Tallgrass prairie has been converted to agriculture over the past century, making it one of the most threatened ecosystems globally. Agriculture conversion of prairie has severely fragmented the landscape contributing to the decline of many grassland birds and some are now threatened with extirpation. Restoration projects seek to increase the quality/size of prairie fragments, hypothetically increasing breeding habitat for grassland birds. Bison are now being reintroduced to prairie restorations as a final step in a complete restoration to increase habitat heterogeneity. The goal of our study was to understand how the immediate impact of bison presence influences nest density and survivorship of all grassland nesting birds at Nachusa Grasslands in northern Illinois. We predicted that nest density, brood parasitism, and survivorship would increase with bison presence. We measured nest density and survivorship of all birds nesting in four tallgrass prairie restoration and two remnant sites from May to July yearly from 2014- 2016. We used a combination of techniques to find nests including systematic walking, haphazard walking, and behavioral analysis. After a nest was found, species, nest contents, brood parasitism, and GPS coordinates were recorded and the nests were monitored every few days until chicks fledged or nests were depredated.
We found 149 nests of twelve different species over the course of three breeding seasons. Nest density and survivorship was compared before and after reintroduction and between sites with and without bison. We found that bison reintroduction did not influence survivorship or total density of grassland bird nests. Brown-headed cowbird (Molothrus ater) parasitism is associated with livestock presence and was predicted to increase with the reintroduction. However, brood parasitism declined from 2014 (0.25) to 2016 (0.02) and was not different between bison and non-bison sites. We will be continuing nest searching for the next two seasons to understand how species are specifically reacting to a grazing pressure over time. In addition, we will continue this research to help understand how the interaction between grazing and fire (pyric herbivory) is influencing the species composition of these sites over time. Such information is critical because these restoration techniques are becoming more common and it is unclear how they will impact higher trophic levels, including declining grassland birds.