How to identify
a Cerulean Warbler in the field?
Unmistakable with their contrasting light-blue back and stark-white underparts, black streaks on the back and flanks and dark necklace.
The wings are darker with blue edging to the primaries and two white wing bars.
Their facial pattern can vary with some individuals showing a strong white supercilium or broad, black streaks on either side of the crown.
The bill on adults is completely black.
Olive-green backs with no streaking, but some individuals can have slightly blue-green rump and forehead.
Underparts are whitish with varying amounts of yellow tinge with the undertail coverts remaining white.
The streaking on their flanks tends to be olive and very faint.
Their wings are darker with two white wing bars.
They have an obvious pale supercilium and a thin, dark line running through the eye.
Adults have a completely dark bill and immatures have a bicolored bill with dark upper mandible and pale lower mandible.
During their first fall migration some immature males can be difficult to distinguish from females, but they tend to show more defined streaking on the flanks and back.
Immatures have a bicolored bill with dark upper mandible and pale lower mandible.
One of the species that can be easily confused with Cerulean Warbler is Blackburnian Warbler (Setophaga fusca).
Here are some tips to help differentiate between them in the field:
The overall color of the upperparts of immature/female Blackburnian Warblers is olive-brown, whereas the upperparts in immature/female CERWs is grayish-blue to greenish.
Blackburnians always show contrasting, dark auriculars whereas CERWs have a noticeable dark line running through the eye.
Two field marks from the underside of the tail can be useful when separating them. The undertail coverts in CERW are much longer, giving the impression of a shorter tail and the shape and extent of the white tail spots is diagnostic.