Social groups of birds and mammals are normally built by delayed dispersal of young. For many species of cooperative breeders, delayed dispersal is associated with delayed reproduction, creating a non-reproductive class embedded in a family structure. When these individuals collaborate in the breeding efforts of others during their tenure as non-reproductive adults, they pose an apparent dilemma for biologists: why should they help to rear others' young instead of dispersing to attempt at least to rear their own (Brown 1983,1987)? This paper reviews a system of cooperative breeding in which few adults produce offspring of their own and in which helping is so effective in improving the reproductive success of breeders that remaining on the natal territory to aid in rearing siblings is an unusually productive alternative method of gene replication compared to dispersing to attempt breeding.

Stripe-backed Wrens (Troglodytidae: Campylorhynchus nuchalis) live in family groups in the savannas of Colombia and Venezuela, where they often co-occur with a congener C. griseus, the Bicolored Wren. A third species, the Fasciated Wren (C. fasciatus) lives in the semiarid scrub and valley woodlands of the coasts of Ecuador and Peru. These three species normally live in groups with (apparently) non-reproductive helpers that remain on their natal territories past physiological maturity. Other species in this genus are known to be cooperative breeders (Selander 1964), including C. turdinus in the Amazon basin (R. H. Wiley, unpublished results) and C. rufinucha in Central America (F. Joyce, personal communication).