For decades it was hypothesized that blackpoll warblers, tiny black-and-white songbirds that breed in boreal forests and winter in South America, migrate south entirely offshore in a nonstop flight. Only recently have technological advancements allowed that hypothesis to be tested and confirmed.
“Blackpolls had been singled out as a migratory champion because this feat is so unbelievable,” said Bill DeLuca, an ecologist at the University of Massachusetts at Amherst. “There were a number of papers that provided indirect evidence that it was true, but it had been debated for years.”
Recent advances in geolocator technology enabled DeLuca and colleagues from the Vermont Center for Ecostudies and elsewhere to capture 40 blackpolls in Vermont and Nova Scotia in 2013 and attach dime-sized devices to the birds’ backs to identify the routes they take during migration. When the birds returned north the next year, five of them were recaptured and were found to have flown offshore for about 1,500 miles to Puerto Rico, Cuba, and the Greater Antilles before continuing on to South America.
“Some of them flew really far offshore, so they’re making a full-on commitment,” DeLuca said. “Once they’re out there, they’re either going to make it or they’re not. They can’t just jump back to the coast if they’re in trouble.”
Because a water landing would be fatal to the birds, the logical question is why do they take the risk of flying over water rather than traveling via a land route like most songbirds? No one really knows, but DeLuca has a guess.
“Migration, for most birds, is the riskiest time of the year, when most of the annual mortality occurs,” he said. “So to compensate, they may just fly over the ocean to get the journey over as quickly as possible. Migrating over land has its own problems: hawks, housecats, development at their stopover sites. Both routes are risky.”
Besides, DeLuca said, blackpolls are physically able to complete the two- or three-day flight. The birds double their weight in fat before departing and are highly efficient fliers. Once they are navigating their way south, they reduce the size of some of the organs they are not using, like their digestive systems. And there is evidence that other physiological adaptations occur to help the birds maximize their oxygen intake; this enables them to fly at higher altitudes than other songbirds.
Now that the migratory pathway of blackpolls from the Northeast is known, DeLuca wants to use the same technology to document the route taken by blackpolls that breed in Alaska and western Canada. Based on records of blackpoll sightings during the migration season, it is believed that those birds fly across the continent and join their eastern counterparts in migrating south over the Atlantic. DeLuca intends to find out if that is true or if they use a different migratory strategy.