In 1978, the total population of the Least Bell's Vireo (Vireo bellii pusillus) was estimated at 90 pairs (Goldwasser et al. 1980). With the expected listing of the subspecies as endangered by the California Fish and Game Commission, and interest in listing it at the federal level, information on the vireo’s breeding biology and population dynamics was needed.
No such data were available, and studies of the nominate subspecies (V. b. bellii) addressed only reproductive rates and nesting substrate, with data gathered from small samples over many years (Overmire 1962), or reported sample sizes too small (Mumford 1952; Nolan 1960; Barlow 1962) to be useful for analysis of population dynamics.
For five breeding seasons (1979–1983), I conducted a banding study of a Least Bell’s Vireo population at Gibraltar Reservoir, Santa Barbara County, California, in order to assess the population’s dynamics.