Birding: Blackpoll warblers perform an amazing flying feat

A small black and white bird is passing through local back yards, parks and woodlands on its way to breeding grounds in Canada. If you notice him at all you may see him poking around tree leaves or twigs, searching intently for insects. There's nothing to alert you to the fact that this is an Olympian among migratory birds, traveling one of the longest and most dangerous routes in our hemisphere.

Blackpoll warblers, weighing about half an ounce, are migration champions, a fact long suspected by those who study birds, and now they have proof.

Tiny tracking devices attached to their backs showed that blackpolls take off in autumn and fly — without a rest stop — to their winter grounds, which can be as far away as South America, a journey of 1,500 or more miles. Even more amazing is the fact that they depart from staging areas in the northeastern United States and fly far out over the Atlantic Ocean to catch prevailing winds. This gives them a big southward boost but also presents a death-defying challenge, since their route is over open water. They must fly for up to three days and nights without anywhere to land.

Researchers recently published a description of their blackpoll work, "Transoceanic migration by a 12 g. songbird," a prosaic title for a study that's creating quite a buzz.

"The results may not be entirely surprising, since people had been guessing for some time that this is what blackpolls did, but it is amazing to have real numbers," says Bill DeLuca, lead researcher based at the University of Massachusetts, Amherst. His American team trapped about 20 blackpolls and attached geolocators to them.

Ryan Norris, who led a team in Nova Scotia, noted that this means that blackpolls have the longest over-water flight for a bird of its body size. If driven down by exhaustion or severe weather, the birds will perish in the ocean, all but the few who happen upon a passing ship or remote island.

The birds we see in the springtime accomplished this amazing over-water journey last fall. In spring, because the winds have shifted, the returning warblers fly over land to reach their breeding grounds.

The tracking devices record sunlight levels at regular intervals, which indicate a bird's position in the world. Since they're not transmitters, researchers must retrieve the devices by recapturing the birds in order to have access to a wealth of information about where and how far a bird has traveled.

Recapturing the same tiny birds half a year later sounds like an impossibility, but DeLuca says this wasn't hugely difficult in the case of the highly territorial blackpoll. The teams had placed distinctive leg bands on "their" birds, to tell them apart from others in the spring.

"The birds are very site-faithful," says DeLuca, "and often return to the same exact breeding territory each spring." The two teams regrouped in the areas where they'd banded the blackpolls the previous year. When they spied the distinctive leg bands they unfurled their nets to trap the birds and remove the geolocators.

Since they won't be stopping to refuel, they bulk up, "basically becoming little fat balls with wings," as DeLuca put it.

Why do they do it? No one really knows. Researchers speculate that the over-water route may be a remnant of an ancient migratory path. Or it could be safer than a route over land, filled with dangers like glass buildings, cats and cars. Or perhaps compressing the migratory period is safer, in the long run.

St. Paul resident Val Cunningham, who volunteers with the St. Paul Audubon Society and writes about nature for local, regional and national newspapers and magazines, can be reached at valwrites@comcast.net