Confronted with threats of habitat loss and climate change, birds that inhabit open-lands such as grasslands and shrublands are among the most rapidly declining groups of birds across North America. One such bird is the Northern bobwhite (Colinus virginianus; hereafter bobwhite) which inhabits heterogeneous landscapes that typically consist of shrubland, grassland, and early-successional forests. Bobwhites are declining at an alarming rate across most of their range, largely due to habitat loss from intense production agriculture. Populations along the northern periphery of bobwhite range are secondarily imperiled by high mortality from severe winter weather. Because bobwhites are a socially and economically important gamebird, there has been great interest from a variety of stakeholders to maintain suitable habitat and promote sustainable populations. As a result of the conservation attention they receive, bobwhite conservation has the potential to benefit non-target songbirds with similar land cover requirements. The umbrella species concept suggests that benefits will accrue to non-target species if viable bobwhite populations are maintained through habitat conservation. The future viability of bobwhite populations is in question in Ohio, so a comprehensive demographic study is needed to estimate probability of population persistence under a variety of future weather and population viability analysis (PVA) scenarios. The first part of my thesis focuses on landscape-scale characteristics that influence bobwhite occupancy in Ohio, and whether occupancy by other open-land species is positively predicted by bobwhite occupancy. The second part of my thesis focuses on projecting population dynamics of bobwhite in Ohio in response to future winter weather and PVA scenarios.
I obtained bird survey data from the Ohio Breeding Bird Atlas (2006–2011). I used land cover data (NLCD 2011) to calculate compositional and configurational landscape metrics within a 630 m radius (mean bobwhite home-range size) of breeding bird atlas survey points where bobwhites were detected and not detected. I restricted the extent of my study to 25 counties in southern Ohio that comprise the core of bobwhite range in Ohio. I then constructed single-season occupancy models using the R package `unmarked’ to determine which landscape metrics influenced occupancy after accounting for detection covariates. I predicted occupancy of other open-land species with predicted bobwhite occupancy probability as the sole covariate. I fit additional occupancy models with landscape variables for species whose occupancy was positively predicted (P ≤ 0.05) by bobwhite occupancy. I then compared the fit of bobwhite-only occupancy models with multi-variable landscape models for the same species.
A total of 244 individual bobwhites were recorded at 187 unique points among 990 points selected. The highest-ranked bobwhite occupancy model contained the following site variables after controlling for detection: cohesion of forest (β= 0.17, 95% CI: -0.07, 0.41), percent agriculture (β= 0.62, 95% CI: 0.16, 1.07), percent barren (β= 0.13, 95% CI: 0.05, 0.20), and percent grassland (β= 0.13, 95% CI: 0.02, 0.24). Bobwhite occupancy was a significant (P ≤ 0.05) positive predictor for 12 of 35 focal species, and a significant negative predictor for 11 of 35 species. Models with only bobwhite occupancy probability as a predictor was the best supported model for just two species: grasshopper sparrow (Ammodramus savannarum; β= 0.496) and willow flycatcher (Empidonax traillii; β= 0.450). Bivariate plots of cover type space with percent agriculture and amount of forest-agriculture edge as variables revealed that locations occupied by bobwhites contained too much agriculture and not enough edge for shrubland specialists, and insufficient agriculture and too much edge for grassland specialists. These results suggest that bobwhite occupancy is more influenced by percent of open-land cover classes in the landscape. Cover type requirements of bobwhites may be too specialized for effective conservation of an entire suite of species, affording instead conditions that favor a narrower set of species that share similar requirements for composition and configuration of cover types. The broad cover requirements of bobwhites likely result in co-occurrence with several species from different guilds whose needs are partially met by those of bobwhites.
I created an integrated population model (IPM) for a declining bobwhite population in southwestern Ohio for the second part of my thesis. The IPM was implemented in a Bayesian framework in program JAGS using the `jagsUI’ package in program R. The IPM combined two sources of count data, five years of winter survival data, and two years of breeding season demographics to quantify the effect of snowfall on winter survival, and to project the population ten years into the future under 4 different winter weather scenarios and 6 different population viability analysis (PVA) scenarios. The IPM estimated that the bobwhite population declined precipitously during 2007–2015 based on fall covey counts (λ= 0.634, 95% CRI: 0.499,0.821) and spring whistle counts (λ= 0.596, 95% CRI: 0.544,0.690). Winter snowfall was confirmed to have a negative effect on winter survival for both adults (β= -0.058, 95% CRI: -0.122, 0.008) and yearlings (β= -0.048, 95% CRI: -0.121, 0.019), and also had the largest influence on population growth rate (λ).
Population projections revealed that the bobwhite population had mean yearly extinction probabilities of 0.384–0.410 across all winter weather scenarios. Extinction probability decreased to 0.327 when nest success and summer survival were increased by 20%. However, when winter survival, nest success, and summer survival were all improved by 20%, extinction probability was reduced to 0.287, a ~30% reduction compared with no change in vital rates. Results demonstrated that this population of bobwhites is highly sensitive to effects of severe winter weather, and the population is at a high risk of extinction without management intervention. Improving protective cover to enhance survival during extended periods of snow cover remains necessary to stem the decline of this population, although management efforts should also focus on creating high-quality breeding habitat to improve fecundity and summer survival.