Tiny songbird's mysterious vanishing act solved

Scientists have figured out how a tiny songbird called the blackpoll warbler mysteriously vanishes from most of Canada each fall.

It turns out that when the bird disappears from human sight, it embarks on one of the most amazing migrations in the world.

The blackpoll warbler weighs just 12 grams – less than two loonies. But the new tracking study shows it is capable of flying up to 2,770 kilometres over the western Atlantic Ocean over three days, without stopping to eat or rest.

That's the longest overwater flight for a bird of this size in the world, said Ryan Norris, a biologist at Ontario's University of Guelph.  

"We were totally amazed when we found out," said Norris, co-author of a study describing the feat.

The study was published this week in the journal Biology Letters.

The study published in this week's Biology Letters journal, and co-authored by a Canadian scientist, was made possible when manufacturers released a new geolocator - a tracking device that birds can wear like a backpack - that weighs just half a gram. (Vermont Centres for EcoStudies)

The blackpoll warbler breeds in the boreal forest that stretches across Canada from Newfoundland and Labrador to the Yukon, and spends its winter in northeastern South America. On its way north each spring, it passes through Canada's more populated south, singing its distinctive high-pitched song along the way, Norris said.

But each fall, it disappears without a trace from all except the easternmost part of the boreal forest.

"That got people wondering: 'Where do they go in the fall?'"

Reports of blackpoll warblers landing on ships in the Atlantic Ocean during bad weather led some scientists to suspect that perhaps they migrated over water instead of land in the fall. But researchers had no direct evidence.

Finally, a few years ago, a new technology became available to biologists like Norris – tiny tracking devices that could be attached to the back of a blackpoll warbler like a backpack.

The trackers had a mass of just half a gram, making them light enough for a 12-gram bird to carry.

They measure light levels, providing data about latitude from the timing of sunrise and sunset, and about longitude from the timing of solar noon, which varies with longitude.

Unfortunately, the devices need to be physically recovered in order to get the data.

"You can't sit at your desk and monitor the flight of the blackpoll warbler," Norris said. "You have to leave your office the next year and go find the blackpoll warbler, and that can be kind of a needle in a haystack."

Norris later learned that other biologists at the University of Massachusetts had come up with the same idea. The researchers decided to work together – the Canadians tracked birds from Nova Scotia, and the Americans tracked birds from Vermont.

In total, they put geolocators on 37 birds, and managed to recover five the next year.

Those five birds flew an average of 2,540 kilometres during a non-stop flight over the western Atlantic Ocean that averaged 62 hours. The birds landed in the Caribbean, where they spent a few days refuelling before moving on to Venezuela.

Norris said that when he saw the results, "I was jumping up and down… I consider this a 50-year unsolved mystery."

Norris suspects that blackpoll warblers from as far west as the Yukon gather in the Martimes and New England to make the non-stop flight over the Atlantic Ocean. However, it will take more research to find that out.

Scientists aren't yet sure why the birds do this instead of taking a three-week journey over land, stopping regularly to refuel, as they do in the spring.

Norris says it may be because the non-stop flight makes them less prone to being eaten by predators along the way. The express journey also gets them to their wintering grounds earlier, and that may help them get prime real estate there, with better access to food, for example.

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