Counting Animals Is a Sloppy Business – Facts So Romantic

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 same: Groups of trained counters walked along backcountry trails, stopping at prescribed points to count all the birds they saw or heard. However, they included some additional data points, such as the approximate distance between the observer and each bird counted. This extra information allowed them to more accurately calculate the probability of detecting individual birds—a factor that could influence the final estimate. When they analyzed their data, they found reason to question the magnitude of the…

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