Extramarital and Pair Copulations in the Cattle Egret

--The copulatory behavior of the Cattle Egret (Bubulcus ibis) was observed in a heronry in central Japan. In 38 of 147 extramarital copulation (EC) attempts with seven females, the males seemed to succeed in ejaculation. There were relatively few complete ECs prior to egg laying, because females responded aggressively to approaching males and because the mates of the females stayed in the territories for 78.8% of the time and protected the females. After egg laying, the aggressiveness of females against approaching males decreased and their mates spent more time outside the territories, resulting in an increase in the occurrence of complete ECs. Fighting males that were approaching a female exhibited a dominance hierarchy. The most dominant male stayed in the territory for the longest time and copulated most intensively with his mate and other neighboring females. Extramarital copulations are likely to occur in colonial herons, but mate-guarding by the male and aggression by the female mate enhance the probability that copulations will be performed only between members of the pair during the fertilizable period of the female. Thus, the monogamous pair bond is maintained. Received 24 April 1980, accepted 3 September 1980. IN recent years there has been a growing interest in extramarital or promiscuous copulations ("rape"; McKinney 1965, MacRoberts 1973, Barash 1977, Bailey 1978, Wishart 1978, Beecher and Beecher 1979, Gladstone 1979, Mineau and Cooke 1979). In herons, there is little quantitative information on the frequency of extramarital copulations (ECs), although qualitative observations note its occurrence (Meanley 1955, Blaker 1969, Lancaster 1970, Wiese 1976, Gladstone 1979). Inoue (in prep.) found that ECs comprised 23% of 121 complete copulations in the Little Egret ( E gretta garzetta ). We observed many ECs during a breeding study of the Cattle Egret (Bubulcus ibis). Here we describe the behavior accompanying both pair copulations and ECs, analyze quantitative data on the temporal variation in the frequency of ECs, and consider underlying causal factors. STUDY AREA AND METHODS Our study was conducted in a heronry located 15 km north of Tsu City, Mie Prefecture, central Japan (34ø50'N, 136ø35'E). The altitude is 20 m above sea level. The heronry has been established for many years on a small island (50 x 300 m) in the Ishigaki Pond (ca. 400 m in diameter), which is mostly surrounded by rice fields. The island is mainly covered with a thick forest of the Japanese black pine (Pinus thunbergii), the crowns and branches of which furnish suitable nesting sites for herons. During our study, the heronry consisted of more than 1,000 birds of five ardeid species: Little Egrets, Blackcrowned Night Herons (Nycticorax nycticorax), Intermediate Egrets (Egretta intermedia), Cattle Egrets, and Great Egrets (Casmerodius albus), in order of decreasing abundance. All of our observations were made from a blind on a 3.6-m-high scaffold at the southern edge of the heronry. Copulatory behavior was observed with the unaided eye or with a binocular telescope (9 x 35) as necessary. Observations were carried out for 34 days, from 10 May to 20 June 1978, amounting to a total of 455.5 h; the longest observation period was from 0330 to 2000. Because B. ibis lacks sexual dimorphism, the individuals that laid eggs or were repeatedly mounted by their mates were considered females. No "reverse mounts" were observed in two pairs (369.1 observation-h) in which the sexes were determined by egg laying. All members of the 10 pairs that we observed were individually recognizable by idiosyncracies in the bill, leg, lore, plume, and so on. Seven pairs 134 The Auk 98: 134-144. January 1981 January 1981] Copulations in the Cattle Egret 135