Spatial ecology of Western Ratsnakes (Pantherophis obsoletus) : implications for bird nest predation
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[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT AUTHOR'S REQUEST.] In fragmented Midwestern forests, many birds face high rates of nest predation, to the point where this region may serve as a population sink for some species. That is, so many of their eggs and young are eaten by predators that these birds cannot produce enough offspring to replace themselves, rendering regions like central Missouri a drain on some bird populations. One important difference between fragmented and contiguous forests (e.g., the Ozarks of southern Missouri where birds seem to be doing better) is that for some bird species, western ratsnakes (Pantherophis obsoletus) are by far the most important nest predator in the fragmented forests. Nest predation rates have been indirectly linked to ratsnake activity patterns or habitat, and recent nest camera studies have directly attributed nest predation by ratsnakes to increased forest fragmentation and weather conditions that are likely favored by ratsnakes. We conducted a large-scale radio-telemetry study of ratsnakes to evaluate their activity patterns and resource selection in study sites where they have been identified as the predominant bird nest predator. First, we used local weather data to model seasonal ratsnake movements as they relate to bird nesting phenology and climate change. Next, we used spatio-temporal temperature models to investigate the thermal conditions available to snakes. We used remote temperature sensors and high-resolution spatial data to develop a dynamic temperature surface for our study area. Models predicted near-surface air temperatures with high accuracy during the winter and growing season, and permitted us to include thermal conditions in resource selection models. Finally, we used discrete choice models to evaluate and predict ratsnake resource selection. We found strong evidence that ratsnakes preferentially select forest-field edges and other habitats that provide structural heterogeneity. Our findings support the hypothesis that edge effects explain increased predation rates in Midwestern forest fragments.
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