Changing Habitat Use Associated with Distributional Shifts of Wintering Raptors

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There is widespread evidence that multiple drivers of global change, such as habitat degradation, invasive species, and climate change, are influencing wildlife. Understanding how these drivers interact with and affect species may be difficult because outcomes depend on the magnitude and duration of environmental change and the life history of the organism. In addition, various environmental drivers may be evaluated and managed at different spatial scales. We used a historical dataset from 1991 to 1994 and current information from 2010 to 2012 to examine whether occupancy patterns of wintering raptors were consistent with regional changes in distribution or habitat conditions within a local management unit, the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA). We predicted that if local populations reflected regional shifts, then rates of raptor occupancy within the NCA would be higher compared to historical estimates and birds would use different habitats compared to historical use. Alternatively, if local populations were determined by habitat conditions, then we predicted that occupancy rate of raptors within the NCA would be lower compared to historical estimates and current habitat use would be consistent with historical use. Results support the hypothesis that northward distributional shifts influenced wintering raptor populations in southwest Idaho to a greater extent than local habitat conditions. Wintering raptors had higher occupancy rates in 2010–2012 compared to 1991–1994, whereas invasive grasses have increased and native shrubs have decreased suggesting that habitat suitability for raptors has declined over time. On the species level, changes in habitat use were associated with greater increases in occupancy rates in 2010–2012 compared to 1991–1994. Organisms flexible in their habitat use may be better able to respond to continental forces driving distribution shifts. Conversely, habitat or prey specialists may be poorly equipped to handle such rapid, large-scale global change. Further, Grinnellian niche models predicting species response to change by mapping current habitat use to forecasted vegetation maps should consider plasticity in habitat use and changes in the cost-benefits of life-history strategies.