Wood thrush diet, genetics, and population ecology

Ladin, Zachary S.
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University of Delaware
Rapid anthropogenic environmental and landscape change broadly influences ecosystems, communities, and populations. This research is focused on the response of the wood thrush (Hylocichla mustelina ), a forest-breeding songbird, to effects of urbanization, habitat loss, and fragmentation. I tested hypotheses dealing with food resource use, genetic structure, and metapopulation dynamics associated with potentially limiting factors for breeding wood thrushes that have been suggested to be contributing to observed range-wide population declines over the past 50 years. These findings can improve our understanding of how limiting factors may be related to wood thrush population declines within urbanized and fragmented landscapes. This work is unique in that it builds upon an ongoing long-term study of wood thrushes in and near Newark, DE, and provides new insight into their breeding ecology. Using stable carbon (δ 13 C) and nitrogen (δ15 N) isotopes, I demonstrated that wood thrush diets contained higher proportions of calcium-rich snails than was previously described. I also found that avian brood parasitism by the brown-headed cowbird ( Molothrus ater ) is related to nestling diet, which may lead to negative effects on nestling development. I used five microsatellite markers to explore the effect of forest fragmentation on fine-scale genetic structure of wood thrushes. Despite relatively low levels of genetic differentiation among sites, I found patterns of genetic flow that were not well explained by inter-site distances (m). Like many songbird species, wood thrushes are socially monogamous, however paternity analysis of wood thrush nestlings revealed that 29 % of offspring were the result of extra-pair mating events. These findings suggest that wood thrush are potentially engaging in inter-site forays during the breeding season, or may be shifting breeding locations annually. Wood thrushes with nests closer to non-forested edges, male wood thrushes with longer wing chord (mm), and males with lower body mass (g) had a higher probability of having offspring through extra-pair matings. Finally, I modeled the metapopulation dynamics of wood thrushes within the study area by integrating several empirically-based approaches to test wood thrush population responses to conservation actions. I found that the probability of wood thrush site occupancy, survival, fecundity, and recruitment could be increased over the next 30 years by reducing the proportion of impervious surface within 500 m buffers around forest sites, and by removing brown-headed cowbirds from wood thrush breeding areas. However, only when these two conservation strategies were applied simultaneously, did simulated wood thrush populations attain positive annual growth rates. In summary, my findings suggest that cowbird parasitism is related to wood thrush nestling diet, and in general, wood thrushes do not appear to be calcium limited. Wood thrushes in and near Newark, DE are not experiencing loss of genetic diversity due to habitat fragmentation, and could benefit from reductions of impervious surface on the landscape and brown-headed cowbird parasitism rates. Collectively, my findings provide important new information that can help guide conservation strategies for wood thrushes breeding in the mid-Atlantic United States, a region where observed rates of breeding population declines are at their greatest.