Type of Culminating Activity
Master of Science in Raptor Biology
James R. Belthoff, Ph.D.
Nesting density in birds is influenced by a number of factors including food availability, predation, and breeding site availability. As a result of a species’ nesting density, individuals incur certain costs and benefits related to predation rates, parasite levels, and productivity. I evaluated hypotheses related to these costs and benefits in burrowing owls (Athene cunicularia hypugaea) that nested within the Morley Nelson Snake River Birds of Prey National Conservation Area in southern Idaho. I calculated territory overlap, an index of nesting density, of actual nests by creating a 200m radius buffer around each nest and measuring the percentage that a focal nest buffer was overlapped by neighboring nest buffers. Territory overlap varied from 0% to 100. Nesting density interacted with distance to agriculture to influence predation rates and with hatching date to affect productivity. Lower odds of predation attempts were realized in higher nesting densities, and nests without an observed predation attempt produced nearly twice the number of fledglings. Reduced predation in nests from high-density areas may partially be explained by owls receiving early warning about predators through the actions of neighbors, which I documented in experiments using a mock predator. These same experiments documented a case of mutual defense against predators by 6 neighboring owls. Increased productivity in higher densities may also be the result of grouping around a localized food source. In addition, I found that increasing fleas reduced productivity but not through reduced nestling body condition. Finally, increased adult female body condition during the nesting season resulted in more fledglings. These results suggest that rather than being forced to live in high-density configurations because of limited resources such as the availability of nest burrows, burrowing owls can benefit from nesting in higher densities under some circumstances.
Welty, Justin Lamar, "Costs and Benefits of Variable Nest Density in Burrowing Owls: Effects on Predation, Ectoparasites, Egg Yolk Hormones, and Productivity" (2010). Boise State University Theses and Dissertations. 126.