The Field Sparrow is a common songbird of eastern North America, breeding in brushy pastures and second-growth scrub, but avoiding similar habitat in developed areas, such as suburbs. Its breeding range covers much of the eastern United States and southern Canada. Winter range is slightly south of the breeding range and greatly overlaps it. Wintering habitat appears similar to breeding, but is not well studied. A partial migrant, some individuals remain on or near their breeding grounds in winter while others move farther south. This sparrow is well recognized for its distinctive song of pure tones, heard throughout the summer – an accelerating series of soft, sweet whistles that start with long duration tones and increase in rate to a trill. Both sexes are alike in plumage, rusty brown on the back and crown with white to light gray unstreaked breasts. Its pinkish bill and legs are considered distinctive. Typically males are slightly larger than females.
The nests of this species, composed almost exclusively of grasses, are located near the ground in early spring, typically at or near the base of woody vegetation. Later nests are built in small saplings and shrubs (usually < 1 m high) as ground cover increases in height. Pairs renest rapidly following predation or desertion. Only females incubate eggs, but both sexes share roughly equally in the feeding of young. On average ~40% of nests fledge young, and double brooding is common. While adults are faithful to breeding sites (~50% return in the following year), young rarely return to their natal area. Although by no means threatened, the Field Sparrow appears to be declining in numbers, most likely due to changes in their breeding habitat as shrubby old fields succeed to forest or are cleared for agriculture or suburban growth.
The species was the subject of a long-term breeding study: 1938 through 1948 in Calhoun County, Michigan (Walkinshaw 1978a), with less focused studies covering the period from 1919–1968 (Walkinshaw 1936, Walkinshaw 1939c, Walkinshaw 1945, Walkinshaw 1978a). A short term in-depth study of breeding biology was carried out in Platt County, Illinois from 1971–1972 by Best (Best 1974c, Best 1974a, Best 1974b, Best 1977b, Best 1977d, Best 1977c, Best 1978, Best 1979). More recently, Burhans, Thompson, and colleagues have closely studied the impacts of predators and the Brown-headed Cowbird (Molothrus ater) on the species, primarily focusing their work in central Missouri (Burhans 1997, Burhans 2000a, Burhans 2001b, Burhans and Thompson III 2006, Burhans et al. 2000, Burhans et al. 2001, Burhans et al. 2002, Strausberger and Burhans 2001, Thompson III and Burhans 2003). Structure and function of the song was the subject of a series of studies carried out by Nelson and colleagues (Nelson 1988a, Nelson 1989b, Nelson 1989c, Nelson 1992b, Nelson and Croner 1991). MC has carried out a long-term breeding study covering the years 1987 through 2006 (and still ongoing) in Lackawanna County, Pennsylvania (see Carey [Carey 1990]). This study remains largely unpublished, but all references to MC in this account refer to data from that study.