I'm not a bird guy at all, but I thought that there were some birds that migrated transatlantic - I did a quick check on google and there do seem to be some banded birds that have made the trip.
I'm not a migratory bird expert, so I don't know if this makes any sense at all, but... Perhaps the ancestors of these birds that perform a transatlantic migration lived during a time in which there was no ocean between N. America and Europe. (These may even have been flying reptilian ancestors). Perhaps the destination of that migration was somehow retained even after the American and European plates separated (not sure how this might have happened, maybe something to do with the magnetic field, height of the sun, etc.) If this happened, then the endpoints of the migration would be programmed instinctively into these birds. Again, I don't know if this makes any sense or not, so I am donning my asbestos suit and await my flaming.