FILE under: strange consequences of having sex. The sedentary Audubon's warbler is starting to migrate – possibly because it is mating with a cousin that flies south in the winter.
Audubon's warbler (Setophaga coronata auduboni) lives in western US and Canada. Some head south for the winter while others don't. David Toews of the University of British Columbia in Vancouver, Canada, and his team decided to investigate why.
When the team looked at the bird's mitochondria – the energy-producing structures within cells – it found that those in northern birds were unusually energy efficient. This is normally only seen in the myrtle warbler (Setophaga coronata coronata), a closely related migratory bird whose range overlaps with the Audubon's in the north. They also found that northern Audubon's warblers were more likely to migrate.