12-gram songbird flies for days non-stop over Atlantic

Think you're good over long distances? You're not. At least not compared to a tiny bird. Come the first signs of winter, the blackpoll warbler (Setophaga striata) takes flight from its home in the forests near of the north-eastern coast of the US and Canada. Three days later, the birds arrive in the Caribbean after a non-stop southwards flight over the western part of the Atlantic Ocean.

Depending on the starting point, the total journey can be as long as 2770 kilometres. It's no mean feat if you weigh only 12 grams. But the journey is made all the more remarkable by taking in part of the Atlantic. For such a small species, flying over an ocean is a huge risk.

The bird's route to the Caribbean and what happens next has been the subject of much speculation, based on local sightings.

So William DeLuca from the University of Massachusetts in Amherst and his colleagues decided to clear things up. Attaching geolocators weighing about half a gram to the backs of five blackpolls, the team was able to track the flights. They found that the tiny birds made non-stop flights over the Atlantic to the Caribbean for a short stop-over, before reaching northern Columbia and Venezuela.

Absorbing organs

It is probably one of the longest non-stop flights ever recorded for a songbird. For the return leg, the team found that the birds take a different route, sticking to land with pit stops for refreshment along the way.

"It all depends on the fuel load when they start their flight," says Anders Hedenström of Lund University in Sweden. The percentage of the bird's body mass that can be used as fuel determines how far it can fly. A tail wind from Nova Scotia in Canada probably further assists their flight, he says.

Although these birds usually weigh about 12 grams, DeLuca and his team found that the average weight of a blackpoll in Nova Scotia, before they started their migratory journey, was about 16 grams, showing that the birds took on extra weight to fuel their flight.

"It does fatten up, by almost doubling body weight and absorbing many of its digestive organs. It turns into a lean, mean flying machine, just wings, fuel and a small orientation computer," says DeLuca.

Journal reference: Biology Letters, DOI: 10.1098/rsbl.2014.1045





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