People kill birds, not theories, claim biologists.

At the beginning of this month when about 5,000 red-winged blackbirds fell from the sky in one night in Arkansas, biologists were called on to put a damper on public speculation about pesticides and secret military tests by reminding everyone how many birds there are and how many die. They often do so as a result of human activity, but in far more mundane and dispiriting ways than conspiracy buffs might imagine.

"Five billion birds die in the US every year," said Melanie Driscoll, a biologist and director of bird conservation for the Gulf of Mexico and Mississippi Flyway for the National Audubon Society. That means that on average, 13.7 million birds die in this country every day. This number, while large, needs to be put into context. The federal Fish and Wildlife Service estimates that a minimum of 10 billion birds breed in the US every year and that as many as 20 billion may be in the country during fall migratory season.

Even without humans, tens of millions of birds would be lost each year to natural predators and natural accidents. But according to the Fish and Wildlife Service, people have severely complicated the task of survival.

Some of the biggest death traps are surprising. Almost everyone has an experience with a pet proudly bringing home a songbird in its jaws. Nationally, domestic and feral cats kill hundreds of millions of birds each year, according to the government. One study done in Wisconsin found that domestic rural cats alone (thus excluding a large number of suburban and urban cats) killed roughly 39 million birds a year.

Pesticides kill 72 million birds directly, but an unknown and probably larger number ingest the poisons and die later unseen. Orphaned chicks also go uncounted. And then there is flying into objects, which is most likely what killed the birds in Arkansas. The government estimates that strikes against building windows alone account for anywhere from 97 million to nearly 976 million bird deaths a year. Cars kill another 60 million or so. High-tension transmission and power distribution lines are also deadly obstacles. Extrapolating from European studies, the Fish and Wildlife Service estimates 174 million birds die each year by flying into these wires. None of these numbers take into account the largest killer of birds in US: loss of habitat.

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