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-Pair Copulations (EPCs) from field observations on a colour-marked population 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 classified as EPC's. (3) Many pairs first copulated more than 2 months before egg-laying. The rate at which breeders engaged in Within-Pair Copulations (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 was 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 EPCs. DNA-fingerprinting confirmed that of 65 chicks (from 26 clutches), only 1 was not fathered by the male partner, but by a neighbouring male, which was seen to copulate with the female before egg-laying. Thus, extra-pair paternity was extremely rare, comprising 1.5% of all chicks. Fingerprinting provided no evidence for intraspecific brood parasitism or quasi-parasitism. (5) Members of new pairs (a minority in the population) were observed to copulate with more mates than were members of old pairs, for the same number of copulations observed. (6) Two case studies suggested that EPCs by males and females of old pairs may be attempts to change mate. One female switched to a new mate after 2 years of EPCs with this bird, while the other female is expected to switch to a neighbouring male in 1992, after 3 years of EPCs with this neighbour. (7) The majority of EPCs by male breeders were in their own territory, while female breeders more often moved to the territory of the male, often the neighbour. This sex difference resembles the sex difference in breeding dispersal: female breeders are more likely to switch territory when switching mate (ENS et al., 1993). (8) Male breeders whose mate was absent sometimes evicted soliciting female intruders instantly. This suggest that EPCs were not necessarily beneficial, even when there was no apparent risk of a penalty by the mate. (9) If EPCs 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 relationship, because our record of EPCs was incomplete and because attempts at mate change apparently often failed due to intra-sexual competition.