Influence of local and landscape characteristics of Prairie Dog colonies on Burrowing Owl nest ecology in South Dakota
Bly, Kristy Lee Sydney
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In the Great Plains, sylvatic plague (Yersinia pestis), poisoning, and habitat conversion continue to reduce and fragment black-tailed prairie dog (Cynomys ludovicianus) habitat in which burrowing owls (Athene cunicularia) seek shelter and nest sites. Consequently, burrowing owls are experiencing population declines at the periphery of their range. The objective of this research was to evaluate habitat factors affecting a population of burrowing owls at the eastern extent of their range. I modeled hypothesized relationships between nest density (Chapter 2) and colony productivity (Chapter 3) and prairie dog habitat characteristics at two scales on the Bad River Ranches, South Dakota. Colony size, prairie dog and total burrow density, mean nearest neighbor and mean nest-to-colony edge distances, cover of warm-season grasses, and soil texture were variables measured at the colony level. Landscape level variables included the degree of colony isolation and the topographic location of colonies. In Chapter 4, I evaluated the spatial distribution of nests within colonies. Twenty-six prairie dog colonies were surveyed for burrowing owl nests during the 2005 (n =10) and 2006 (n = 16) breeding seasons. I evaluated competing models of nest density using a likelihood cross-validation approach. The model selection results in Chapter 2 suggested the top a priori predictor of nest density was colony size. Nest density was greatest on small colonies (<20 ha) and lowest on large colonies (>40 ha); in contrast, owl numbers were higher on large colonies. Exploratory analysis identified the interaction between cover of bare ground and colony habitat as the best approximating model of nest density. In Chapter 3, competing models of colony productivity were assessed. Although an additive model containing the degree of isolation, habitat, and colony size had a positive effect on colony productivity, exploratory analysis suggested colony size and the degree of isolation had the strongest influence. In Chapter 4, results showed that owls displayed a strong preference for nesting near colony perimeters, but this did not translate to higher productivity. The associations I observed lend support for the value of maintaining prairie dog colonies to ensure the persistence of burrowing owl populations in the Great Plains ecosystem.