Adoption and infanticide in birds: a review

Infanticide and adoption are two behaviors seen in a variety of animals that are initially hard to explain by natural selection. These behaviors are two courses of action an adult can take when faced with someone else’s hungry young. The idea, though, that an individual would take the time and energy to feed and defend these offspring when they could instead ignore or eat them seems maladaptive. However, studies of birds show that adoptions and infanticides do occur and shed light on how these behaviors have been selected and when each action is advantageous. In European starlings, a replacement male’s decision to adopt an entire brood or get rid of it corresponds to their expected paternity. Meanwhile, in tree swallows, males will generally adopt an entire brood as long as they father a threshold number of young. Male yellow-headed blackbird territory owners demonstrate their fatherly skills to females by adopting unrelated broods. In species that live in areas with few nesting sites, such house sparrows, females compete for nesting sites much like males compete for territories in other species. Due to this competition, when females replace other females at a nest, the replacement females kill the brood. Finally, infanticide also occurs in communal breeders, such as the guira cuckoo. Individuals that did not reproduce in a given reproductive cycle force the entire group to renest when they kill the young hatchlings, thereby giving themselves a chance to reproduce. By studying the variations in adoption and infanticide behaviors in birds, the adaptive value of each can be determined.