Habitat Preferences and Competition of Wintering Juncos and Golden‐Crowned Sparrows

Trapping and banding of wintering juncos and golden—crowned sparrows was carried out from 1964 to 1968 along the opposite edges of an old field. The juncos ranged freely along the field borders but the golden—crowns were largely restricted to dense willow thickets in one part of the field. An attempt to assess possible competition between the two species by removing juncos in 1966—67 was unsuccessful because of infiltration of new birds from outside the study area. Removal of golden—crowns in 1967—68 reduced their population markedly and resulted in significant increase in use of the willow thickets by juncos. Although juncos are primarily seed eaters in winter, and seeds are a steadily dwindling resource at that season, competition for food between the two species was alleviated because the sparrows subsisted in large part on sprouting annuals, a resource renewed continuously after sprouting started following the fall rains. Further, the green food eaten by the sparrows undoubtedly satisfied their water requirements to a great extent and reduced competition for water sources. The competition demonstrated by removal experiments resulted not from direct competition for the same set of resources but from the proximity of a water trough, an important junco resource, to the heavy cover preferred by the sparrows. Although outnumbered about 5:2, the larger sparrows were able to exclude the juncos from this cover to an appreciable extent.