Ancient murrelets, Synthliboramphus antiquus, are hefty little seabirds that are mostly black and white but with a gray back that looks like a draped shawl. (That’s where the “ancient” part of their name comes from.) And according to a new study published in Ibis, they’re the only marine bird that’s known to make east-west migrations across the entire width of the North Pacific – though researchers aren’t sure why. As New Scientist puts it, they travel 8,000 kilometers (5,000 miles) across the ocean and then again in reverse for no clear reason.

In 2013, Environment Canada’s Anthony Gaston and colleagues deployed geolocators on ancient murrelets breeding in Haida Gwaii, British Columbia. Four of these gizmos were retrieved the following year. Based on their longitude positions, all four birds quickly moved westwards after breeding in July, and three of them reached the waters between Japan and China by November. The return trip was even more rapid: It began in February, and by March, they had touched down in Haida Gwaii. 

But why did they traverse the vast Pacific only to migrate between areas with similar climate? The waters around Japan are as temperate as North American waters, which support many other marine birds. “I don’t know of any other bird that covers such a long distance from east to west,” Gaston tells New Scientist, “especially when it winds up in waters that are very similar to those it just left.”

The birds likely flew at least a couple hundred kilometers every day over several hours. Theirs is also the longest migration recorded in any bird from the family Alcidae, which also includes puffins and auklets. 

“The whole thing is a bit of a mystery,” Gaston adds. “It seems like an awful long way to go for no obvious profit.” Based on genetic analyses, the ancient murrelet originated in Asia and only recently colonized North America, New Scientist reports. And it’s possible that they’re just retracing the route their ancestors took to colonize a new continent. 

Top image credit: Eric Ellingson/Flickr