Bird banders can be scientists or amateur naturalists, but all are federally licensed.
They catch birds in specially designed nets and, once the birds are in their hands, much can be learned.
Banders measure tail and wing lengths to help determine the birds age, and they record its sex, weight and physical condition.
Before a bird is released unharmed, it is fitted with a small, ID bracelet on its leg.
This bird band makes it possible to identify that specific bird in the future.
When banded birds are recaptured, researchers gain information about migration routes, population trends and longevity.

http://www.pwrc.usgs.gov/BBL/