Nest size and aromatic plants in the nest as sexually selected female traits in blue tits

Besides the direct functionality of nests driven by natural selection, accumulating evidence shows that nest building behaviors and nests may also evolve under sexual selection. Empirical research on the potential role of nests or nest features as sexual signals, however, is comparatively scarce for avian species in which the female is the only sex involved in its construction because of a male bias in the study of sexual traits, even though maternally built nests may be more common than paternally and biparentally built nests. In blue tits Cyanistes caeruleus, females alone build nests and add aromatic plants to them. We manipulated nest size and amount of aromatic plants in the nest to assess subsequent male effort and risk taking during provisioning of nestlings as indices of differential allocation. Risk taking was assessed through trappability indices of males at the nest-box when provisioning nestlings. Although male provisioning rates did not differ between experimental groups, male risk taking during provisioning was significantly lower in nests reduced in size than in control and enlarged nests, and it was significantly higher in nests supplied with aromatic plants than in control nests. Females showed nonsignificant trends to increase their provisioning effort in reduced nests, probably to compensate reduced male investment. Finally, female provisioning rates and especially male risk taking had a major positive impact on reproductive success and thereby on female fitness. In summary, this study (and previous evidence) suggests that nest size and aromatic plants in blue tit nests are used by females as sexual signals to elicit differential allocation in males. This study adds to the scarce evidence in favor of nests or nest materials acting as sexually selected traits regarding female signaling.