Costs and benefits of parental care in eastern kingbirds

Abstract

We manipulated brood sizes of eastern kingbirds (Tjrannus tyrannus) to measure the costs and benefits of parental care and to test whether kingbirds showed evidence of individual optimization of reproductive effort. We found that the number of feeding trips (trips/h) increased and that per capita feeding rates (trips/nestling/h) declined as brood size increased. The decline in per capita feeding rates was mostly due to high feeding rate* to broods of one: parents made roughly equal number of trips to feed each nestling in broods of two to five. Nonetheless, nestling mass declined with brood size, probably because large broods were fed more small prey. Nesding condition (mass adjusted for structural size) differed only between broods of one and five. After controlling for effects of brood size, feeding rates had no supplementary influence on either nesding size or condition, but productivity and feeding rate were positively and significantly related. Adult male condition did not vary with brood size, manipulated brood size, or total feeding rate, but declined as the pair's per capita feeding rates increased. In addition, males that returned to breed were in better condition before leaving for migration than those that failed to return. Female condition tended to decline, and the probability of returning to breed dropped when broods were enlarged. However, female condition was independent of the probability of returning. Our results show that high feeding rates were costly, but that they carried benefits (greater productivity). Some evidence for individual optimization of reproductive effort existed: variability in nestling and adult female condition were better explained by changes in brood* size than by the actual number of young in the nest. However, most evidence supported die alternative that increased brood size was equally costly for all birds.