Introduction

The Mourning Warbler, a common breeder in cleared but regenerating areas of North America’s boreal forest, winters in Central and South America, where it also prefers disturbed areas with thick undergrowth. In favoring such clearings caused by logging or forest fires, this warbler may be one of North America’s few Neotropical migrants that has benefited from human settlement.

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 other three species. The Mourning Warbler is closest in plumage and behavior to its western counterpart, the MacGillivray’s Warbler. Based on morphological similarities and suspected cases of hybridization, they have been regarded as subspecies by some authorities. However, erratic contact on the breeding grounds, distinct song types, different patterns of geographic variation in song, and differences in skeletal dimensions support their distinction as separate species. Unlike most wood warblers, the Mourning Warbler has a single song repertoire, which varies little across the breeding range. In contrast, the plumage of this species varies across its breeding range, primarily in the amount of white feathering around the eye and the presence or absence of dark lores through the eye, but the variation is haphazard and not concordant.

A majority of the literature on this species has focused on taxonomic problems, identification of probable hybrids or extreme specimens, and extralimital range expansions. The most comprehensive study of its breeding life history was conducted by Cox (1960) in Minnesota. Most aspects of the Mourning Warbler’s general biology deserve further study, including migration, behavioral ecology on the wintering grounds, population dynamics, molt, parasites, and physiology.