Male detention affects extra-pair copulation frequency and pair behaviour in western bluebirds

Male western bluebirds, Sialia mexicana, show patterns of mate following consistent with mate guarding. The hypothesis that males prevent extra-pair copulations by guarding their mates was tested using I-h male detention experiments. Males were caged on territory in view of the female, visually occluded on territory or removed from the territory altogether (visually+vocally occluded). Extra-pair males were more likely to join females within 15 m and attempt to copulate with them in the three detention treatments than in treatments where males were captured and released or not manipulated at all. Detention also increased the number of females accepting one or more extra-pair copulations. The increased frequency of extra-pair copulations was not caused by movement of females away from their nests, nor by increased rates of vocalization by resident males or females. Extra-pair events occurred shortly after the resident male was caught, suggesting that extra-pair copulations can occur during brief lapses in mate following. Intruding males were usually neighbours with mates and nests of their own. Half intruded;when their own mates were at peak receptivity, suggesting that males leave their own fertile mates to seek extra-pair copulations. Detained males did not compensate for detention by increasing the frequency with which they copulated with their mates after they were returned to the territory. Acceptance of extra-pair copulations by females did not increase with;he number of extra-pair copulation attempts and females were no more likely to participate in extra-pair copulations when resident males were visually occluded than when they were visually present. Only half of the females that experienced extra-pair copulation attempts accepted one or:more extra-pair copulations. Although male detention resulted in an increase in the frequency of extra-pair copulations in western bluebirds, females appeared to be selective about participating in extra-pair matings. (C) 1997 The Association for the Study of Animal Behaviour.