Cooperative breeding is a reproductive system in which one or more members of a social group provide care to young that are not their own offspring. The aid-givers may be non-breeding adults, in which case they are usually called ‘helpers’ or ‘auxiliaries’, or they may be co-breeders, and share reproduction with other group members of the same sex. Although the care given usually includes providing food, it may involve other parental-type behaviors as well, including territorial defense, nest or den construction, incubation and defense against predators.

Cooperative breeding is relatively rare: it is currently known to occur in about 220 of the roughly 9000 species of birds, and a smaller number of mammals and fish (for recent compilations, see Emlen 1984; Brown 1987). However, several theories of cooperative breeding suggest it should occur most frequently in tropical and warm temperate habitats, rather than in cold temperate or highly seasonal areas (see e.g. Fry 1972; Brown 1974; Stacey and Ligon 1987), and the reproductive biology of most species in these regions is presently unknown. In some habitats, such as the savannahs and woodlands of Australia, cooperative breeding is very common among some groups such as ground-foraging insectivorous birds (Ford et al. 1988). It also appears frequently in certain taxonomic groups, such as the New World jays (family Corvidae: Hardy 1961, Hardy et al 1981), the Old World babblers (family Timaliidae: Gaston 1978), mongooses (subfamily Herpestinae: Rood 1984), and some primates such as tamarins and marmosets (family Callitrichidae: Terborgh and Goldizen 1985; Sussman and Garber 1987).