-In many bird species with biparental care, each parent takes the exclusive 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. In this paper, we describe brood division in a Norwegian population of Bluethroats (Luscinia s. svecica). In general, brood division arose once the young became spatially dispersed after fledging. The only exceptions to the rule occurred when the male was polygynous and provisioned 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 spacing out by an enclosure around the nest. Young fed by the same parent were more clustered than young fed by different parents. Experimental switching of young among single-parent groups suggested that parents were able to recognize individual offspring outside the nest. However, there were no indications that parents divided the brood by sex, size, or genetic parentage. Our data are consistent with hypotheses that assume a parental benefit from brood division when the young are spatially dispersed. Received 13 September 1996, accepted 7 April 1997. IN MANY BIRD SPECIES with biparental care, the parents divide the brood after fledging so that each parent takes sole care of some young (Skutch 1976). This phenomenon is known as brood division (Smith 1978, McLaughlin and Montgomerie 1985), and the association between a parent and its young is termed a family unit (Nolan 1978). Many reports on brood division are anecdotal and provide no data. However, some studies of brood division are more detailed and well documented (e.g. Smith 1 Address correspondence to this author. E-mail: j.t.lifjeld@toyen.uio.no 1978, Moreno 1984, Harper 1985, McLaughlin and Montgomerie 1985, Kopachena and Falls 1991). In many altricial species, particularly open nesters, the young leave the nest early (Maher 1964), often before they are completely developed and able to fly (Tinbergen 1939, Skutch 1976, Knapton 1978, Kopachena and Falls 1991). Early nest departure coupled with spatial dispersion of young is viewed as a strategy to minimize the risk of predation (Tinbergen 1939, Maher 1964, Willis 1972, Knapton 1978, Nolan 1978) and/or to help parents reduce the energetic costs of parental care (McLaughlin and Montgomerie 1989a,b). Spatial dispersion