Skill acquisition and the timing of natal dispersal in the white-throated magpie-jay, Calocitta formosa

Offspring may remain associated with parents to gain the skills required for independent survival and reproduction. The ontogeny of foraging and breeding behaviour was followed in a cohort of cooperatively breeding white-throated magpie-jays from fledging through to dispersal. Young jays approached adult levels of foraging proficiency within 1 year of fledging. During their first year of age, young jays provisioned nests as helpers at a lower rate than did older individuals. No improvement occurred during that first year, however, suggesting that lower provisioning was the result of other factors than the lack of foraging or nesting skills. Natal dispersal was extremely sex-biased: males dispersed between 4 and 23 months of age, but females remained in their natal group. Male dispersal coincided with both the maturation of foraging skills and the start of a breeding season. Males from larger groups dispersed at an earlier age than those from smaller groups. Therefore, although offspring must associate with parents during some minimum period to acquire skills, the decision of whether and when to disperse from the natal territory is determined by other factors in the white-throated magpie-jay. ? 1996 The Association for the Study of Animal Behaviour In many species, offspring that appear to have reached physical maturity delay dispersal and help parents breed. The conventional view is that delayed dispersal is a result of ecological constraints that limit the dispersal opportunities of young (Emlen 1991). Most debate on the evolution and maintenance of cooperative breeding has centred on the relative importance of direct benefits of philopatry (e.g. access to territory resources, increased survivorship, increased probability of territorial inheritance) versus indirect fitness benefits from augmenting the production of kin (Brown 1987; Emlen 1991; Koenig et al. 1992). One additional direct benefit of philopatry, which also predicts the age of natal dispersal, has been termed the skill hypothesis (Brown 1987). This hypothesis proposes that offspring are constrained to remain with parents until they have become proficient in behaviour necessary for successful dispersal and independent breeding (Lack 1968; Brown 1987; Koenig et al. 1992). The skill hypothesis is a possible explanation for the occurrence of delayed dispersal if three conditions are met. First, offspring must show improvement in ‘skills’ (e.g. foraging success, offspring provisioning, predator detection) throughout the period they remain with parents. Second, they must disperse as they approach adult proficiency. Third, those individuals that disperse first from a cohort should have reached adult proficiency earlier than those that delay dispersal. If the timing of dispersal is variable due to natural manipulation, however (e.g. death of parents, ejection from the territory), or experimental removal from parents, those who remain with parents longer must become more skillful than those who are forced to disperse early. If offspring remain philopatric after reaching adult proficiency, or if dispersal occurs whenever a reproductive opportunity occurs, then other direct benefits of philopatry also influence the timing of dispersal. The need to learn skills may provide a minimum period that offspring must associate with parents, but other factors influence the timing of dispersal in this case. If all the necessary conditions for the skill hypothesis are present, Correspondence: T. A. Langen, Department of Biology, University of California, Los Angeles, 405 Hilgard Drive, Los Angeles, CA 90095, U.S.A. (email: [email protected]). 0003–3472/96/030575+14 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour