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Hybrid songbirds' sense of direction as mixed as their genes

A new study of hybrid birds shows that they don't always follow the best migratory paths. The study looked at hybrid birds' migratory routes and compared them to those of their parents.

The results suggest that birds with mixed genetic backgrounds follow more difficult, intermediate migratory routes than their parents.

Zoologists at the University of British Columbia conducted the study. UBC researcher Kira Delmore put geolocators on a flock of hybrid Swainson's thrushes and monitored their migration to Central and South America from the U.S. The geolocation devices recorded the times of sunrise and sunset everyday. A year later, Delmore downloaded the information from the devices and used the times to extrapolate the latitude and longitude of the birds' locations. Delmore and colleagues then compared the migration routes to those of the thrushes' parent populations.

They found that the hybrid thrushes followed an intermediary route between the paths taken by their parent populations. Some of these intermediary routes were more challenging than the parents' routes.

"Instead of taking well-trodden paths through fertile areas, these birds choose to scale mountains and cross deserts," Delmore explained in a statement.

This is the first study examining in detail the migratory routes of free-flying hybrid birds and their parents. The link between hybrid genes and mixed migratory routes suggests that migratory behavior is strongly controlled by genetics, according to Darren Irwin, a professor at the UBC Department of Zoology.

"These thrushes will allow us to actually look for the genes responsible for migratory behavior," says Irwin, who is also the senior author of the study, which was published in Ecology Letters.

According to Delmore, the idea that hybridization leads to differentially mixed migration routes is a good thing. Hybridization in many populations can cause the two parent populations to collapse, over time, into one. If the hybrids are following more difficult migratory routes, they may exhibit a lower survival rate. While the idea seems morbid, it is actually a positive thing overall for the preservation of songbird diversity. The two separate parent populations remain thus, thanks to the "self-destructive behavior" of the offspring population with the mixed genes.

Swainson's thrushes are olive-brown songbirds with lighter eye-rings and are not endangered. The attached geolocation devices weighed only 4 percent of birds' individual body weights, so they did not disrupt the birds' natural migration behaviors.

To hear the Swainson's thrush's song, listen here.

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