Stacey and Koenig discuss the phenomenon of cooperative breeding among birds, an unusual kind of social behavior common to only a few hundred species worldwide, in which individuals other than the male-female pairs help to raise the young of a single nest or den. Because certain individuals aid in raising offspring which are not their own, cooperative breeding gives rise to some of the clearest examples of altruism among animals. This unique breeding behavior is of interest to evolutionary biologists and behavioral ecologists since such species exhibit some of the most unusual and bizarre...