Size of nest-cavity entrance influences male attractiveness and paternal provisioning in house wrens.

In altricial birds, parental provisioning is plastic and can respond to a variety of environmental stimuli. In this study, we manipulated the size of entrances into artificial nest cavities (i.e., nestboxes) in a population of house wrens (Troglodytes aedon) as a means of manipulating a male's sexual attractiveness, and examined changes in parental provisioning. Nest cavities with large entrances are less desirable as nesting sites, and the males at these sites are less attractive to females. Therefore, we predicted that males at boxes that had large entrances would invest more in parental care (i.e., those that succeeded in finding a mate would provision their offspring at a higher rate) than males at nestboxes with small entrances. As predicted, males provisioned their offspring with food at the highest rates at nestboxes with enlarged entrances, and male provisioning effort positively predicted the number of fledglings they produced per egg. Males at these boxes provisioned more than their mates and more than females and males at nestboxes with small entrances. At nestboxes with small entrances, males provisioned at the same rate as females, and female provisioning did not differ significantly between treatments, on average. Male and female provisioning rates were negatively correlated, such that the increase in provisioning by males at nestboxes with enlarged entrances did not enhance nestling condition, likely because food delivery by females declined with increased provisioning by males. However, the amount of time females spent providing warmth for their ectothermic young increased with increases in male provisioning, suggesting that levels of male parental care altered the mode, not necessarily the extent, of care provided by females. These findings suggest that male provisioning is related to sexual attractiveness, and that sexual conflict over biparental care may not be as simple as the assessment of food provisioning might otherwise suggest.