Polygyny in the dusky warbler, Phylloscopus fuscatus: The importance of female qualities

The polygyny threshold model states that secondary females gain benefits from high territory quality that outweigh the costs of sharing a male. We aimed to test this prediction using the dusky warbler as a model species. We first showed that neither the shifted sex ratio hypothesis nor the no-cost models were likely to apply to our study. Secondary females settled in territories of higher quality (high food abundance, low predator density) and had a nonsignificantly higher reproductive success than simultaneously settling monogamous females. However, there were strong indications that these two groups of females differed intrinsically. Secondary females were older than late monogamous females, and while they replaced lost clutches more often, they also suffered from a nonsignificantly higher winter mortality. Consequently, it was impossible to tell whether differences in reproductive success were caused by differences in territory quality or in female qualities. Our study suggests that female choice may also depend on characteristics that are specific to the individual, and may therefore be more sophisticated than allowed for in traditional polygyny models. In other words, the polygyny threshold may be not the same for all individuals. We suggest that prior breeding experience may help older females to profit more from the benefits and to suffer less from the costs of polygyny than young females. (C) 2001 The Association for the Study of Animal Behaviour