Avian Population Collapse

A column by Carl Zimmer in the New York Times bears the alarming news that scientists studying the populations of over 500 species of birds in North America reported last week that the number of individual birds has fallen by 29% since 1970. There are almost 3 billion fewer birds in North America today than there were fifty years ago.

If this is correct it's deeply disturbing. I don't doubt that habitat loss, both in the birds' breeding grounds and in their wintering grounds in Central and South America have taken a toll. It's interesting that among the species hardest hit were grassland species in the midwest (717 million fewer birds) where vast tracts of grassland acreage have been sacrificed to development and agricultural production.

I do have a concern about the methodology of the study, however. It relies heavily on estimates of numbers by amateur observers and among the species showing severe declines are blackbirds (440 million fewer since 1970) which are so numerous and which in the non-breeding season flock together in such huge numbers, that accurate estimates of their numbers are very difficult to assess.

More distressing, than the drop in numbers of abundant species like blackbirds, though, is the decline in woodland species like warblers which breed in the boreal north. The warbler population has shrunk, according to the study, by some 617 million birds.

Cape May Warbler

Oddly, however, vireos, which are similar to warblers and which share similar habitat, have shown a jump of 53% in their population. Why that should be is apparently a mystery.

Whatever the explanation of this mystery, it seems obvious that habitat loss, and perhaps diseases like West Nile virus and even feral house cats are taking a toll. Every new shopping center and housing development eliminates acres of habitat, and every new highway is a killing field for birds and other wildlife.

But if these stressors really are what's causing the collapse it's very hard to imagine a solution.