Oct 15 2014
If you’re a Swainson’s thrush of mixed parentage you’ll probably pick a bad migration route. It’s in your genes.
In eastern North America we see only one subspecies of Swainson’s thrush, the olive-backed (above), but in British Columbia there are two. The russet-backed subspecies breeds along the Pacific coast and follows the coast to spend the winter in Mexico and Costa Rica. The olive-backed subspecies breeds in the interior and migrates across the continent and the Gulf of Mexico to winter in South America.
Where their breeding ranges meet the thrushes pair up without regard to these distinctions. Their hybrid offspring inherit a mixture from their parents, including mixed coloration.
Kira Delmore at the University of British Columbia wondered if the hybridization extended to their migration routes so she tagged hybrid Swainson’s thrushes with light-level geolocators to track their routes.
The data proved that their migration routes are inherited and that those of mixed parentage inherit a blend. While each parent would have followed the Pacific coast or a safe route across the continent, the hybrids chose novel and dangerous compromises between the two paths.
“Instead of taking well-trodden paths through fertile areas, these birds choose to scale mountains and cross deserts,” said Delmore.
The dangerous routes probably cause more hybrids to die on migration than their pure counterparts, thus keeping the subspecies distinct. Says Delmore, “The self-destructive behavior of hybrids could be helping to maintain the great diversity of songbirds we enjoy.”
Read more about this study here at Science Daily.
(photo by Matt Reinbold from Wikimedia Commons. Click on the image to see the original)