Every autumn migrating birds in the Northern Hemisphere fly south to escape the cold. If we humans were to make such a journey, we would need a map. But each bird has its route stored at least partially in its genes. Rather than relying solely on external cues, it has an innate flight plan.
Most individuals within a single population follow the same migration path, taking advantage of favorable winds and optimal topography. But some birds are hybrids; their parents come from different populations and so have different paths. How do the birds choose?
Early experiments suggested that hybrids take an intermediate route relative to the ones their parents follow. The tests used laboratory-raised birds whose preferences were assessed using a cage designed to record the direction the birds wanted to fly. “These studies were fantastic, but what we really needed to do was follow [wild] birds over an entire year,” says Kira Delmore, a graduate student at the University of British Columbia.
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Delmore and her colleagues outfitted 97 wild Swainson’s thrushes with tiny GPS trackers. Some of the subjects belonged to a coastal subspecies that flies along the west coast of North America to winter in Mexico, Guatemala and Honduras. Others belonged to an inland subspecies that flies through east-central North America to Colombia and Venezuela. The hybrids of the group were born in a small area where the two populations overlap in the coastal mountains of western Canada.
The team recovered useful data from 21 of the birds and found that some hybrids flew intermediate routes compared with their parents, confirming the earlier lab findings. Others took mixed routes, following one parent’s path in spring and then switching to the other’s in fall. Still others stuck with the route of one parent. Intriguingly, some of those hybrids that took intermediate routes also settled in intermediate destinations. “This is the first paper to show that both [the] route and destination of hybrids can be inter- mediate,” says Bridget J. Stutchbury, a bird researcher at York University in Toronto. The study was published in October in Ecology Letters.
Delmore suspects that hybrids may have a harder time surviving because they fly inefficient routes over arid or mountainous terrain: in this case, the American Southwest. Researchers will have to run another tracking study to determine whether that is the case. If Delmore’s hunch turns out to be true, then migration pathways may be a driving factor of bird speciation. For the Swainson’s thrushes, should the hybrids have trouble surviving their trips, then the coastal and inland groups might eventually evolve into separate species.