Welty, Justin L. – Costs and benefits of variable nest density in Burrowing Owls: Effects on predation, ectoparasites, egg yolk hormones, and productivity. 2010.
COSTS AND BENEFITS OF VARIABLE NESTING DENSITY IN BURROWING OWLS: EFFECTS ON PREDATION, ECTOPARASITE LEVELS, AND OVERALL PRODUCTIVITY
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 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.
EFFECTS OF NEST DENSITY, LAYING DATE, AND EGG ORDER ON YOLK HORMONES IN BURROWING OWL EGGS
Avian egg yolk hormones are of interest to scientists because variation among eggs within and among clutches may be adaptive. Using Radioimmunoassay I analyzed concentrations of egg yolk androgens (testosterone, dihydrotestosterone, and androstenedione), estradiol and corticosterone from one early and one late-laid egg in 46 western burrowing owl (Athene cunicularia hypugaea) nests located in the Morley Nelson Snake River Birds of Prey National Conservation Area in southern Idaho. I evaluated hypotheses relating hormone levels to density of nesting pairs, laying date of the first egg in a nest, and laying order within a nest. Estradiol and corticosterone concentrations were generally below detection levels of the assay, so my subsequent analyses focused on the three egg yolk androgens. Nesting density, calculated by measuring territory overlap with a 200m buffer around each nest, varied from 0% to 100% but had no significant effect on yolk androgens. However, egg yolk androgens varied with laying date, peaking in the middle of the laying season while remaining low early and late in the breeding season. This pattern appears unusual among bird species and may be a result of an interaction between high male quality early in the laying season and a decline in fledgling success as the nesting season progresses. Moreover, within nests, late-laid eggs had higher testosterone, dihydrotestosterone, and androstenedione than early-laid eggs. This rise in androgens from early to late-laid eggs within a nest is present in many bird species. In burrowing owls it could reflect a mechanism to assist nestlings from late-laid eggs that hatch one to several days after their siblings to compete for resources within the nest, avoid being cannibalized by older siblings, or promote survival in the presence of larger siblings.