In 1960-63 populations of the song sparrow (Melospiza melodia (Wilson)) were studied on some of the islands along the Pacific Coast of southern British Columbia. The general problem was to find the factors that might be responsible for the regulation of numbers of any small passerine species. A more specific problem was to explain the extremely high population density of the song sparrows on Mandarte Island compared with densities elsewhere. Most individuals on Mandarte Island were colour marked, and changes in population density and behaviour were observed throughout the breeding season and at intervals during the rest of the year. Environmental conditions on Mandarte Island were compared with those on other islands in the area. All islands were alike in having similar weather and few predators; they differed in the kind of vegetation. The high density of the Mandarte Island population seems to have been a result of the simplicity of the habitat, which favours song sparrows and not their potential competitors, and of the adaptability of song sparrows in utilizing common feeding grounds in undefended areas outside their usual habitat. The critical period in the regulation of numbers was the autumnal territorialism, when increased territorial activities resulted in heavy losses and emigration of the young to areas with lower densities. / Science, Faculty of / Zoology, Department of / Graduate