The difference in seasonal range is because of the difference in seasonal temperature range. (drtimball.com)
They have seasonal ranges for many categories, but so does SPG. (pointchaser.com)
The Song Sleuth app was just released for iOS, with an Android version in the works for this fall, and it not only helps people become better birders by helping them identify birds by their songs, but it also includes access to The David Sibley Bird Reference, which offers additional details about the birds, including the birds» seasonal range maps, song samples, and illustrations of their appearance. (treehugger.com)