UC Irvine

Compared reproductive performance of 5 species of seabirds at St. George Island (2.5 million birds) and St. Paul Island (250 000 birds), in the SE Bering Sea. All species had lower chick growth rates at the larger colony; differences were statistically significant in 4 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 U. lomvia on St. George were only 74% of those for chicks from St. Paul. No significant differences were found in clutch size or breeding success between populations breeding at the 2 colonies. For black-legged kittiwakes Rissa tridactyla common murres, and thick-billed murres, analysis was extended to include published data from other colonies. There were 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.-from Authors