PARENTAL-CARE RESPONSES BY YELLOW WARBLERS (DENDROICA-PETECHIA) TO SIMULTANEOUS MANIPULATIONS OF FOOD ABUNDANCE AND BROOD SIZE

Theoretical models view biparental care as a state of equilibrium that can be maintained only when the amount of parental care provided by o ne parent depends on the amount provided by its mate. According to the se models, biparental care results when a decrease in the contribution of one parent is partially, but not completely, compensated for by an increase in the contribution of the other parent. Furthermore, this e quilibrium state can only be stable if any other external factor affec ts male and female effort equally. We used yellow warblers (Dendroica petechia) to examine whether changes in brood size and food abundance affect the parental contributions of the sexes equally. Supplemental f ood did not affect parental care by either sex, but brood size did. Bo th males and females provided more to larger broods, and in large broo ds only, their provisioning rates increased with nestling age. Parenta l effort per nestling was similar in the two sexes, being higher for s maller broods and increasing with nestling age. Based on brood biomass , parental effort was greater for smaller broods, and decreased with n estling age in females only. Therefore, in agreement with current mode ls of the maintenance of biparental care, the effects of brood size an d nestling age on parental care did not differ significantly between t he sexes. Nonetheless, data from other species and theory indicate tha t the costs and benefits of providing parental care differ between the sexes, so it is unlikely that biparental care can be maintained solel y by a partial compensation response.