Graduate Thesis Or Dissertation

Double-crested Cormorants (Phalacrocorax auritus) and Brandt’s Cormorants (P. penicillatus) Breeding at East Sand Island in the Columbia River Estuary : Foraging Ecology, Colony Connectivity, and Overwinter Dispersal

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  • Double-crested cormorants (Phalacrocorax auritus) and Brandt’s cormorants (P. penicillatus) nest sympatrically in a large mixed-species colony on East Sand Island (ESI) in the Columbia River estuary. Ecological theory predicts that such morphologically similar species will partition prey resources when faced with resource limitations. During the summer of 2014, I investigated local movements, foraging dive behavior, and foraging habitat selection by breeding adults of both cormorant species using GPS tags with integrated temperature and depth data-loggers (GPS-TDlog, Earth & Ocean Technologies). The overall foraging areas and core foraging areas (defined as the 95% and 50% kernel density estimates of dive locations, respectively) of double-crested cormorants were much larger and covered a broader range of estuarine habitats than those of Brandt’s cormorants. Only 26% and 27% of the overall and core foraging areas, respectively, for double-crested cormorants overlapped with those of Brandt’s cormorants. Most of the overall and core foraging areas of Brandt’s cormorants (59% and 89%, respectively) overlapped with those of double-crested cormorants, however. Within areas of overlap, Brandt’s cormorants tended to dive deeper (median depth = 6.48 m) than double-crested cormorants (median depth = 2.67 m), and selected dive locations where the water was deeper. After accounting for local water depth, Brandt’s cormorants utilized a deeper, more benthic portion of the water column than did double-crested cormorants. Our results indicate that species-specific patterns of foraging habitat utilization likely reflect past evolutionary divergence in foraging niche and evolved differences in behavioral and physiological adaptations, resulting in some partitioning of prey resources that would mitigate interspecific competition. Nevertheless, the substantial overlap in foraging habitat between the two cormorant species, particularly for Brandt’s cormorants, suggests that superabundant prey resources allow these two large and productive cormorant colonies to coexist on a single island near the mouth of the Columbia River. Annual consumption of millions of out-migrating juvenile salmonids (Oncorhynchus spp.), including smolts from populations listed under the U.S. Endangered Species Act, by double-crested cormorants nesting at ESI motivated natural resource managers to investigate potential management techniques to reduce cormorant predation by reducing the size of the breeding colony. To better understand potential dispersal of cormorants from the ESI colony due to management to reduce colony size, satellite transmitters were fitted on 83 double-crested cormorants captured on the ESI colony before egg-laying in 2013. Dispersal from ESI immediately following tagging was nearly ubiquitous, but temporary, and provided limited information on where cormorants might prospect for alternative nest sites if prevented from nesting on ESI. During this initial pre-nesting period, tagged cormorants were detected at colonies and roost sites as far from ESI as the Puget Sound region of coastal Washington; nevertheless, all but 4% of tagged cormorants returned to ESI within 2 weeks of being tagged. Following the subsequent breeding season, tagged cormorants staged at several nearby estuaries before migrating both north and south from ESI to overwinter in areas from British Columbia to northwestern Mexico; only 3% overwintered in the Columbia River estuary. Tracking data revealed substantial connectivity between the ESI colony and other colonies and regions within the range of the western North America population, suggesting the potential for widespread dispersal throughout the population’s breeding range if nesting habitat on ESI was reduced or eliminated. Dispersal did not extend across the Cascade-Sierra Divide, however; greater connectivity existed with estuary locations throughout the range and particularly with more proximate estuaries that served as post-breeding staging areas. Surprisingly low regional and colony connectivity was observed with the Oregon Coast, despite numerous active and historical colonies in that region, as well as proximity to the colony at ESI. The strong philopatry to ESI that nearly all tagged cormorants exhibited, however, suggests that few alternative nesting opportunities are as attractive for prospecting double-crested cormorants as the ESI colony site.
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