Blackpolls breed across northern Canada and central Alaska and winter in South America. Alaskan birds wintering in Brazil may migrate over 8,000 km. During the fall migration, some birds fly from the northeastern United States to the Antilles, a route that carries them over some 3,000 km of open water, a journey that probably takes up to 88 hours.
Blackpoll breeding behavior is virtually unknown. At least some Blackpolls are polygnous. Early in the breeding season, if some males are slow to arrive from the south, females will mate with already mated males; furthermore, males with rich territories attract multiple mates. Blackpolls are also poorly studied in South America. Erika and I observed Blackpoll Warblers in the winter when we birded in a coffee plantation in eastern Peru. In South America, Blackpolls travel with other migrants in mixed species flocks. Much of the information in this post is from Hunt and Eliason (1999).