The darker the color, the more favorable the climate conditions are for survival. The outlined areas represent approximate current range for each season. More on reading these maps.
There are two different populations of this species. Birds in the West, often called “Calaveras Warbler,” are currently separated from their eastern cousins by hundreds of miles of Canadian prairie. But if Audubon's climate model is correct, that may change. The species as a whole is projected to spread northward (losing about half of its current range) and the two populations to converge, at which point future ornithologists should have a better idea of whether they are different species, as some now suspect.