Increasingly, though, both casual bird-watchers and ornithologists note a steady decline in numbers—not just of endangered species, but also of common birds not usually considered to be at risk. Study after study, survey after survey show a worrisome downward trend in populations.

A National Audubon Society report called "Common Birds in Decline," for instance, shows that some widespread species generally thought to be secure have decreased in number as much as 80 percent since 1967, and the 19 others in the report have lost half their populations. The figures reflect an array of threats faced by birds throughout North America. (Read about the decline of European songbirds in National Geographic magazine.)

Migrants return from Central America to find that the