A Bird's Life.(The Talk of the Town)

If you're a small bird, bound for, say, the Caribbean in the autumn, your stay in New York City will begin around dawn. You'll be worn out from flying through the night and desperate for a safe place to rest and to eat, well, like a bird, which is to say, like a pig. A forest by a river would be perfect. Asphalt, steel, and glass are not. If you happen to land in actual woodland by an actual river in the Bronx, you could be snatched, alien-abduction style, by intelligent beings. Beings who then weigh and measure you, poke you with a needle, and maybe scan you with a super-magnet. And blow hard on your chest. (They always do that.) Also, fit you with a tiny ankle bracelet, to track you for the rest of your life.

If this has happened to you in New York City, it is the work of Chad Seewagen and Eric Slayton, ornithologists who want to understand how birds like you cope with cities, which, inconveniently, have been built under your Atlantic migration route. Birds in an urban park, Seewagen said, are in a tough spot: "They're concentrated into a small area, food is limited, there are lots of people and dogs and other animals all over the place.''

As usual during migration season, he had a bird in hand. Five inches long, and weighing about the same as two nickels, the bird stuck its beaked head out above the turret of Seewagen's gently closed fingers and kept completely still except for the thumpa-thumpa-thumpa of its heart, beating five hundred times a minute. Seewagen carefully parted the bird's blue-gray head feathers. "See the red on its crown? That's a Nashville warbler. This bird was probably flying all night long from Connecticut. That's like a person running at top speed from sundown to sunup.''

After such a night, a warbler drops out of the predawn sky in a condition called "hyperphagia,'' from the Greek for "eat'' and "a lot.'' It must rebuild its body fat to stay alive and to fuel the next stage of its journey. To know how the city affects birds, Seewagen and Slayton need to know whether they are able to get enough food to plump up to flying weight.

So, for the past four years, in woodland on the grounds of the Bronx Zoo, Seewagen, who is twenty-eight and taciturn, with close-cropped red hair and broad shoulders, and Slayton, who is forty-two and talkative, shaggy, and slight (and also ...