The route was inferred for decades from radar echoes, coastal fattening and the near-absence of autumn blackpolls from the southeastern United States. Direct proof waited for tracking devices light enough for a bird under fifteen grams. Geolocators recovered from returning individuals recorded an autumn track leaving the northeast coast and continuing over open water for roughly 2,300 to 2,700 kilometres before landfall in the Greater Antilles or northern South America.
The physiology is the enabling part. Before departure, birds roughly double their body mass by depositing fat, and they also reduce digestive organs that are useless in flight. They depart on the tailwinds behind an autumn cold front, which converts a marginal energy budget into a feasible one.
The route is not symmetrical. Spring return is over land, up through Central America and the eastern United States, because the wind fields that make the autumn ocean crossing possible do not run the other way in spring.