Aspects of the breeding biology of an expanded population of glaucous-winged gulls in British Columbia

Verbeek, NAM.

Journal of Field Ornithology 571: 22-33


ISSN/ISBN: 0273-8570
DOI: 10.2307/4513086
Accession: 037885739

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The population of Glaucous-winged Gulls in British Columbia has doubled between 1960 and 1974. It was assumed that this increase would have resulted in a decrease in egg size, clutch size, incubation period, and hatching success, and an increase in the laying interval between eggs in a clutch. A comparison of data obtained on Mandarte Island in 1961-1962 and 1979-1980 does not support the assumption. The additional birds apparently exploit food sources that have heretofore been largely untapped. Indirect indications are, however, that the food supply at other times, particularly the later part of the breeding season, may no longer be adequate, because the peak of mortality of birds less than 1-yr-old has shifted from mid-winter prior to the population increase to early autumn at present.