Impacts of a Recent Bison Reintroduction on Grassland Bird Nests and Potential Mechanisms for These Effects

Abstract

The majority of tallgrass prairie has been converted to agriculture over the past two centuries. Restoration projects have sought to increase the quality and size of prairie fragments, with the potential to increase grassland breeding bird habitat. American bison (Bison bison) are being reintroduced to prairie restoration sites and the immediate impacts of these reintroductions are unclear. Our goal was to understand how bison reintroduction influences grassland bird nests and the potential mechanisms behind any change. We measured nest density, nest success, and brood parasitism with a before-after-control-impact (BACI) design to determine if bison influenced pooled nesting success of breeding passerines and field sparrows (Spizella pusilla), the most common nester. In addition, we measured visual obstruction and avian species composition to determine if they changed after the reintroduction. Bison did not influence nest density, brood parasitism, or visual obstruction. Pooled nest success was higher during the first 2 y in bison sites after the reintroduction but decreased the following 2 y. Bird species composition differed slightly between areas with and without bison, but this was not due to the reintroduction and was likely due to other site characteristics. Field sparrow nest success was higher before and after reintroduction in bison sites and drove the pooled nest success results in bison sites. Caution should be taken when interpreting pooled nest data, but these results suggest that bison did not negatively impact nest success of grassland nesting birds immediately following their reintroduction (<5 y).