The Smith's Longspur has one of the most unusual social breeding systems known among passerines. Unlike the majority of songbird species that form socially monogamous relationships for breeding, the Smith's Longspur is polygynandrous—each female pairs and copulates with two or three males for a single clutch of eggs, at the same time that each male pairs and copulates with two or more females. Males do not defend territories, but instead guard females by following them closely and compete for fertilizations by copulating frequently in order to dilute or displace sperm from other males. Over a period of one week in the early spring, a female longspur will copulate over 350 times on average; this is one of the highest copulation rates of any bird. Males are well-equipped to deliver such large numbers of ejaculates—their testes are about double the mass of those of the monogamous and congeneric Lapland Longspur (C. lapponicus). As expected from their mating behavior, most Smith's Longspur broods contain chicks of mixed paternity. At such nests, two or more males may assist females in feeding nestlings although the amount of investment provided by a given male depends on the number of young he has sired within a nest. The mating system of the Smith's Longspur is much more promiscuous than that found in other longspurs and exactly why it differs is still a mystery. Perhaps the advantages females obtain from extra male help in raising offspring may explain why they pair and mate with more than one male.
Smith's Longspur was first described by William Swainson as the “Painted Bunting” in 1831 from a specimen collected by John Richardson in Saskatchewan. Audubon later renamed this species in honor of his friend Gideon B. Smith of Baltimore in 1844 from new specimens collected in Illinois. Kemsies 1968 summarized the life history of this species in Bent's classic series, but the remoteness of the subarctic breeding range of the Smith's Longspur meant the first systematic descriptions of breeding biology did not appear until Jehl's pioneering work in northern Manitoba (Jehl 1968a, Jehl 1968c). Details of the unusual social mating system of the Smith's Longspur was later worked out by Briskie (Briskie 1992, Briskie 1993a, Briskie 1999), and Briskie et al. 1998 reported the first genetic profiling of paternity and the relationship between paternity and parental care. Systematic studies of the wintering ecology of the Smith's Longspur were first undertaken by Gzybowski (Grzybowski 1982, Grzybowski 1983b, Grzybowski 1983c), with later work by Dunn and Dunn 1999 and Ormston 2000.