Graduate Thesis Or Dissertation
 

Site fidelity and colony dynamics of Caspian terns nesting at East Sand Island, Columbia River Estuary, Oregon, USA

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/fb494c738

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  • Fidelity to breeding sites in colonial birds is an adaptive trait thought to have evolved to enhance reproductive success by reducing search time for breeding habitat, allowing earlier nest initiation, facilitating mate retention, and reducing uncertainty of predator presence and food availability. Studying a seabird that has evolved relatively low colony fidelity, such as the Caspian tern, allowed me to explore the influence of stable nesting habitat on fidelity and nest site selection. The Caspian tern (Hydroprogne caspia) breeding colony on East Sand Island (ESI) in the Columbia River estuary is the largest colony of its kind in the world. This colony has experienced a decade of declining nesting success, culminating with the failure of the colony to produce any young in 2011. The objective of my study was to understand the dynamics of this Caspian tern super-colony by investigating the actions of breeding individuals over two seasons, as well as the behavior of the colony as a whole from 2001-2011. I was interested in (1) the degree of nest site fidelity exhibited by breeding terns in successive years and its relationship to reproductive success, and (2) how the interaction of top-down and bottom-up forces influenced average nesting success across the entire colony, and caused the observed trends in nesting success at the East Sand Island colony from 2001 to 2011. My study investigated the potential influence of bottom-up and top-down drivers for the declining productivity at this once thriving colony of Caspian terns. I used sophisticated surveying equipment to test for nest site fidelity and group adherence between two consecutive breeding seasons for 80 Caspian terns marked with field readable leg bands. Available bare sand nesting habitat at this colony site declined steeply between the two years, displacing some focal individuals from their previously held nest territories. Terns whose former nest site was no longer in suitable habitat had twice the inter-annual distance between nests when compared with terns whose former nest site was still in suitable habitat. There was a negative association between inter-annual distance between nests and the number of neighbors retained from the previous year. Displaced terns retained few, if any, neighbors, indicating that group adherence by nesting terns was largely a result of individual philopatry to nesting areas within the colony, rather than adherence to neighboring nesting pairs. There was a tendency for displaced terns to nest in closer proximity to the edge of the colony, and to have nesting attempts that ended earlier than did non-displaced terns. After all nesting attempts failed during year 2 of the study, terns displaced from year 1 nest sites paradoxically exhibited higher fidelity to the colony site after colony failure in year 2 than terns that retained their year 1 nesting area. Failure of the previous nesting attempt and the novelty of the nest site and its neighbors, factors that should have resulted in low philopatry, were out-weighed by the scarcity of suitable alternative nesting habitat for Caspian terns within the region. I also investigated the potential influence of bottom-up and top-down drivers for the declining productivity at this once thriving colony of Caspian terns. Since 2001 the decline in reproductive success of Caspian terns at ESI has been associated with a significant increase in average river discharge during May and June. I also found a significant increase in kleptoparasitism rates of terns by glaucous-winged/western gulls (Larus glaucescens x L. occidentalis) since 2001, and a significant negative relationship between average annual rates of gull kleptoparasitism and Caspian tern nesting success at ESI. There has also been a significant increase in disturbance rates by bald eagles (Haliaeetus leucocephalus) during June for terns nesting at the ESI colony, and eagle disturbance rates were positively associated with May river discharge. The abundance of forage fish for terns in the estuary was inversely related to river discharge, which also influences the reliance of tern nest predators on the tern colony as a food source, resulting in increased disturbance and decreased reproductive success at the tern colony. Although correlational, our results support the hypothesis that the decline in Caspian tern nesting success at this large estuarine colony is primarily initiated by bottom-up factors, both as they directly affect tern productivity through the food supply, and indirectly as they affect the alternative food supply of potential tern nest predators.
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