Formerly considered 2 species, the Myrtle Warbler in the East and Audubon's Warbler in the West (see Systematics, below), the Yellow-rumped Warbler is one of the most common warblers in North America. With a broad breeding range, stretching from Alaska south to Guatemala and east to the northeastern United States, it is often abundant in winter in the southern United States, Mexico, and the western Caribbean.
Among warblers, this species is one of the most ecologically generalized. Although it is confined largely to coniferous breeding habi-tat, individuals forage in a broad range of microhabitats and employ a variety of foraging techniques, from fly-catching to foliage-gleaning for insects. During the nonbreeding season, this warbler is found in almost any habitat and expands its diet to include a substantial amount of fruit. Its ability to digest the waxes in bayberries (Myrica spp.) make it unique among warblers, and allows populations to winter in coastal areas as far north as Nova Scotia.
Myrtle and Audubon's warblers hybridize in the southern Canadian Rockies. On the basis of this evidence, as well as genetic similarities, the 2 were combined into a single species. These 2 former species are very similar in most aspects of their biology, including breeding, foraging, habitat, and song.
Despite the wide range and abundance of the Yellow-rumped Warbler, many aspects of its biology remain largely unstudied¿particularly its breeding biology, owing in part to the remoteness of much of its breeding range and to its low breeding densities. Perhaps because the species is so numerous during migration and winter, recent advances have centered on this part of the annual cycle, and often make use of captive birds. These investigations include studies of migration behavior and physiology (Yarborough and Johnston 1965, Hussell and Caldwell 1972, Terrill and Ohmart 1984, Moore and Simm 1985, Moore and Phillips 1988, PDH), digestive physiology (Place and Stiles 1992, Afik and Karasov 1995), and metabolism (Yarborough 1971, Hussell and Caldwell 1972). Work in the hybrid zone in Canada has greatly expanded our understanding of the systematics of the complex (Hubbard 1969, 1970, Barrowclough 1980). A great deal of work has also been done on the species' ecological niche in both breeding and wintering warbler assemblages, particularly in the areas of foraging ecology and behavior (Morse 1968, 1976a, 1989a, Katusic-Malmborg and Willson 1988).
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