BROOD DIVISION IS ASSOCIATED WITH FLEDGLING DISPERSION IN THE BLUETHROAT (LUSCINIA S. SVECICA)

In many bird species with biparental care, each parent takes the exclu sive care of some of the young after fledging. Some of the hypotheses that have been put forth to explain brood division behavior state that it is advantageous for a particular parent to care for a particular ' 'type'' of young, e.g. with respect to sex, size, or parentage. Other hypotheses claim a benefit to the parents (e.g. reduced foraging costs or risks of predation) only when the young are spatially dispersed. I n this paper, we describe brood division in a Norwegian population of Bluethroats (Luscinia s. svecica). In general, brood division arose on ce the young became spatially dispersed after fledging. The only excep tions to the rule occurred when the male was polygynous and provisione d the young at a low rate. No brood division was found when the young were still in the nest, nor when they were physically prevented from s pacing out by an enclosure around the nest. Young fed by the same pare nt were more clustered than young fed by different parents. Experiment al switching of young among single-parent groups suggested that parent s were able to recognize individual offspring outside the nest. Howeve r, there were no indications that parents divided the brood by sex, si ze, or genetic parentage. Our data are consistent with hypotheses that assume a parental benefit from brood division when the young are spat ially dispersed. .