Factors affecting the growth of a Black Guillemot colony in northern Alaska

Abstract

Annual variation in breeding populations at seabird colonies has been well documented, but there have been few long-term attempts to examine the environmental and demographic forces responsible. I studied breeding chronology and demography Black Guillemot in northern Alaska from 1975-1997 to identify the factors responsible for colony establishment and growth. The Black Guillemot is a cavity-nesting seabird whose populations are frequently limited by nest-site availability. Snowmelt in spring and snow accumulation in autumn had major effects on annual nesting initiation and success, respectively. Annual arrival at the colony and median date of egg laying was well correlated with the date of snow disappearance, with annual clutch initiation advancing 4.5 days per decade in response to regional climate amelioration. Successful breeding requires a snowfree cavity for >80 days. Decreased breeding success and post-fledging survival occurred in a year with a snow-free period <80 days. Historic weather records indicate annual snowfree periods >80 days were uncommon until the 1960's, when the species was first recorded breeding in northern Alaska. When additional nest sites were provided, growth of the colony was rapid, increasing from 18 pairs in 1975 to 225 pairs in 1989. Breeding numbers then decreased to 150 in 1996 as factors other than nest-site availability controlled population size. Annual population growth averaged 37% from 1976-1982 when nest site occupancy was low, 3% from 1983-1989 when all or most nesting cavities were occupied, and -6% from 1990-1996 as breeding productivity decreased and mortality of adults increased. Without immigration and with the average annual vital rates the colony would have had an annual rate of growth of 4% during this study. Contrary to published models of seabird colony growth, I found immigration important (>60% of annual recruitment) in all phases of growth. Philopatry showed previously unreported large variation among cohorts related to variation in nest-site vacancies at the natal colony and estimated recruitment opportunities at regional colonies. Philopatry was highest (>80%) for cohorts maturing when most regional recruitment opportunities were at the study colony but low (15%) when nest-site availability was likely similar at the natal colony and other colonies in the region.