Brown-headed Cowbird

Brown-headed Cowbird - Spec. Name: Molothrus ater

This is a small brood parasitic * icterid of temperate to subtropical North America. They are permanent residents in the southern parts of their range; northern birds migrate to the southern United States and Mexico in winter, returning to their summer habitat about March/April.

Brown-headed Cowbird (M)

They resemble New World orioles in general shape but have a finch-like head and beak. Adults have a short finch-like bill and dark eyes. The adult male is mainly iridescent black with a brown head. The adult female is grey with a pale throat and fine streaking on the underparts.

They occur in open or semi-open country and often travel in flocks, sometimes mixed with Red-winged Blackbirds (particularly in spring) and Bobolinks (particularly in fall), as well as Common Grackle or European Starlings. These birds forage on the ground, often following grazing animals such as horses and cows to catch insects stirred up by the larger animals. They mainly eat seeds and insects.

Females

Before European settlement, the Brown-headed Cowbird followed bison herds across the prairies. Their parasitic nesting behaviour complemented this nomadic lifestyle. Their numbers expanded with the clearing of forested areas and the introduction of new grazing animals by settlers across North America. Brown-headed Cowbirds are now commonly seen at suburban birdfeeders.

Brrown-headed Cowbird (M)

* Explaining Brood Parasitism

Brood parasites are organisms that use the strategy of brood parasitism, a kind of kleptoparasitism found amongbirds, fish or insects, involving the manipulation and use of host individuals either of the same (intraspecific brood-parasitism) or different species (interspecific brood-parasitism) to raise the young of the brood-parasite. This relieves the parasitic parent from the investment of rearing young or building nests, enabling them to spend more time foraging, producing offspring etc. Additionally, the risk of egg loss to raiders such as raccoons is mitigated, by having distributed the eggs amongst a number of different nests.

As this behaviour is damaging to the host, it will often result in anevolutionary arms race between parasite and host.

In many monogamous bird species, there are extra-pair matings resulting in males outside the pair bond siring offspring and used by males to escape from the parental investment in raising their offspring.

This form of cuckoldry is taken a step further when females lay their eggs in the nests of other individuals. Intraspecific brood parasitism is seen in a number of duck species with females laying their eggs in the nests of others for example in the Goldeneye, Bucephala clangula.Interspecific brood-parasites include the Old World cuckoos in Eurasia and Australia, cowbirds and Black-headed Ducks in the Americas, and indigobirds, whydahs, and the honeyguides in Africa. Seven independent origins of obligate interspecific brood parasitism in birds have been proposed. While there is still some controversy over when and how many origins of interspecific brood parasitism have occurred, recent phylogenetic analyses suggest two origins in Passeriformes (once in New World cowbirds: Icteridae, and once in African Finches: Viduidae); three origins in Old World and New World cuckoos (once in Cuculinae, Phaenicophaeinae, and in Neomorphinae-Crotophaginae); a single origin in Old World honeyguides (Indicatoridae); and in a single species of waterfowl, the black-headed duck (Heteronetta atricapilla).

Eastern Towhee mouth feeding a female cowbird

Most avian brood parasites are specialists which will only parasitize a single host species or a small group of closely related host species, but four out of the five parasitic cowbirds are generalists, which parasitize a wide variety of hosts; the Brown-headed Cowbird has 221 known hosts. They usually only lay one egg per nest, although in some cases, particularly the cowbirds, several females may use the same host nest.The Common Cuckoo presents an interesting case in which the species as a whole parasitizes a wide variety of hosts, but individual females specialize in a single species. Genes regulating egg coloration appear to be passed down exclusively along the maternal line, allowing females to lay mimetic eggs in the nest of the species they specialize in. Females are thought to imprint upon the host species which raised them, and subsequently only parasitize nests of that species. Male Common Cuckoos will fertilize females of all lines, maintaining sufficient gene flow among the different maternal lines.

The mechanisms of host selection by female cuckoos are somewhat unclear, though several hypotheses have been suggested in attempt to explain the choice. These include genetic inheritance of host preference, host imprinting on young birds, returning to place of birth and subsequently choosing a host randomly (“natal philopatry”), choice based on preferred nest site (nest-site hypothesis), and choice based on preferred habitat (habitat-selection hypothesis). Of these hypotheses the nest-site selection and habitat selection have been most supported by experimental analysis.

Common adaptations of avian brood parasitesAmong specialist avian brood parasites, mimetic eggs are a nearly universal adaptation. There is even some evidence that the generalist Brown-headed Cowbird may have evolved an egg coloration mimicking a number of their hosts.

Most avian brood parasites will remove a host egg when they lay one of their own in a nest. Depending upon the species, this can happen either in the same visit to the host nest or in a separate visit before or after the parasitism. This both prevents the host species from realizing their nest has been parasitized and reducescompetition for the parasitic nestling once it hatches.Most avian brood parasites have very short egg incubation periods and rapid nestling growth. This gives the parasitic nestling a head start on growth over its nestmates, allowing it to outcompete them. In cases where the host nestlings are significantly smaller than the parasite nestling, the host nestlings will often starve to death. Some brood parasites will eliminate all their nestmates shortly after hatching, either by ejecting them from the nest or killing them with sharp mandiblehooks which fall off after a few days.

Text is available under the Creative Commons Attribution-ShareAlike License; additional terms may apply. See Terms of Use for details.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profit organization.