Sharing mates and nestboxes is associated with female “friendship” in European starlings Sturnus vulgaris
Résumé
Breeding decisions in birds involve both mate and nest choice, and there is increasing evidence that social influences may modulate individual choices. Female preferences may be affected by other females’ preferences and mutual choice cannot always be excluded, which makes the whole pattern more complex than assumed by most sexual selection models. Social transmission may be facilitated by particular social bonds, therefore pre-breeding social networks may influence later mate choices. The other case where females share mate or resources is polygyny, generally viewed to only benefit males. If mutual benefits may arise then mechanisms should evolve to reduce the reproductive cost for females such as to reduce the cost of aggression by sharing their mate with a preferred same sex social partner. We tested the hypothesis that females’ mating decisions may be influenced by the pre-breeding social network and that social partner relations established prior to breeding may share decisions (mate/sites) in a facultative polygynous species, the European starling. Two experiments were designed to test the relative importance of male or nest by following the whole dynamics of the breeding cycle from the pre-breeding period until mate and nest selection. In both cases socially isolated females tended to be excluded from breeding, while pre-breeding social partners tended to share mates and to nest in close proximity, mate copying leading in some case to polygyny. The final pattern resulted both from female “likes and dislikes” and male preferences for some females. Aggressive interactions between females were rare. Vocal sharing between females may have been a clue for males as to the degree of social integration of these females.