Fat, Flight and Tenacity in a Fall Warbler

Two wings and a prayer will carry a Blackpoll Warbler on a remarkable journey to South America this autumn. Well, two wings, a prayer, and the energy packed into a scoop of Ben & Jerry’s ice cream.

Since June, this wisp of a bird, with a high, thin pulsing song, has been raising young in boreal woods from Alaska, across Canada, and into high forests of the Northeast. With their offspring on the wing and independent, many songbirds now begin migration to the tropics. But the Blackpoll’s route is where few would dare flutter: a long-distance flight over the Atlantic Ocean.

Songbirds generally avoid ocean crossings. During migration, many species choose to stop and go, to touch down among trees where they can rest and refuel. But the Blackpoll is a mariner this season. From its boreal breeding grounds, it moves southeast toward the coasts of maritime Canada, New England, and shoreline points south. Then something dramatic happens.

When crisp autumn winds blow from behind, the Blackpoll – in what is less a leap of faith than a fine example of evolution – launches out to sea for three days and more than 2,000 miles of non-stop flapping toward the northern shores of South America.

News Alert (September 23): Your blogger-in-chief, Bryan Pfeiffer, is on Monhegan Island, Maine. Reports on the fall migration begin Monday morning.

At least that’s the prevailing hypothesis. Although we can’t track these warblers as they fly, we have ample evidence to support this iconoclastic route: lots of Blackpolls at coastal sites in the fall, Blackpolls spotted from ships at sea, and relatively few Blackpolls in southeastern states and Central America (where we would expect to find them on a more terrestrial southbound route).

The sea and its trade winds welcome the Arctic Tern and American Golden Plover, celebrated transoceanic migrants. But a warbler? How can a woodland bird make a similar journey?

It does so with the same currency as the tern and the plover: fat. The Blackpoll gains weight – a lot of weight. After breeding in July, the average Blackpoll Warbler weighs in at about 10 to 12 grams (no more than half an ounce) which means you could mail two Blackpolls anywhere in the US for the price of a first-class stamp. But before the fall flight, Blackpolls nearly double their mass as they binge on insects, spiders, seeds, and fruit. They store the feast as fat and then burn it efficiently at sea.

Generally, the greater a bird’s fat reserves (as a percentage of its body weight) the farther it can fly without pausing to refuel. After all, risks await birds during those rest stops, including uncertain habitat and predation by hawks, house cats, and other animals.

Yet, the benefits of fat reserves and non-stop flight carry their own risks. A bird focusing on food before migration may be less alert and an easier mark for predators. A fatter bird may lack the acceleration and agility necessary to avoid hawks and falcons in flight. And for a warbler over the ocean, there is no rest stop, no port in a storm.

On balance, the Blackpoll Warbler, more fat than feather, takes a course evolution has charted at sea. Making the flight from, say, Maine to Brazil, a Blackpoll burns less fat than we find in a single serving of Ben & Jerry’s Coffee Heath Bar Crunch.

If only the rest of us had such good use for ice cream.