The MacGillivray's Warbler is the western counterpart to the eastern Mourning Warbler. Fortunately, the breeding ranges of the two species do not overlap. Male MacGillivray's typically have broken eye rings while they are absent in male Mourning Warblers. Females and immatures of the two species are extremely difficult to tell apart and are best separated by range. Connecticut Warblers are similar to MacGillivray's Warblers but have complete eye rings, and are larger. Female and immature Common Yellowthroats are also quite similar but lack the gray or brownish hood of the MacGillivray's (although this can be indistinct in some immatures) and have white, not yellow, bellies.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.