When John James Audubon found a Fox Sparrow for the first time on its breeding grounds in southern Labrador in 1834, he had no idea that he was looking at one of North America's most geographically variable birds. With 18 subspecies divided into 3 or 4 distinct groups, this species shows extensive variation that has been the focus of 3 intensive monographs on external morphology, skeletal characteristics, and genetics ( Swarth 1920b , Linsdale 1928b , Zink 1986 ). Its life-history characteristics also vary greatly across its range.
The Fox Sparrow is a common but shy inhabitant of streamside thickets and chaparral across the northern boreal and western montane regions of North America. Habitat preference varies geographically, with each of the major groups preferring nesting locations with different plant communities. Fox Sparrow populations also vary in migratory distance and route; individuals nesting in the Sierra Nevada of California migrate only short distances, mostly altitudinally, while those from Alaska migrate long distances, with some traveling over open ocean. Populations also vary in their song types; while northern and eastern Fox Sparrow populations sing 1 or 2 song types each, western populations sing 3 or 4; commonly heard calls (contact notes) also vary geographically, differences largely correlating with the major groups. This sparrow's large geographic distribution, relative abundance, and extreme geographic variation in plumage, morphology, habitat preference, migratory behavior, and vocalizations make it ideal for comparative evolutionary studies. While several modern studies have focused on Fox Sparrow systematics, song, and migration, many aspects of the life history of this species remain unknown. Its shy habits and preference for dense vegetation have made the collection of life-history data difficult.