Wow. That Blackpoll migration is one of those things that make you go hmm. How the heck did they ever figure out that there was an airstream out there to take them back to Brazil? Pretty dang cool.
Trial and error apparently, involving the death of innumerable individual birds. In songbirds timing, distance and direction is all genetic, implicit in these genes is some degree of random mutation.
Every year a few migrating songbirds inherit the wrong directions, almost always ending in death. Years ago when I was in college a little painted redstart from Northern Mexico turned up at some lady’s feeder in Upstate NY, throngs of birders went to see it, until the lady’s cat ate it.
Sorta like that, in that book I recommended Weidensaul relates the case of a migratory Amazonian rail, that ordinarily migrated around the rain forest, found dead under a bird feeder on Long Island in December.
I had a friend who was a fisheries observer in the Pacific, every fall a few migratory North American songbirds, heading west instead of south, would land on the boats to rest for awhile, before heading out in a doomed attempt to cross the Pacific.
Once in a great while it works, and if this new direction results in more offspring than the old, over subsequent generations this new destination replaces the old.
