It sounds like the setup to a bad joke told by zoologists: What do you get when you cross a bird that always flies to the west with one that always flies east? But the punch line is weirder than you’d guess. Birds’ migratory routes are partly coded into their DNA. A baby that inherits genes for two different routes doesn’t commit to either path. Instead it bounces between them and may take a wild zigzag straight through the middle—even if that route is perilous.
Laboratory tests in the past