Orange-crowned and Tennessee Warblers are two fairly non-descript species often confused with each other. Regional and seasonal differences add to the confusion. Males and females also show subtle differences.
The Orange-crowned in particular can show a range of plumages from a bright green to a very drab olive with grayish head.
Sharp-looking male Tennessee Warblers have a distinctive gray head, green back and white underparts and can even be confused with the Red-eyed Vireo.
The following images provide comparisons of the two species in similar poses.
Tennessee Warblers have a straighter, perhaps thinner looking bill.
Undertail coverts on the Tennessee are almost always white, yellowish/green on the Orange-crowned Warbler.
Underparts of Orange-crowned always yellow-green to dingy olive green. Tennessee shows variable amount of white underneath, from almost pure white to yellow-green with with limited white on belly or the undertail coverts.
Orange-crowned always on the left. Photos © Greg Lavaty.