Since the first mass bird deaths were reported on New Year's Day, a new round of proposed explanations has accompanied each additional bird kill – some plausible, others far-fetched, still others downright loony. The reality is that there are many bird deaths every day. Some birds die from natural causes or predation, and others are killed because of dangers introduced by humans. This article puts the deaths into some perspective:
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.It is unfortunate that conservationists and responsible reporters have had to counter misinformation in the aftermath of several large bird kills. However, it is good to see more information on human-induced bird deaths making it into mainstream publications. Most of the numbers above were familiar to me, as I expect they will be to many readers. But that is a result of my reading bird blogs and conservation-oriented magazines over the past decade. It would be helpful for bird conservation if more people became aware of how many bird deaths are caused by common hazards.
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 America: loss of habitat to development.
All of this explains why about a quarter of the 836 species of birds protected under the Migratory Bird Treaty Act are in serious decline. For a third of the other birds there is not enough information to be sure about the health of their populations.