An alternative route to coloniality in the bearded tit: Females pursue extra-pair fertilizations

We combine behavioral studies with DNA-fingerprinting of a population of bearded tits Panurus biarmicus and test the idea that the pursuit of extra-pair copulations by females acts as selective pressure that may contribute to the formation of colonies in this socially monogamous species. We show that the rate of extra-pair paternity in colonial breeding pairs is very high, whereas no extra-pair chicks were detected in solitary nests. We found no cases of extra-pair maternity and intraspecific brood parasitism in solitary nests, whereas it did occur in dense breeding situations. We further found no difference in the quality of males between colonial and solitary individuals. However, colonial females were of significantly higher quality in terms of body size and condition. We propose that high-quality females settle in colonies to increase their opportunities to adjust the choice of their social partner by obtaining extra-pair copulations. We suggest that colony formation enables high-quality females to incite male-male competition for extra-pair copulations and consequently secure extra-pair fertilizations by high-quality males and that high-density nesting is controlled by female bearded tits.