Abstract
Abstract We compared reproductive performance of five species of seabirds at two colonies, St. George Island (2.5 million birds) and St. Paul Island (250,000 birds), in the southeastern Bering Sea. All species had lower chick growth rates at the larger colony, and the differences were statistically significant in four species. Fledge weights of Common Murres (Uria aalge) on St. George Island were 84-88% of those on St. Paul. Average fledge weights of Thick-billed Murres (Uria lomvia) on St. George were only 74% of those for chicks from St. Paul. We found no significant differences in clutch size or breeding success between populations breeding at the two colonies. For three species, Black-legged Kittiwakes (Rissa tridactyla), Common Murres, and Thick-billed Murres, we extended our analysis to include published data from other colonies. We examined breeding performance as a function of colony size, population size (suggestive of intraspecific competition), and "effective colony size," the sum of the populations of species with considerable dietary overlap (suggestive of interspecific competition for food). We found consistently negative relationships between population size and several measures of breeding performance (clutch size, growth rate, fledge weight, and breeding success). In addition to the lower breeding success at colonies that support large populations, chicks from these colonies may be subject to higher postfledging mortality because of fledging at lower weights.
Highlights
THERE is mounting evidence that reproductive performance in seabird colonies may vary with population or colony size
Comparison of reproductive performance at the Pribilof Islands. - We found no significant differences in clutch size or breeding success of seabird populations at St
Density-dependence of kitliwake and murre reproduction.-When we examined the effect on reproductive performance of population size, colony size, and effective colony size, reproductive measures showed the most consistently negative relationships with population size (Fig. 2)
Summary
THERE is mounting evidence that reproductive performance in seabird colonies may vary with population or colony size. In a review of recent work, Birkhead and Furness (1985) concluded that negative relationships between population size and several aspects of reproductive performance support the hypothesis that intraspecific competition for food near colonies may regulate seabird populations. To partition the possible effects of intra- and interspecific competition, we examined reproductive performance for negative relationships with population size (indicating intraspecific competition) and "effective colony size," the sum of species with considerable dietary overlap (indicating interspecific competition for food). The results of these comparisons suggest whether reproduction in Northern Hemisphere seabirds generally shows density-dependence and the relative importance of intra-and interspecific competition
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