Intraspecific Predation and Chick Survival: Comparison among Colonies of Slaty-Backed Gulls

Abstract

Relationships between the occurrence of intraspecific predation, population size and nesting density were studied in slaty-backed gulls Larus schistisagus on Teuri, Daikoku and Yururi Islands around Hokkaido, Japan. Chick survival on Daikoku was higher than that on Teuri and Yururi. The main causes of death of chicks were intraspecific predation on Teuri and attacks by neighbor adults on Yururi. Slatybacked gulls frequently preyed upon conspecific chicks on Teuri, where the population size and nesting density were the smallest, while they rarely did on Daikoku, where the population size was the largest and the nesting density was medium and on Yururi, where the nesting density was the highest and the population size was medium. Therefore, intraspecific predation was not a density dependent mortality factor of chicks, hence, could not regulate population dynamics. The regional difference in the occurrence of intraspecific predation did not always relate to breeding synchrony or to chick guarding by parents. Twenty percent of individual male gulls probably were responsible for most of the intraspecific predation. Reproductive success of pairs including these cannibal males was not significantly different from that of non cannibal pairs including males that fed their offspring with seabird chicks but did not prey upon conspecific chicks. Therefore, pairs including cannibalistic males possibly did not have selective advantage over others.