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Tiny bird's journey 'extraordinary'

Published 01/04/2015

A blackpoll warbler is being fitted with a miniaturised light-sensing geolocator (Vermont Centre for Ecostudies/PA)
A blackpoll warbler is being fitted with a miniaturised light-sensing geolocator (Vermont Centre for Ecostudies/PA)

A tiny songbird that makes its home in forests routinely undertakes non-stop flights of up to 1,700 miles across open ocean, scientists have confirmed.

The heroic flight of the blackpoll warbler, which weighs just half an ounce (12 grams), is "on the brink of impossibility" according to one of the researchers, who attached miniature tracking devices to the birds.

Experts had long wondered about tantalising clues that suggested the bird was capable of epic migrations between North and South America.

Now they have proof that the warbler really does fly without stopping to rest, eat or drink across the wide expanse of the Atlantic Ocean.

Unlike other migratory songbirds travelling south for the winter, the blackpoll warbler ignores the safe option of hugging the coastline through Mexico and Central America.

Instead it risks certain death by choosing to fly hundreds of miles from the nearest land, despite being adapted for life in trees. Unlike gulls and other seabirds, the warbler cannot land on water and live.

Of all the birds that complete the journey from New England and Canada to the Caribbean, Venezuela and Colombia, only about half survive to make the return trip the following spring.

US scientists were astonished when they analysed geolocator data from five of 40 birds carrying the tracker devices, which weighed just half a gram.

Dr Bill DeLuca, from the University of Massachusetts at Amherst, said: "It was pretty thrilling to get the return birds back, because their migratory feat in itself is on the brink of impossibility.

"When we accessed the locators, we saw the blackpolls' journey was indeed directly over the Atlantic. The distances travelled ranged from 2,270 to 2,770 kilometres (1,410 to 1,721 miles)."

He added: "For small songbirds, we are only just now beginning to understand the migratory routes that connect temperate breeding grounds to tropical wintering areas. We're really excited to report that this is one of the longest non-stop over-water flights ever recorded for a songbird, and finally confirms what has long been believed to be one of the most extraordinary migratory feats on the planet."

For the study, published in the Royal Society journal Biology Letters, the scientists captured 20 birds in Vermont and another 20 in Nova Scotia and fitted them with geolocator packs.

The devices, each about the size of a 5p piece, were attached using leg harnesses, much like a rucksack.

Too small to house GPS receivers, they operated by means of "solar geolocation" - a method used for centuries by mariners and explorers. It is based on the fact that day length varies with latitude and the time of solar noon - when the sun is at its highest point in the sky - varies with longitude.

Recordings of the date and length of daylight allowed the scientists to calculate where on Earth the devices were at regular intervals.

Of the original 20 birds, only three from the Vermont group and two from the Nova Scotia group were recaptured after making return flights.

Canadian team leader Dr Ryan Norris, from the University of Guelph, stressed how important it was for the birds to build up their fat stores before starting the migration.

He said: "They eat as much as possible, in some cases doubling their body mass in fat so they can fly without needing food or water.

"They don't have the option of failing or coming up a bit short. It's a fly-or-die journey that requires so much energy."

Why the birds choose to make such a perilous journey rather than follow a longer but safer coastal route remains a puzzle. The scientists believe that from the warbler's point of view, it might make sense to get the flight over with as quickly as possible.

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