Size Selective Hunting by Slaty-backed Gulls Larus schistisagus and Influence on Fledging Success of Black-tailed Gulls L. crassirostris

Abstract

(1) The influence of predation by Slaty-backed Gulls Larus schistisagus on the fledging success of Black-tailed Gulls L. crassirostris was studied on Teuri Island off Haboro, Hokkaido, in 1980 and 1981. (2) Four quadrats were set in the breeding colony of Black-tailed Gulls in which 326 chicks were colour ringed and their biomass was measured. The analysis of food remains collected from forty nests of Slaty-backed Gulls was made for the estimation of the amount of predation. (3) The amount of predation reached a maximum later than the time when the biomass of prey reached a maximum. (4) Slaty-backed Gulls selectively hunt smaller chicks because of low hunting successs of large prey. (5) Later broods of Black-tailed Gulls were exposed to higher predation pressure by Slaty-backed Gulls. (6) The fledging success of Black-tailed Gulls was lower in late (14%) than in early (31%) and middle broods (41%). This trend could partly be explained by predation by Slaty-backed Gulls.