|Abstract or Summary
- 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.