They are very similar to the MacGillivray's Warbler in appearance, especially in females and immature birds, but their breeding range does not overlap into the west.

Mourning warblers are small songbird with yellow underparts, olive-green upperparts, a thin pointed bill and pink legs.

Mourning warblers are low or ground nesters, and prefer disturbed forests with a thick under-story and covered canopy.

These birds mainly eat insects, also some plant material including fruiting bodies from the Cecropia tree in winter.

Their breeding habitat is thickets and semi-open areas with dense shrubs across Canada east of the Rockies and the northeastern United States.

[6] Mourning warblers typically lay 2–5 eggs, which are white or speckled brown and black in appearance.

[5] According to the North American Breeding Bird Survey, Mourning Warbler populations have declined by about 43% in the last 50 years.

[1] As they have a preference for disturbed forests, they may be benefitting from human development that may be negatively effecting the habitats of other bird species.