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 U.S. 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 United States every year and that as many as 20 billion may be in the country during the fall migratory season.

Even without humans, tens of millions of birds would be lost each year to natural predators and natural accidents  millions of fledglings die during their first attempts at flight. But according to the Fish and Wildlife Service, people have severely complicated the task of survival. Although mortality rates are difficult to calculate for certain, using modeling and other methods like extrapolation from local research findings, the government has come up with estimates of how many birds die from various causes in the United States.