Jones, Holly P.
Ph.D. (Doctor of Philosophy)
Department of Biological Sciences
The majority of tallgrass prairie has been converted to agriculture over the past two centuries, making it one of the most threatened ecosystems globally. Agricultural conversion of prairie fragmented the landscape, causing declines and local extirpation of many grassland birds. Restoration projects have sought to increase the quality and size of prairie fragments by converting cultivated land back to prairie through revegetation and management with prescribed fire, hypothetically increasing breeding habitat for grassland birds. Bison and other grazers are now being reintroduced to prairie restorations as a final step in the restoration process to increase habitat heterogeneity.
The goal of my dissertation was to determine how a recent bison reintroduction at Nachusa Grasslands impacted grassland bird nest success, nest predator composition, and grassland bird detection frequency and to see if it is similar to other studies. I measured daily survivorship rate, visual obstruction of vegetation around nests, and species composition of fourteen species of known avian breeders in areas with and without bison from 2014 to 2018 (Chapter 2). Reduced vegetation obstruction and Brown-headed Cowbird parasitism resulted in decreased nest success overall. Generalist nesters did not differ with respect to any measured variables, including presence of bison. In contrast, facultative and obligate grassland nesting birds occurred at lower nest densities and experienced reduced and more variable nest success in areas with bison. In addition, I constructed artificial nests and placed them at a set density in sites with and without bison and with varying fire frequencies over the same time period (Chapter 3). I found that nest success decreased, and total proportion of depredation events and proportion of Cricetid depredation events increased with prescribed fire. Yearly differences influenced nest success the most and were not related to precipitation during the experiment. I found less nest depredation in the sites with bison, but I found no interaction between fire and grazing. Lastly, I sought to quantify overall species richness and the detection frequency as a proxy for relative abundance of certain grassland bird species that are thought to be impacted by bison grazing (Henslow’s Sparrows, Grasshopper Sparrows, Eastern Meadowlarks, Dickcissels, and Brown-headed Cowbirds) from 2016- 2018 (Chapter 4). I used stationary bioacoustics recorders to record the soundscape just after dawn and just before dusk during the summer breeding season in areas with and without bison. I found that species richness and detection frequency of the majority of my target species were not influenced by bison presence. Grasshopper Sparrows were the only species to respond to bison presence and prescribed fire with increase detection frequency in areas burned and grazed. Henslow’s Sparrows also showed a species-specific response, but only to prescribed fire, where detection frequency was lower in areas burned. These results are consistent with other research in remnant prairie with both cattle and bison grazing. Site age was the predominant influence on detection frequency of Henslow’s Sparrows, Dickcissels, and Eastern Meadowlarks likely due to the vegetation differences in sites that are planted versus remnant sites, with Brown-headed Cowbird abundance only influenced by year.
My results suggest that nesting success and detection frequency may be being impacted by bison < 5 years after reintroduction. Nachusa Grasslands has a very diverse suite of grassland bird nesters and it is hard to quantify if any impact of bison will be detrimental to these species. In addition, some grassland bird species and their nest predators are not responding to this reintroduction as expected. This suggests a possible time-lag to this response or that these species will not be impacted by the recent bison reintroduction at Nachusa Grasslands. Continued research before, during, and after land management changes is needed to make sure species are not unduly impacted by these decisions.
Herakovich, Heather, "Eastern Tallgrass Prairie Management and Its Impact on Grassland Birds" (2019). Graduate Research Theses & Dissertations. 7107.
Northern Illinois University
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