Breeding seasons of oceanic birds in a subarctic colony

Abstract

We studied the breeding seasons of marine birds on the Semidi Islands, western Gulf of Alaska, from 1976 to 1983. Distributions of laying or hatching observed in 11 species during 1–7 years are presented; less detailed information is available on the breeding schedules of three species. The combined laying period of 14 species lasted 3 months from mid-April to mid-July; the first eggs of the earliest and latest species were laid about 9 weeks apart. Mean laying dates varied by 2–16 days in nine species observed in 2 or more years. Although the egg-laying sequence of species was largely preserved from year to year, we found little evidence of concordant annual variation in breeding seasons. Species that fed lower in the food chain tended to breed earlier than those that fed at higher trophic levels. Early laying was correlated with longer laying periods, both within and between species. With the exception of three puffin species, birds with similar food habits exhibited less overlap in hatching and fledging dates than laying dates. A test for nonrandom dispersion of breeding times failed to find evidence for competitive avoidance of breeding overlap in piscivores or planktivores. Species may not have timed their egg laying to provide for maximum food availability during chick rearing. Rather, the comparative analysis of breeding schedules suggests that breeding times were determined more by the food requirements of laying females. Young females laying relatively late probably account for the right-skewed distributions of egg laying observed in this and other studies of colonial seabirds.