Editor's Note: Phylogenetic analyses of sequences of mitochondrial and nuclear DNA indicate that several species formerly placed in Oporornis (tolmiei, philadelphia, and formosus) are more closely related to Geothlypis species than to Oporornis sensu stricto. See the 52nd Supplement to the AOU Checklist of North American Birds for details. Future revisions of this account will reflect these changes.
A relatively common breeder in disturbed areas of North America's boreal forest, the Mourning Warbler is an early successional species that nests in clearings caused by humans or by natural disturbances. Second growth in these clearings is dense, dominated by shrubs, thickets, and deciduous and conifer saplings, with little or no canopy. This warbler feeds primarily on insects by gleaning foliage, on or near the ground, where it also nests. It winters in Central and northern South America, where it also prefers disturbed areas with thick undergrowth. A secretive species, very little is known about its migratory behavior and ecology.
A member of the genus Oporornis—along with the Kentucky (O. formosus), Connecticut (O. agilis) and MacGillivray's (O. tolmiei) warblers—the Mourning Warbler is the only member of this genus whose breeding range comes into at least minimal contact with breeding members of all these other three species. The Mourning Warbler is closest in plumage and behavior to its western counterpart, the MacGillivray's Warbler. The species status of these two 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. (2009) described a hybrid zone in the southern Peace Region of British Columbia near Dawson Creek. Analyses of mitochondrial DNA, chromosome markers and plumage are consistent with extensive hybridization in the zone of overlap. This region deserves special 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.
Much of what we know about the breeding biology and life history of this species is based on an early study by Cox's (Cox 1960) in Minnesota. More recently, Mourning Warblers have been the subject of many studies of ecological forestry and the impact of different forest management practices on bird communities in boreal Canada and the northeastern United States. These studies have shown this species to be a resilient one, capable of adapting to a variety of different silvicultural practices. It immediately colonizes new clearings, usually within 1–2 yr of tree harvesting, but numbers begin to decline 7–10 yr later. Some ornithologists have expressed concern over this and other fugitive species that chase early second growth, rapidly appearing and disappearing with natural ecological succession in boreal forests. Extensive human exploitation in the form of logging, mining, oil exploration, agriculture and prescribed burning continues to provide a constant supply of breeding habitat throughout much of the breeding range. This warbler may be one of North America's few Neotropical migrants that has benefited from human disturbance.