They are a migratory species, with part of the population migrating southeastwards to the Greater Antilles (where it overwinters in the Blue Mountains of Jamaica for example[5]) and the other southwestwards to the Yucatán Peninsula region in winter.

Nests are fairly large and bulky, constructed from moss, grass, and small leaves situated above ground in a tangle of tall reeds or vines.

This does fit a pattern one would expect from genetic drift, but there seem to be no geographical or ecological barriers restricting gene flow.

Even during the last ice age, when average temperatures, precipitation and sea levels were lower, there seems to have been ample contiguous habitat.

Offspring of pairs comprising birds of different subpopulations will, in such species, attempt to migrate into an intermediate direction.

More research such as analyzing bird banding data is needed to determine whether this mechanism applies in Swainson's warbler.