Extra-pair Copulations in Little Terns Sterna albifrons

Abstract

EPC (extra-pair copulation) attempts in Little Terns Sterna albifrons were studied from April to July in 1988 at the Sai River, Nagano Prefecture, central Japan.1) 114 and 62 birds were trapped and marked individually with color leg rings, respective in 1987 and 1988. EPC attempts were observed for 492 hrs and breeding behavior for 630 hrs.2) Little Terns mated monogamously (31 nests were examined).3) EPC's were attempted in 114 approaches by extra-pair males holding a fish. Complete EPC's were 4 (3.5%), mounting 11(9.6%), and failed EPC's 15 (13.2%).4) In the normal pair copulation, males approached to their mate without fish. Females made the hunched posture to extra-pair males approaching with fish. Most extra-pair males could not copulate, as females moved out to grab the fish curried by the males when the males attempted to mount.5) Females made the hunched posture and lying posture to extra-pair males with fish, and grabbed his fish by jumping before mounting. 17 (14.9%) cases of grabbing behavior by jumping were observed, and each of four females succeeded to grab once.6) Pretending to accept the male's EPC attempts, females grabbed food male carried during the incubation and fledging periods. As time of leaving nest of females increased during these periods, the contact chance to extra-pair males may increase. Courtship feeding his rate decreased during these periods, becoming females to be hungry. Grabbing food may be induced by food shortage.7) Females depend on their mates for plenty of food during the egg-laying period. Males did not guard their mates during this period, but their feeding may function mate guarding.8) Extra-pair males did not seem to judge the fertile period of females, and failed to copulate usually by females' grabbing behavior. They contribute little to fertilizing in EPC.