The American Tree Sparrow is a familiar migrant and winter visitor in many settled areas of North America, where winter flocks are seen in in weedy fields, marshes, open groves of trees, and often attend backyard feeders. “American Tree Sparrow” is somewhat of a misnomer as the species breeds in remote northern areas of the continent, often north of the treeline. The name was given by early European settlers for the superficial resemblance of this species to the Eurasian Tree Sparrow (Passer montanus). The species had long been included in the genus Spizella, but recent genetic work shows the taxon to be sister to the Fox Sparrow (Passerella iliaca) and less closely related to other Spizella ( Barker et al. 2013 , Klicka et al. 2014 ), and thus, best treated in its own genus Spizelloides ( Slager and Klicka 2014 ).
Because the American Tree Sparrow breeds in remote areas, its natural history has received rather limited attention. Most of what is known about this sparrow's breeding biology is based on general observations of 26 nests and detailed observations of 9 nests made by A. Marguerite Baumgartner near Churchill, Manitoba (e.g., Baumgartner 1937b , Baumgartner 1938b , Baumgartner 1968b ). During the breeding season, males sing persistently to proclaim their territory. Individual males sing only a single song type, although song types are widely shared among males. Females build a nest on or near the ground and incubate the eggs alone. Both parents help to raise a single brood of typically 3 to 5 young. During winter, the American Tree Sparrow occupies the more populated regions of the continent where the species has served as a model organism in studies investigating the mechanisms of physiological control and the timing of gonadal development and regression, and of molt and migration (e.g., Wilson 1985a , Wilson and Reinert 1996 ).
Although the American Tree Sparrow is an abundant species that breeds in relatively undisturbed areas and seems to adapt well to human settlements in the overwintering range, the population was estimated to have declined by 53% between 1970 and 2014 ( Rosenberg et al. 2016 ). The reasons for the decline are yet to be determined, but broad-scale changes in land-use (e.g., agricultural intensification and forest maturation) have likely resulted in extensive loss of weedy old-fields and other open habitats used by overwintering birds. This highlights the need to understand more about the biology of the American Tree Sparrow, especially the factors that regulate populations.