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Even Close Subspecies of Migrating Birds Can’t Agree on the Best Route

Scientists in British Columbia attached tiny ‘backpacks’ to birds and mapped their winter migration from Canada to Central America and back again

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Captured Swainson’s thrush wearing a geolocator. Photo credit: Kira Delmore, UBC

The continental divide doesn’t just apply to water anymore. Scientists in British Columbia attached tiny “backpacks” to birds and mapped their winter migration from Canada to Central America and back again.

What they found was surprising. They tagged two different subspecies of Swainson’s thrush, both of which live in British Columbia, with small geolocators. Though the subspecies are closely related and don’t live that far apart, they took two very different paths down towards their summer homes. One took the coast road, skirting the Pacific, while the other went towards the other side of the Rocky Mountains, right through middle America.

Map of diverging migration routes in fall (top) and spring. Cool colours represent coastal subspecies, warm colours inland species. Dashed lines represent dates around the equinox where researchers were unable to estimate latitude. Image: University of British Columbia

The stark contrast between the two has scientists wondering what would happen if the two subspecies mated to create hybrids. In the press release released by the researchers, one scientist says:

“Given that migratory behavior is under genetic influence in many species of birds, these results raise the question of what hybrids between these two subspecies would do,” says Darren Irwin, associate professor of Zoology at UBC and co-author of the paper. “One possibility is that hybrids would take an intermediate route, leading to more difficulties during migration. If so, the migratory differences might be preventing the two forms from blending into one.”

The geolocators carried by the birds are about the size and weight of a penny, and track the bird’s location by measuring the sunrise and sunset times each day. With that data, scientists could figure out the exact position of the bird, though figuring out their precise latitude became difficult around the equinox (when the length of night and day are approximately equal).

 

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