A relatively common breeder in disturbed areas of North America's boreal forest, the Mourning Warbler is an early-successional species that nests in forest clearings created by humans or by natural disturbances. Vegetation in these clearings is dense, dominated by shrubs and deciduous and conifer saplings, with little or no higher canopy. This warbler feeds primarily on insects by gleaning foliage, on or near the ground, where it also nests. It also prefers disturbed areas with thick undergrowth on overwintering areas in Central America and northern South America.
Very little is known about the annual cycle of the Mourning Warbler because this secretive species goes largely unnoticed during migration and overwintering periods. However, researchers are addressing some information needs. Using stable-isotope analysis, Hobson et al. (1) found that birds from the southeastern portion of the breeding range overwinter in Venezuela. There is new information on the causes of mortality during migration, such as hazardous weather and collisions with buildings (2, 3). In addition, researchers have a better understanding of the relationship between physiological requirements, habitat selection, and habitat quality during fall migratory stopover and overwintering periods (4, 5, 6).
For many years the Mourning Warbler was considered as 1 of 4 species in genus Oporornis, but a comprehensive molecular phylogeny (7) merged the Mourning, Kentucky (formerly Oporornis formosus), and MacGillivray's (formerly O. tolmiei) warblers into the genus Geothlypis. The Mourning Warbler is the only Geothlypis whose breeding range comes into contact with all other congeners. The Mourning Warbler is closest in plumage and behavior to its western counterpart, the MacGillivray's Warbler. The species status of these 2 warblers has intrigued ornithologists for years. Some authorities considered them subspecies based on morphological similarities and suspected cases of hybridization. Others have considered them distinct species because of their erratic contact on the breeding grounds, distinct song types, different patterns of geographic variation in song, and differences in skeletal dimensions. Recently, Irwin et al. (8) described a hybrid zone in the southern Peace Region of British Columbia near Dawson Creek, where analyses of mitochondrial DNA, chromosome markers and plumage are consistent with extensive hybridization in the zone of overlap. Further, Kenyon et al. (9) documented changes in song within the hybrid zone. Songs of both species were beginning to converge, possibly due to males learning and incorporating characteristics of each species’ song into their adult songs in the areas of hybrid contact. This study system deserves additional research attention, especially analyses of song variation within the contact zone and reactions of females and territorial males to playback of allopatric versus sympatric song types.
Research on song variation has shown that there is a macrogeographic pattern of 4 regiolects across the breeding range (10). A Newfoundland regiolect is found primarily on the island and southeastern coast of Labrador. The smallest regiolect occurs within the province of Nova Scotia. A large Eastern regiolect extends from western Ontario across Quebec and south along the Appalachian Mountains to West Virginia. It is also found along the Great Lakes from northern Minnesota to Michigan. A Western regiolect ranges from northeastern British Columbia east through western Ontario and northern Michigan. There is evidence of behavioral discrimination of these songs by males within the regiolects based on song-playback experiments (11).
Much of what we know about the breeding biology and life history of this species is based on an early study by Cox (12) in Minnesota. The Mourning Warbler continues to be a subject of many studies of ecological forestry and the impact of forest management on bird communities in northern forests. These studies have shown this species to be resilient, capable of adapting to a variety of silvicultural practices. It rapidly colonizes new clearings, usually 1–2 years since harvesting, but numbers decline by 7–10 years. Some ornithologists have expressed concern over this and other fugitive species that 'chase' early-successional forest, appearing and disappearing with ecological succession in boreal forests. Exploitation in the form of logging, mining, oil exploration, agriculture, and prescribed burning continue to create new breeding habitats across much of the breeding range. The Mourning Warbler is one of few Nearctic–Neotropical migrants that has benefited from human disturbance in forested landscapes.