We are in the middle of one of the great ornithological spectacles of the year, the fall migration. It’s fun to track the appearance and disappearance of migratory species as they traverse the continent. We can learn much by our collective observations of the pace and route of bird migration at the species level.

But ornithologists are also interested in populations (isolated groups of one species) of birds. As an example, do black-throated green warblers breed in Maine winter in the same tropical areas as the black-throated green warblers nesting in Michigan or in the Great Smokies? And what are the migratory pathways for these populations starting from different longitudes? To answer these questions, individuals have to be identified. A tried and true method is bird banding.

The Bird Banding Lab (henceforth, BBL), a federal agency in the U.S. Geological Survey, coordinates banding activities of native North American birds. To band native birds, one must obtain a Master Bander Permit, possible only after extensive experience in assisting a licensed bander. The BBL provides banders with aluminum bands, each with a unique, nine-digit number. The bander captures birds either in mist-nets or traps and fits the bird with a numbered band using special banding pliers. The banding process can be done quite quickly, minimizing the stress to the captured bird.

The bander sends the BBL records for all birds banded, including age and sex. The banded bird thus provides a record that a particular bird was at a particular place at a particular time. But the real value of banding comes when a banded bird is recovered. Sometimes banded birds are found dead while others are captured by banders.

The re-encountered bird is reported to the BBL workers who close the loop, letting the finder know where the bird was banded and letting the original bander know where the bird was re-encountered.

The power of bird banding relies on the re-encounter of banded birds. As you might imagine, the odds of re-encountering many species of birds are pretty slim. As an example, 745,000 purple finches have been banded in the U.S. and Canada, but only about 20,000 have ever been re-encountered (2.7 percent). As a scientific tool, banding requires that many individuals be banded.

The band numbers for most bands are too small to be read through binoculars (as if a bird would hold still for you to read its digits) so most birds must be recaptured. However, the numbers of bands on large birds like swans can sometimes be read with a spotting scope, obviating recapture of the bird to discover its identity.

Here are a few of the ornithological discoveries that have been made possible by banding birds. Arctic terns are known to migrate from pole to pole, twice a year. We have learned much about where different populations of birds winter. For instance, palm warblers do an interesting crisscross in migration. Populations breeding in the upper Midwest and Canada’s prairie provinces migrate southeast to winter in Florida while our eastern palm warblers migrate southwestward along the Gulf Coast.

Re-encountered birds provide us with information on the longevity of birds. Recently, a ring-billed gull that had been banded 53 years earlier was found alive, blowing away the previous longevity record.

Some banding records can cause your jaw to drop. For instance, a semipalmated sandpiper banded in Nova Scotia was recaptured four days later at the mouth of the Amazon River. It had flown 2,800 miles, non-stop, in just 96 hours.

John James Audubon was the first bird bander in North America. He tied some aluminum wire to some nestling eastern phoebes in Pennsylvania and found that the birds returned the following year to nest.

Herb Wilson teaches ornithology and other biology courses at Colby College. He welcomes reader comments and questions at

whwilson@colby.edu