This long-distance neotropical migrant breeds at low to moderate elevations along the Rocky and Sierra mountains from the Northwest Territories and southeastern Alaska south to northeastern Mexico. Breeding populations throughout much of the United States can best be described as remote and disjunct, located in mountain ranges isolated by large regions of arid, sagebrush steppe, prairie grasslands, salt deserts, agricultural floodplains and livestock grazing. Three breeding populations located outside the main range of this species are in the Cypress Hills along the Saskatchewan – Alberta border, the Black Hills of South Dakota and in northeastern Mexico.
MacGillivray's Warbler is often referred to as a shy skulker, making it difficult to detect on migration and on the breeding grounds. This elusive behavior and remote distribution makes it a highly sought after trophy species by birdwatchers. However, these birds are relatively common in appropriate breeding habitat. They are most often associated with disturbed second growth and riparian vegetation, composed of dense thickets, shrubs, willows, and saplings of various tree species. Their wintering range is primarily along the Pacific slope of Central America from southern Mexico to northern Panama, with winter habitat also composed of vegetation characteristic of early successional stages including dense brush, scrub, field edges and riparian corridors.
Some interesting insights into the evolution of MacGillivray's Warblers were revealed by two recent studies of their population genetics. Research on variation in mtDNA by Mila et al. (2000) showed that an isolated breeding population from ne. Mexico was genetically distinct from Rocky Mountain populations in the United States. They estimated that the Rocky Mountain and Mexican populations had diverged during early Pleistocene glaciation, approximately one million years ago. In the northern part of the breeding range, Irwin et al. (2009) described the first known hybrid zone between the MacGillivray's Warbler and its eastern counterpart, the Mourning Warbler (Geothlypis philadelphia), in the Peace Region of British Columbia. Song types of each species that differed in the allopatric parts of their respective breeding ranges (Pitocchelli 1988, 1990) failed to prevent hybridization in British Columbia (Kenyon et al. 2011).
Migratory behavior of MacGillivray's Warblers has received the attention of ornithologists. Kelly and Hutto (2005) noted that MacGillivray's Warblers migrate entirely over land and there is no overlap with the migration route of the Mourning Warbler. These closely related species are isolated from each other throughout their entire annual cycles, except for the recent contact in British Columbia. Spring migration in MacGillivray's Warblers is now known to exhibit a leapfrog pattern based on hydrogen isotope analyses. Populations from southernmost parts of the wintering range leapfrog over intermediate populations to reproduce at the northern extremes of the breeding range (Paxton and van Riper 2006).
Because of its dependence on riparian habitat during migration and breeding, the MacGillivray's Warbler has been an important subject species of many studies of conservation and management of this declining ecosystem in the western United States.