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Birds of a different
feather nest together IN north America the brown-headed cowbird has taken over the role of cleaner-bird. Before white men came to America this bird used to service American bisons who used to roam throughout the continent in massive herds. When white settlers came to America and started slaughtering these native bovines and pushed them to almost total extinction, these cleaner birds shifted to imported domestic cattle. At the height of bison’s glory these birds had to stay with their hoofed partners throughout the year, who used to migrate from one place to another in search of greener pastures. Consequently, cowbirds could not stay long enough at one place to nest and raise their own young, hence they adopted the life-style of cuckoo and became brood parasites. They used to lay their eggs in the nests of any birds that happened to be breeding nearby. In the changed situation, however, brown-headed cowbirds adopted different partners but their breeding habit remained unchanged. Now this bird of North America and Mexico is recorded as parasitising as many as 206 bird species, some regularly. |
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Brood parasitic cowbirds choose host species whose eggs have an appearance similar to their own and to avoid detection they throw out one of the host’s eggs for each egg they lay. Relatives of cowbirds, new world blackbirds are very common and familiar birds of Americas. Females have buff-brown plumage speckled with darker brown and black throughout the year. Males have a similar plumage in all times of the year except the breeding season, but when spring comes the back of their necks get bright yellow patchs and rumps and the shoulders turn white with black body. This patterning of colours gives them their local name of skunkbirds. American blackbirds have evolved unique a technique of ‘gaping’ for finding their food which includes small insects. It pushes its closed bill into the hollow stem from the side, and then opens it with a great force which splits the stem. Thus it reaches small insects, grubs and spiders which are hiding inside the hollow, away from the ever searching eyes of predators. Redwinged blackbirds are America’s most numerous landbirds and also the greatest bird pests. The US fish and Wildlife Service conducted a survey in the winter of 1976-77 in the states of Kentucky and Tennessee and found 25 roosts which had over ten lakh (one million) birds in each roost. According to the estimate these 25 roosts, that too in only two states, contained about 4.8 crore (48 million) birds. Another estimate reveals that roosts in reed-beds sometimes contain birds at a density of more than 25 lakhs individuals per hectare. As far as the the role of this bird as bird pest is concerned, it inflicts heavy losses on agriculture. A survey conducted in the year 1970 and 1971 in USA showed nationwide losses to the tune of about 2 crore dollars to ripening corn only. New world orioles, another relative of these birds, are very clever. They build their hanging nests, similar to those of the weaver-birds, near wasp nests which gives them security against predators and mischievous animals like monkeys. |
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There are five
species of cowbirds of which all except one are brood parasites.
Bay-winged cowbird which raises its own young is parasitised by its
own cousin, screaming cowbird. During breeding season bay-winged
cowbird, acting as a professional house-grabber, takes over other bird’s
nest, throws away the eggs of the rightful owner and lays its own.
Seeking an opportune moment, while the bay-winged is away, screaming
cowbird visits its relative and quietly lays its one egg in the nest.
In this way chicks of one relative are raised by another.