This is bird note. Every year hundreds of bird species migrate between North and South America.

But what came first? Northbound migration or southbound migration?

Biologists believe this bright orange Baltimore Orioles descended from a South American ancestor that began migrating north in search of new breeding habitats.

This supports the southern home hypothesis that most birds originated in a southern territory and then evolved to migrate north. But there's a northern home hypothesis too. The family tree of warblers in the Western

hemisphere shows that most of these songbirds likely originated in North America, then evolved to extend

their winter range by migrating south.

Many birds have likely gained and lost the ability to migrate multiple times in either direction over the course of

their evolutionary history.

And it's still happening today. Barnswallows normally breed across North America before spending their winters in the

tropics. But recently a small group of these acrobatic birds began staying in Argentina year round.

Researchers suspect that given enough time,

they could eventually become a genetically distinct subspecies of their own.

For Bird Note, I'm Michael Stein.