Why do the young of cooperative breeders--species in which more than two individuals help raise offspring at a single nest--delay dispersal and live in groups? Answering this deceptively simple question involves examining the costs and benefits of three alternative strategies: (1) dispersal and attempting to breed, (2) dispersal and floating, and (3) delayed dispersal and helping. If, all other things being equal, the fitness of individuals that delay dispersal is greater than the fitness of individuals that disperse and breed on their own, intrinsic benefits are paramount to the current maintenance of delayed dispersal. Intrinsic benefits are directly due to living with others and may include enhanced foraging efficiency and reduced susceptibility to predation. However, if individuals that disperse and attempt to breed in high-quality habitat achieve the highest fitness, extrinsic constraints on the ability of offspring to obtain such high-quality breeding opportunities force offspring to either delay dispersal or float. The relevant constraint to independent reproduction has frequently been termed habitat saturation. This concept, of itself, fails to explain the evolution of delayed dispersal. Instead, we propose the delayed-dispersal threshold model as a guide for organizing and evaluating the ecological factors potentially responsible for this phenomenon. We identify five parameters critical to the probability of delayed dispersal: relative population density, the fitness differential between early dispersal/breeding and delayed dispersal, the observed or hypothetical fitness of floaters, the distribution of territory quality, and spatiotemporal environmental variability. A key conclusion from the model is that no one factor by itself causes delayed dispersal and cooperative breeding. However, a difference in the dispersal patterns between two closely related species or populations (or between individuals in the same population in different years) may be attributable to one or a small set of factors. Much remains to be done to pinpoint the relative importance of different ecological factors in promoting delayed dispersal. This is underscored by our current inability to explain satisfactorily several patterns including the relative significance of floating, geographic biases in the incidence of cooperative breeding, sexual asymmetries in delayed dispersal, the relationship between delayed dispersal leading to helping behavior and cooperative polygamy, and the rarity of the co-occurrence of helpers and floaters within the same population. Advances in this field remain to be made along several fronts.(ABSTRACT TRUNCATED AT 400 WORDS)

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We examined the fitness consequences of helping behavior in the western bluebird (Sialia mexicand) at Hastings Reservation in Carmel Valley, California, USA, and tested hypodieses for how helpers benefit from engaging in alloparental behavior. Both juvenile and adult western bluebirds occasionally help at the nest. During a 12 year period, all adult helpers and most juvenile helpers were male. Helpers usually fed at nests of both their parents and rarely helped when only one parent was present. The frequency of pairs with adult helpers was only 7%, but nearly one-third of adult males helped among those with both parents on the study area. At least 28% were breeders whose nests failed. The propensity to help appears to depend upon parental survival, male philopatry, and die breeding success of potential helpers. Feeding rates were not increased at nests with juvenile helpers, apparendy because breeding males reduced their feeding rates. In contrast, adult helpers increased die overall rates of food delivery to die nest in spite of a reduction in die number of feeding trips made by both male and female parents. Helpers did not derive any obvious direct fitness benefits from helping, but diey had greater indirect fitness than nonhelpers due to increases in nesding growdi rates and fledging success at dieir parents' nests. Helpers fledged fewer offspring in their first nests dian did nonhelpers, suggesting diat they were birds widi reduced reproductive potential. Aldiough we have not yet measured the effect of extrapair fertilizations on die fitness benefits of helping, we calculated die difference in fitness between helpers and nonhelpers as a function of die potential helper's paternity when breeding independendy and his fadier's paternity in die nest at which he might help. In conjunction with constraints on breeding and indirect fitness benefits, we predict that relatedness of males to die young in dieir own as well as dieir parents' nests will influence helping behavior in western bluebirds.