The Influence of Woody Cover and an Exploration of Raptor Density on Northern Bobwhite Survival in Southern Illinois

Author

Blake Baum, Southern Illinois University CarbondaleFollow

Date of Award

5-1-2024

Degree Name

Master of Science

Department

Biological Sciences

First Advisor

Eichholz, Michael

Abstract

Northern bobwhites (Colinus virginianus) are a culturally and economically significant game bird characteristic of grassland landscapes throughout their range. Despite the variety of research conducted on this species for nearly a century, populations continue to steadily decline, raising concern for future populations and highlighting the need for region-specific research. My research was conducted at Burning Star State Fish and Wildlife Area in northeastern Jackson County, Illinois from 2020-2022. My research addressed the following two objectives to provide insight on the population ecology of bobwhites at this site: (1) investigate the impact of multiple woody cover types within the home ranges of wintering bobwhites, how that cover may interact with winter severity, and their impacts on pre-breeding season survival, and (2) investigate the density of over-wintering diurnal raptor species between landscape types and discuss how this may be influencing predation risk. For objective 1, I radio-marked 141 individuals of which 102 had ≥ 15 relocations for estimating home-range size, and 127 individuals had ≥ 5 relocations and were used in survival analyses. I used an extended cox proportional hazards global model with continuous variables at the home-range scale. Variables considered included interspersion and juxtaposition index of blackberry, aggregation index of blackberry, and total area of blackberry, early successional forest, trees, and autumn olive. In addition, I used year, winter severity index (a time-dependent covariate), and an interaction term for year and winter severity index. The global model including all variables was retained as the most parsimonious model. Increases in the aggregation index of blackberry, area of autumn olive, and area of early successional forest were associated with increased hazard while increases in the interspersion and juxtaposition index of blackberry, area of blackberry, and area of trees were associated with a reduced hazard (α<0.05). For objective 2, I conducted 34 point-count surveys of predatory raptors throughout 6 landcover types every two weeks from October 2021 – January 2022. Bald eagles and barred owls were removed from analyses as I either did not credit them as significant predators of bobwhites or were nocturnal species, respectively. Observations from the 6 landcover types were split into two detection models based on whether the landcover type was classified as “open area cover” with a 125 m buffer distance or as “closed canopy cover” with a 75 m buffer distance. A uniform detection function was identified as the most parsimonious model for both the open area cover and closed canopy cover type models. Open area cover had a higher but not statistically significant difference in raptor density. My study demonstrates that woody cover types within the home-ranges of over-wintering bobwhites have strong relationships with the survival of those individuals and that diurnal raptors do not appear to be heavily concentrated in woody landscapes as predicted.