Individual Diet Difference, Parental Care and Reproductive Success in Slaty-Backed Gulls

Abstract

In Slaty-backed Gulls, Larus schistisagus, some males prey on seabird chicks and deliver these prey to their own broods. Females rarely prey on seabird chicks. Effects of this individual diet variation on reproductive success in relation to parental care behavior were studied on Teuri Island, Hokkaido. Pairs delivering more seabird chicks to their broods raised more fledglings. Their chicks grew faster than those of pairs delivering mostly fish, possibly because energy value of food-loads with seabird chicks was greater than those with fish or marine invertebrates. Timing of breeding, territory size and egg volume also affected reproductive success. The diet variation was not, however, related to these factors. Diet overlap between mates did not affect division of parental care between mates nor reproductive success directly. Males tending to prey on seabird chicks remained more on territory, probably because their food was easily accessible and they foraged more efficiently. This may explain why their mates attended less than other females without decreasing reproductive success.