Grassland birds are one of the most imperiled guilds of North American birds, and their populations continue to decline in Michigan. Habitat loss and fragmentation due to agricultural conversion have been the most important causes of decline, with losses of native temperate grasslands >83% in the Midwest. Farmland set-aside programs such as the Conservation Reserve Enhancement Program (CREP) increase the area of native and introduced grasslands in the U.S., contributing to the conservation of grassland birds. Local evaluations (e.g., within major watersheds or ecoregions) of the impacts of farmland set-aside programs on grassland bird populations are important because grassland bird habitat requirements vary among geographic regions and there is geographic variation in the effect of farmland set-aside lands on wildlife. I examined avian populations of CREP grasslands in the Saginaw Bay watershed of Michigan from 2005-06 to address the following questions: 1) Are grassland birds present? 2) Do densities of grassland species differ in native and introduced CREP grasslands? 3) Is grassland bird occupancy correlated with grassland size, type, and structural variables and/or the extent or proximity of woody vegetation near grasslands (≤100 m)? Results of this study show that CREP grasslands provide habitat for grassland bird species in Michigan. Of 15 grassland species documented to occur in the state in the most recent Michigan Breeding Bird Atlas, 11 species were observed in study grasslands, including 1 Michigan endangered species (Henslow's Sparrow), 3 Michigan special concern species (Dickcissel, Grasshopper Sparrow, and Northern Harrier), and 4 species with significantly declining population trends (p <0.10) in Michigan according to recent analyses of Breeding Bird Survey data (Bobolink, Eastern Meadowlark, Ring-necked Pheasant, and Vesper Sparrow). Detection probability estimates were calculated for observed species. Species with low detection probability estimates (i.e., <0.10) were not included in comparisons of density or in occupancy modeling. Densities of 5 of 8 grassland bird species with detection probability estimates ≥0.10 were higher in native grasslands than introduced grasslands during at least one year of the study (i.e., Grasshopper Sparrow, Ring-necked Pheasant, Savannah Sparrow, Sedge Wren, and Vesper Sparrow). One species, Bobolink, was more abundant in the introduced vegetation during one year of the study. Grassland type was positively associated with the occupancy of Ring-necked Pheasant, Sedge Wren, and Vesper Sparrow, with higher detection-corrected occupancy estimates in native grasslands for all species. Grassland size was positively associated with Sedge Wren occupancy and negatively associated with Dickcissel occupancy. Although grassland type and grassland size were not significant correlates of occupancy for any species, results suggest that these covariates are more than random effects of grassland bird occupancy and hence should be included in future studies and considered in grassland bird conservation strategies. Occupancy modeling results also suggest a diverse response of grassland bird species to habitat features, indicating that species-specific information should be used when managing grassland bird species. Study results demonstrate that CREP grasslands, particularly native grasslands, provide grassland bird habitat, suggesting that maintaining and increasing the area of CREP grasslands should be a grassland bird conservation strategy in Michigan.
Read
