MacGillivray's Warbler breeds commonly in riparian habitat and clearcuts of northern coniferous forests along the Rocky Mountains. Like its close relative the Mourning Warbler (Oporornis philadelphia), it is generally shy and elusive and often difficult to detect, although it is less retiring during migration and on its wintering grounds. Male MacGillivray's Warblers on breeding territories usually become conspicuous, singing loudly from tree branches or shrubs. Cryptic females are often encountered by accident or when scolding intruders near the nest and fledglings.
Although MacGillivray's Warbler is common throughout much of its breeding range, its biology remains poorly understood. This species deserves special attention because of its ecological role in boreal forest ecosystems which are being developed by logging and mining interests in the northwestern United States and British Columbia. Industrial forest development should benefit MacGillivray's Warbler because it inhabits second growth and disturbed habitat; in fact, there is evidence that its breeding range has expanded significantly since the early exploration and development of western North America in the 1800s.
This warbler is the western member of the genus Oporornis. Unlike its eastern relatives¿the Mourning Warbler, Connecticut Warbler (O. agilis), and Kentucky Warbler (O. formosus)¿it is restricted primarily to the Rocky Mountains and their eastern foothills. It also breeds in small numbers on the prairies in the Black Hills of western South Dakota and in the Cypress Hills of Saskatchewan. It closely resembles the Mourning Warbler in morphology, behavior, and habitat preference. Because of their strong similarities, these two species were once considered eastern and western races of the same species. Subsequent studies on vocal and morphological differences, however, have indicated they are separate and distinct.
An interesting but little-known controversy surrounds the common and scientific names of this species. John Kirk Townsend first named it Tolmie's Warbler, Sylvia tolmiei, after Dr. W. T. Tolmie, Esq., an ornithologist, surgeon, and entrepreneur with the Hudson Bay Company and Townsend's close friend, who spent most of his life in the Pacific Northwest. Later, however, in his Birds of North America, John J. Audubon renamed the species MacGillivray's Warbler, Sylvia macgillivrayi, honoring his close personal and professional friendship with Dr. W. MacGillivray, a Scottish ornithologist and professor of natural history who helped Audubon edit his work. Audubon's disregard for Townsend's prior names and MacGillivray's lack of field experience in North America have caused resentment among some veteran western birdwatchers, who prefer reinstating the original common name, Tolmie's Warbler. Some common Spanish names still reflect the conflict over MacGillivray's versus Tolmie: Reinita de MacGillivray, Verderón de Tolmie, and Verdin de Tolmie.
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