Site- and sex-level differences in adult feeding behaviour and its consequences to offspring quality in tree swallows (Tachycineta bicolor) following brood-size manipulation

Abstract

In species with biparental care, males tend to invest less in offspring than do females, likely because of differences in the costs and benefits associated with parental effort. Here I test for sex differences in the response of tree swallows ( Tachycineta bicolor (Vieillot, 1808)) to a brood-size manipulation at two locations differing in food resources, Alaska and New York. I tested sex and habitat differences in how swallows responded to changes in offspring demand. At both sites, both sexes increased effort when feeding enlarged broods, although Alaskan males increased feeding less than Alaskan females. Males decreased feeding effort more to reduced broods than females, but only in Alaska. Food abundance was higher in Alaska than in New York, and Alaskan tree swallows made more feeding visits than New York tree swallows. In New York, food availability did not predict feeding rate and there was no sex difference in the response to brood manipulation. In both sites, male feeding effort was linked with nestling residual body mass, while female feeding effort was correlated with nestling growth rate. This study demonstrates that male tree swallows differ from females by being the first to reduce feeding effort under certain conditions and that male and female feeding rate affects offspring quality differently.