COSTS AND BENEFITS OF PARENTAL CARE IN EASTERN KINGBIRDS

We manipulated brood sizes of eastern kingbirds (Tyrannus tyrannus) to measure the costs and benefits of parental care and to test whether k ingbirds showed evidence of individual optimization of reproductive ef fort. We found that the number of feeding trips (trips/h) increased an d that per capita feeding rates (trips/nestling/h) declined as brood s ize increased. The decline in per capita feeding rates was mostly due to high feeding rates to broods of one: parents made roughly equal num ber of trips to feed each nestling in broods of two to five. Nonethele ss, nestling mass declined with brood size, probably because large bro ods were fed more small prey. Nestling condition (mass adjusted for st ructural size) differed only between broods of or condition, but produ ctivity and feeding rate were positively and significantly related. Ad ult male condition did not vary with brood size, manipulated brood siz e, or total feeding rate, but declined as the pair's per capita feedin g rates increased. In addition, males that returned to breed were in b etter condition before leaving for migration than those that failed to return. Female condition tended to decline, and the probability of re turning to breed dropped when broods were enlarged. However, female co ndition was independent of the probability of returning. Our results s how that high feeding rates were costly, but that they carried benefit s (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 t he alternative that increased brood size was equally costly for all bi rds.