Grassland bird responses to bison and prescribed fire in restored tallgrass prairies

Abstract

Grazing from megaherbivores such as bison (Bison bison) and periodic fire are 2 important disturbance regimes in grassland ecosystems. In restored tallgrass prairies where these processes were previously removed, prescribed fire application and bison reintroduction are tools used by managers to recreate habitat heterogeneity formed by these disturbances. Tallgrass prairie bird communities may be indirectly affected by these disturbances, as bison and prescribed fire alter the structure of critical breeding habitat for grassland birds. The objectives of this research were to determine the effects of bison and prescribed fire on grassland breeding birds in 2 tallgrass prairie preserves in the Midwest region of the United States. We surveyed bird communities, vegetation structure, and bison activity at an Illinois preserve (n = 20 sites) and an Indiana preserve (n = 27 sites) in 2020 and 2021 and compiled a history of restoration activities (e.g., prescribed fire, planting year) at these sites. Grazing and fire disturbances affected grassland bird diversity and abundances, whereas we found little to no evidence that restoration planting age and spatiotemporal factors affected grassland bird populations. Disturbance effects often corresponded to species-specific responses to changes in vegetation structure. Grassland-obligate bird diversity was lower in recently burned and ungrazed management units, in comparison to unburned-ungrazed and unburned-grazed sites. Henslow's sparrow (Centronyx henslowii), a species known to be sensitive to recent prescribed fire, exhibited increased abundance with time since fire, an increase that was further amplified with bison presence. These results highlight the importance of applying varying levels of grazing and fire disturbance to provide variable vegetation structure to accommodate the habitat preferences of a diversity of grassland bird species. Moreover, our results indicate that bison may play a role in mediating the differing effects of variable prescribed fire frequencies on grassland bird species of concern.