Graduate Thesis Or Dissertation
 

Factors affecting colony size, reproductive success, and foraging patterns of double-crested cormorants nesting on East Sand Island in the Columbia River Estuary

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  • The purpose of this study was to investigate the primary factors affecting colony size, reproductive success, and foraging patterns of Double-crested Cormorants (Phalacrocorax auritus albociliatus) nesting at East Sand Island in the Columbia River estuary, the largest colony of this species on the Pacific Coast of North America. This colony grew dramatically over the past 13 years and appears to represent a substantial proportion (>40%) of the West Coast population. Due to increasing concern over avian predation on juvenile salmonids in the Columbia River estuary, there was a need to understand the factors limiting the size and productivity of this large and growing cormorant colony and how breeding adults exploit the available forage fish resources in the estuary. The East Sand Island colony recently fragmented into separate sub-colonies that differed in reproductive success; clutch size, hatching success, brood size at fledging, nesting success, and overall productivity were all higher at a recently-formed satellite sub-colony compared to the main colony. Depredation of cormorant nest contents by Glaucous-winged/ Western Gulls (Larus glaucescens X L. occidentalis) following disturbances caused by Bald Eagles (Haliaetus leucocephalus) appeared to be the primary factor limiting reproductive success. During my study, nesting habitat and food supply did not appear to be limiting colony size or reproductive success. I predict that the colony will continue to expand unless forage fish stocks decline and/or eagle disturbances increase. I used radio-telemetry to investigate the spatial and temporal patterns of foraging male and female Double-crested Cormorants. Nesting adults tended to commute over 5 km from the colony to forage in either the estuarine-mixing zone or the freshwater zone of the estuary, where forage fishes were presumably more available than in the marine zone near the colony. The sexes exhibited striking differences in foraging distribution. Males commuted longer distances to forage in the freshwater zone compared to females, which tended to forage in the estuarine-mixing zone; however, females took longer foraging trips than males on average. Gender differences in foraging patterns may enhance the foraging efficiency of pairs nesting at a large colony such as East Sand Island. The cormorant breeding colony on East Sand Island seems to be avoiding density-dependent constraints of food supply by foraging over a wide area of the estuary on a diversity of marine forage fishes whose stocks are currently high. I predict that in years when stocks of marine forage fish within the estuary are low (e.g., due to poor ocean conditions), Double-crested Cormorants may become more reliant on the more predictable fish resources of the estuary, such as out-migrating salmonid smolts.
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