DO MALES AND FEMALES DIFFER IN THE FEEDING OF LARGE AND SMALL SIBLINGS - AN EXPERIMENT WITH THE BLUETHROAT

Males and females have been reported to differ in their feeding of lar ge 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 exi stence of such a sex difference by manipulating nestling: size hierarc hies of the bluethroat (Luscinia s. svecica) in two directions; half t he broods were ''asynchronized'' to yield large size-differences withi n broods and the other half were ''synchronized'' to yield small size- differences. In all broods, nestlings were categorized as being either large or small according to body mass. We recorded male and female fo od distribution by video early (day 4 after hatching) and late (day 8) in the nestling period. Males and females did not differ in their dis tribution of food among different-sized nestlings. With large size-dif ferences, both males and females fed large nestlings nearly twice as o ften as small ones. In contrast, when the size-differences were small, food was more evenly distributed among nestlings. Early in the nestli ng period, males fed more nestlings during each feeding visit than did females. Our finding that male and female bluethroats do not differ i n the feeding of large and small siblings is in contrast to most previ ous studies. Variation in costs and benefits to males and females from feeding different-sized nestlings, and restrictions to parental choic e due to nestling interactions, may explain interspecific variation.