Individual differences in gull-chick predation by Slaty-backed Gull

Abstract

We investigated the impact of predation by Slaty-backed Gull Larus schistisagus on the chicks of Black-tailed Gull L. crassirostris and Slaty-backed Gull on Teuri Island, Hokkaido, Japan, in 2019. We collected 346 pellets and food remains from 10 Slaty-backed Gull nests (10–140 samples per nest). The average occurrence of seabird remains in the food samples was 47.5%; however, the occurrence of seabirds was much higher (66–90%) at three nests (designated as "seabird-predator nests"). Direct observation of the study area where 122 Black-tailed Gull and 17 Slaty-backed Gull were nesting, showed that the males of these three "seabird-predator nests" accounted for 75% of all predation attempts. Of these three, two males took 21% of Black-tailed Gull chicks and fledglings, and 24% of Slaty-backed Gull chicks in the study area. Black-tailed Gull chicks tended to be more vulnerable to predation if the chick hatched later and in the nests closer to these two seabird-predator nests, whereas these effects were not observed among fledglings. This study highlights the significance of specialist individual Slaty-backed Gull in determining the breeding success of Black-tailed Gull.