Move over monarch butterfly, humpback whale and Serengeti wildebeest. The new most epic of migratory animals is the humble blackpoll warbler (Setophaga striata). The blackpoll is a very small songbird, weighing just a scant 12 grams. That's only slightly more than the mass of a new pencil (10 grams) and slightly less than the weight of an empty soda can (15 grams). The point is it's a tiny, tiny bird. You could probably knock it over just by blowing on it. This bird impresses nobody.
Each summer, the blackpolls show up in the boreal forests of Canada and Alaska to breed. Their summertime migration route, from south to north, is well understood: they fly up the coast of North America from South America, using spots along the US eastern seaboard for rest and relaxation as they make their way to their breeding grounds near Nova Scotia in Canada.
Then, as winter approaches, they pack up, fly off and show up a few days later in South America and the Caribbean. The problem is nobody really knows how they get there. Some suspected that they flew directly over the Atlantic Ocean, which would require a non-stop flight lasting two to three days. That would be quite a long, intense voyage for such a tiny, unremarkable bird. That's why a group of researchers led by William V. DeLuca from the University of Massachusetts outfitted 37 blackpoll warblers with the tiniest of backpacks, each containing an even tinier geolocator, before the birds prepared to fly south for the winter.
The lightest GPS transmitters currently available weigh some three to four grams, which is too heavy for the 12-gram blackpoll. The commonly accepted rule among biologists is that trackers or collars can weigh up to just 5% of the animal's body weight. Then, in 2013, a new device became available that weighed just half a gram, meaning it could be used with the blackpoll warblers. It determines the animal's position – its latitude and longitude – by combining information from a light sensor and an internal clock. In the shadowy forests where the birds breed, it's not a very accurate method since the device calculates latitude based on the amount of time between sunrise and sunset. But over the open ocean, the method is far more accurate, even if it's a bit cloudy.
When the birds returned to North America the following summer, the researchers were able to find and recover data from five of the birds they initially tagged. They published their findings recently in the journal Biology Letters.
And in that data, finally, was proof that the blackpolls fly south directly over the water, without stopping, for up to three days. The blackpoll warbler is suddenly quite the impressive bird. "Our data provide irrefutable evidence that blackpolls fitted with geolocators [made] non-stop flights over the Atlantic Ocean and overwintered either in northern Columbia or Venezuela," write the researchers. That meant a non-stop flight of up to 2,770 kilometres (1,721 miles) over the course of 49 to 73 hours.
“The blackpoll warbler's migration might be one of the most extraordinary migratory feats on the planet.”
Sure, 2,770 kilometres isn't the longest migration pathway on our planet. But when you stop to consider just how small these birds are, the flight becomes so much more remarkable. For each gram they weigh, the blackpolls flap their wings over some 233 kilometres (144 miles) of ocean. The northern wheatear, a bird that flies a non-stop route of 3,400 kilometres (2,112 miles) from the Canadian Arctic to the UK each year, is a larger bird. So it manages to travel 136 kilometres per gram, which makes the blackpolls' journey nearly twice as impressive.
An average human (who weighs 80.7 kilograms, or 80,700 grams, or 177 pounds) would have to trek more than 18 million kilometres (more than 11 million miles) without stopping to maintain an equivalent migration! That's one eighth the distance from the Earth to the Sun. DeLuca argues that this makes the blackpoll warbler's migration "one of the most extraordinary migratory feats on the planet."
The birds can achieve such an impressive feat because they bulk up before the flight, packing on extra fat to give them the energy they need. "They eat as much as possible, in some cases doubling their body mass in fat so they can fly without needing food or water," said Norris. "It's a fly-or-die journey that requires so much energy."
But the researchers are not stopping here – they're now turning their attention to another of the blackpolls' migration mysteries. The birds breed in North America's boreal forests, from Alaska and the Yukon in the west to Nova Scotia in the east. It's thought that all the blackpolls eventually wing their way to Nova Scotia and the surrounding area before heading south, but nobody has ever proven that.
This summer, Norris and his team will be outfitting more birds with geolocators in Whitehorse, Yukon. When the birds return the following summer, they'll come bearing new data. If his suspicions are correct – that they fly all the way to Nova Scotia before turning south to fly across the Atlantic – then the entire migration route could be, gram for gram, the longest for any animal.
Top header image: Stephen Thompson, Flickr