restricting gene flow is at work, but it is not presently known what it is. It is possible that the subpopulations conform to the different wintering areas. In some migrant birds it is known that the initial direction of the migration is set by fairly simple hereditary mechanisms. Offspring of pairs comprising birds of different subpopulations will, in such species, attempt to migrate into an intermediate direction. Such a course would lead a Swainson's warbler deep into the Caribbean where there are no wintering or even stopover points, and the bird would almost certainly perish. More research such as analyzing bird banding data is needed to determine whether this mechanism applies in Swainson's warbler. References Winker, K., Graves, G. R. & Braun, M. J. (2000) Genetic differentiation among populations of a migratory songbird: Limnothlypis swainsonii. J. Avian Biol. 31'''(3): 319–328. PDF fulltext External links Photos - VIREO Sounds - Florida Museum of Natural History Swainson's warbler Native birds of the Southeastern United States Swainson's warbler Swainson's warbler