Brood Parasitism in Birds: Strangers in the Nest

B irds are well known for their parental care, patiently incubating their eggs and then bringing food to their young until they are old enough to look after themselves. Certain birds, known as "brood parasites," however, lay their eggs in the nests of other birds and have no social family life with their own offspring. Obligate brood parasites, such as some cowbirds and cuckoos, give no care to their own young, depending entirely on other species to hatch and care for their young. By contrast, facultative brood parasites, including a number of other cuckoos, colonial swallows, and weavers, occasionally lay their eggs in the nests of their own or other species but usually rear their own young. In both cases, female parasites may remove the eggs of the host, while the young parasites may kill the host young or compete with them for care. Care that the hosts provide to the young parasites is care denied to their own young, with the result that being parasitized often has a detrimental effect on the reproductive success of the nesting hosts and may affect their population numbers as well. Brood parasitism is of interest to biologists for a number of reasons.