Helpers at the nest in some Argentine blackbirds

Systems of communal or cooperative breeding in which several adults help to feed one brood (Lack, 1968, p. 72) have been considered to be relatively rare among birds, but recent evidence suggests strongly that the habit may be very widespread especially in tropical and subtropical regions (Fry, 1972; Harrison, 1969). An excellent review is provided by Brown (1974) who also discusses several theories for the evolution of this and related forms of breeding. He points out that known communal breeders are nearly all permanent residents in relatively stable habitats where their populations are likely to be at or close to the carrying capacity of the environment most of the time. He postulates that a period of K-selection, followed by a period of kin selection are the normal prerequisites for the evolution of communal breeding.