Experimental Evidence for Food Limitation of Egg Production in Gulls

Lesser Black-backed Gulls Larus fuscus nesting on Flat Holm Island in 1989 laid significantly smaller clutches than birds nesting in the same colony in the years 1983-86. Provisioning territories with supplementary food (fish) in 1989 during the prelaying period significantly increased clutch size and egg size compared with unfed controls. Similar experiments carried out on nearby Skomer Island in 1988, where egg production by unfed birds was typical of the species in Britain, did not increase clutch or egg size. The results clearly indicate that egg production can be limited by poor food supply during the laying period, but suggest that such food limitation is not a proximate mechanism for the restriction of the normal clutch to three.