When the field study of the Black-tailed Gull Larus crassirostris was performed at the colony of Kabu island from 1991 to 1993, eggs were often laid outside nests during egg-laying stage (I call them ″eggs laid outside nests″). There were not attendants which took care of them after egg-laying. I recorded some traits of them and discussed a new tactic of intraspecific nest parasitism in the Black-tailed Gull. Eggs laid outside nests were found 25 cases in 1991, 21 cases in 1992 and 10 cases in 1993. Total of them were 56 cases. Traits of eggs laid outside nests were 1) to be laid mainly in the latter of egg-laying stage, 2) to be laid later than the last eggs in the nearest clutch, 3) to be fertile, 4) to be larger egg size (maximum length, maximum width, weight) than the last eggs in the nearest clutch. Anything else, seven cases that eggs laid outside nests were added to the nearest clutch and were rolled around the nearest clutch by neighbors were observed. Results of experiments of reaction of nesting individuals to eggs laid outside nests were showed that the nearer they were placed at nests, the easier they were added to clutches.Results of different egg size and irregular sequence of appearance of eggs in eggs laid outside nests were similar to general characters of intraspecific nest parasitism. Therefore, I strongly suggested that a new tactic of intraspecific nest parasitism was occured in the Black-tailed Gull by eggs laid outside nests.

View full abstract