Counting Animals Is a Sloppy Business

In 1989, scientists at the United States Fish and Wildlife Service and the National Zoo published a study on migratory songbirds with alarming results. The study relied on 22 years of data from annual surveys of more than 60 neotropical species, birds that breed in North America and overwinter in Central and South America. And the numbers showed that more than 70 percent of these populations, many of which had been stable or growing only a decade earlier, were now plummeting.

Ted Simons was a young wildlife biologist with the National Park Service at the time. To help determine the cause of the apparent declines, his team conducted its own surveys, in the Great Smoky Mountains of Tennessee. Their methods were essentially the