This familiar sparrow is one of the most diverse and widespread songbirds in North America, with 24 diagnosable subspecies (52 named) breeding from Newfoundland to the Aleutian islands of Alaska and south to central Mexico. Individuals vary 150% in body mass over this range-the largest breed in beach grass in the Aleutians, the smallest in California salt marshes. Throughout the breeding range, females build and incubate open-cup nests in herbs, grasses, and shrubs near fresh or salt water, at forest margins, in chaparral, marshes, dunes, and hedgerows, wherever suitable cover and insect food are present. Males are recognized everywhere by their distinct melodic song, despite marked individual and geographic variation in vocal repertoire.
Breeders reach peak densities in riparian areas, on islands, and bordering tidal marshes on the Pacific Coast, where individuals often defend territories year-round. In these areas, extreme site fidelity may facilitate genetic divergence and variation in morphological traits subject to natural selection. Elsewhere, birds are partially or completely migratory depending on snow cover and winter temperature, owing to the species' terrestrial habit and reliance on seeds in winter. Genetic studies of migratory populations suggest long-distance dispersal occurs, and breeding-season surveys show that migrants often colonize ephemeral habitats, such as forest openings.
This species is sexually monomorphic in plumage, but males are larger than females on average. Social monogamy is evident in nearly all breeding groups, but site fidelity exceeds mate fidelity, and DNA studies show that 15% or more of young are sired outside the social pair. Thus, simultaneous and serial polygamy are common. Resident populations are regulated by competition for food and space and limited by nest depredation. Population sinks develop where nest depredation exceeds approximately 50%, but populations persist via immigration. Residents are vulnerable to severe weather and epizootics and, when either occurs, accipiter hawks can cause substantial direct mortality. Little is known about the population dynamics of migrants by comparison.
The landmark work of Margaret Morse Nice on Song Sparrows (Nice Nice 1937 , Nice 1943b ) set an early standard for the deft integration of behavioral and ecological studies of individually identified animals in the field, and established the Song Sparrow as a model species. Subsequent work by Smith ( Smith 1981d , Smith 1988b ), Arcese ( Arcese 1989c , Arcese 1989b , Arcese 1989a ), Marler and Peters ( Marler and Peters 1977 , Marler and Peters 1987 ), Beecher et al. ( Beecher et al. 1994 , Beecher et al. 2000a , Beecher et al. 2000b ), Keller ( Keller 1998a ), Nowicki et al. ( Nowicki et al. 1998 , Nowicki et al. 1999 ), and many others has followed Nice to demonstrate the utility of Song Sparrows in research. Future work on the causes of behavioral, morphological, and demographic variation among populations and individuals should prove rewarding.