smaller fields (\u3c 15 ha). White-eyed vireo (Vireo griseus), prairie warbler, blue-gray gnatcatcher (Polioptila caerulea), and Henslow\u27s sparrow were located in older bum class fields. Field sparrow, common yellowthroat, American goldfinch (Carduelis tristis), eastern kingbird (Tyrannus tyrannus), northern cardinal (Cardinalis cardinalis), and indigo bunting were more abundant in fields that had been recently bumed. Key management issues that need to be addressed to more effectively manage for grassland birds at Fort Campbell include 1) more effective burning to control woody vegetation; 2) management to create more large fields (\u3e100 ha); and 3) conversion of cool-season grass hayfields to native warm season grasses