WHY DOES THE TYPICALLY MONOGAMOUS OYSTERCATCHER (HAEMATOPUS-OSTRALEGUS) ENGAGE IN EXTRA-PAIR COPULATIONS

1) We attempted to identify the possible costs and benefits of Extra-P air Copulations (EPCs) from field observations on a colour-marked popu lation of Oystercatchers, a long lived monogamous species, where male and female share parental care and territory defence. (2) Despite our broad definition of EPCs, only 7.0% of successful copulations by male breeders and 5.1% of successful copulations by female breeders were cl assified as EPC's. (3) Many pairs first copulated more than 2 months b efore egg-laying. The rate at which breeders engaged in Within-Pair Co pulations (WPCs) was highest in the month preceding egg-laying: almost 1 copulation per hour during low tide. Males were increasingly likely to initiate WPCs close to the period of egg-laying. Once the clutch w as complete copulation rates dropped to near zero. (4) Most EPCs were observed well before egg-laying. In the month preceding and including egg-laying only 3.3% of all successful copulations by females were EPC s. DNA-fingerprinting confirmed that of 65 chicks (from 26 clutches), only 1 was not fathered by the male partner, but by a neighbouring mal e, which was seen to copulate with the female before egg-laying. Thus, extra-pair paternity was extremely rare, comprising 1.5% of all chick s. Fingerprinting provided no evidence for intraspecific brood parasit ism or quasi-parasitism. (5) Members of new pairs (a minority in the p opulation) were observed to copulate with more mates than were members of old pairs, for the same number of copulations observed. (6) Two ca se studies suggested that EPCs by males and females of old pairs may b e attempts to change mate. One female switched to a new mate after 2 y ears of EPCs with this bird, while the other female is expected to swi tch to a neighbouring male in 1992, after 3 years of EPCs with this ne ighbour. (7) The majority of EPCs by male breeders were in their own t erritory, while female breeders more often moved to the territory of t he male, often the neighbour. This sex difference resembles the sex di fference in breeding dispersal: female breeders are more likely to swi tch territory when switching mate (ENS et al., 1993). (8) Male breeder s whose mate was absent sometimes evicted soliciting female intruders instantly. This suggest that EPCs were not necessarily beneficial, eve n when there was no apparent risk of a penalty by the mate. (9) If EPC s are primarily attempts to change mate, we predict an increase in the probability of mate change when an individual has engaged in EPCs in the previous year. We surmise that we failed to establish this relatio nship, because our record of EPCs was incomplete and because attempts at mate change apparently often failed due to intra-sexual competition .