Do males and females differ in the feeding of large and small siblings? An experiment with the bluethroat

Abstract

Males and females have been reported to diAer in their feeding of large and small siblings in several species of birds. According to recent hypotheses, this phenomenon may be related to a sexual conflict over avian hatching patterns. We designed an experiment to test for the existence of such a sex diAerence by ma- nipulating nestling size hierarchies of the bluethroat (Luscinia s. svecica) in two directions; half the broods were ''asynchronized'' to yield large size-diAerences within broods and the other half were ''synchronized'' to yield small size-diAerences. In all broods, nestlings were cate- gorized as being either large or small according to body mass. We recorded male and female food distribution by video early (day 4 after hatching) and late (day 8) in the nestling period. Males and females did not diAer in their distribution of food among diAerent-sized nestlings. With large size-diAerences, both males and females fed large nestlings nearly twice as often as small ones. In contrast, when the size-diAerences were small, food was more evenly distributed among nestlings. Early in the nestling period, males fed more nestlings during each feeding visit than did females. Our finding that male and female bluethroats do not diAer in the feeding of large and small siblings is in contrast to most previous studies. Variation in costs and benefits to males and females from feeding diAerent-sized nestlings, and restrictions to parental choice due to nes- tling interactions, may explain interspecific variation.