The Tribune - Spectrum



Sunday, June 11, 2000
Nature


Cowbirds of North America
By Nutan Shukla

BROWN-headed cowbird of North America has taken over the role of the cleaner-bird. Before white men came to America this bird used to service American bisons who used to roam throughout the continent. When white settlers came to America, and started slaughtering these native bovines and drove them to almost near extinction, these cleaner birds shifted to the imported domestic cattle. In the bisonís glory-days, these birds used to stay with their hoofed partners round the year, who used to migrate from one place to another in search of greener pastures these birds could not nest and raise their own young, hence they adopted the lifestyle of the cuckoo and became brood parasites. They used to lay their eggs in the nest of any bird that happened to be breeding nearby. In the changed situation, however, brown-headed cowbirds adopted different partners but their breeding habit remained unchanged.

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Now this bird of North America and Mexico is recorded as parasiting as many as 206 bird species, some regularly. Like cuckoos, all the species of cowbirds except one are brood parasites. Only bay-winged cowbird raises its own young, but at the same time it is parasitized by its cousin screaming cowbird. During breeding season bay-winged cowbird acting as a professional house-grabber takes over other birdís nest, throws away eggs of the rightful owner and instead lays its own. When the eggs are laid screaming cowbird visits its relative and quietly lays its own egg in the nest of unsuspecting bay-winged. In this way chicks of one relative are raised by another.

Other cowbirds choose host species whose eggs have an appearance similar to their own and to avoid detection they throw out one of the hostís eggs for each egg they lay.

There are five species of cowbirds which are found in N.C. and S. America and the West Indies. Inhabiting open country and grassland they are medium-sized, black birds having glossy, bronze or blue iridescent plumage. Having stout, conical bills, they feed on insects and seeds.

Orioles of the New World, member of the cowbird family, are very clever birds. They build their hanging nests, similar to those of the weaver-birds, near wasp nests which gives them special security, monkeys and other animals do not dare to raid them.

Most of the bird species have learnt to avoid poisonous insects and butterflies, but some have developed total or partial immunity to toxins present in insect bodies. For example most bird species have learnt to avoid the North American monarch butterfly, which derives poison from the leaves of the milkweed plant. The caterpillars who feed on this plant store the poison in their bodies and it is retained when they pupate and become butterflies. The black-headed oriole has learnt to eat only those parts of the butterfly which contain small amounts of poison. If it ingests too much it is sick.

Orioles are divided in two groups namely Old World orioles and New World orioles. There are 24 species of New World orioles, found in N.C. and S. America and the West Indies. Of these many species are migratory. Mainly arboreal, they are yellow and white or black and orange birds which have narrow, conical bills. They build globular nests which have side openings or open cup-shaped nests which are suspended between branches. These birds feed on berries, insects and fruits.

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