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Mate attendance and copulatory behaviour in western bluebirds: evidence of mate guarding

Abstract The western bluebird,Sialia mexicana, is a socially monogamous passerine with extra-pair offspring in over one-third of nests. Copulatory behaviour and mate attendance were observed for 5 years to determine whether males protect their paternity by mate guarding. Western bluebirds copulated frequently but the frequency of copulation did not increase in the hour after laying, when fertilization is thought to occur. Males attempted to copulate with their mates at a steady rate from 60 days before laying until the onset of incubation. Female acceptance of within-pair copulations increased to 80% 10 days before laying and remained high throughout the laying period. Proximity of paired males and females corresponded with changes in the frequency of within-pair copulation attempts, but following of females by their mates corresponded with peak receptivity of females to copulation. Neither proximity nor following corresponded with temporal changes in the rate ofcourtship feeding. Pairs were closer and following of the female more frequent when nearest neighbour distances were reduced, as expected with mate guarding if the threat of extra-pair copulation increases with local density. Pairs were also closer when vegetative cover around the nest increased, suggesting that one function of mate attendance may be to keep the female in view. Male western bluebirds may ensure their paternity by thwarting extra-pair copulations and by increasing copulatory access; however, paternity assurance provides a more comprehensive explanation than copulatory access for spatial and temporal variation in associative behaviours of pairs.