The cerulean warbler is 4 to 4.5 inches long, has a wingspan of 7.5 to 8 inches and an average weight of 3.2 ounces. Females are slightly smaller than males. Males are azure blue on the back and white on the belly. An identifying mark is a dark blue or black line across the throat. They have dark streaking on the flanks and back.
Females are dull grey-blue above with a yellowish wash below.
Both sexes have two white wingbars. Juveniles are brownish-gray above, with a pale center crown stripe and entirely white underparts. Cerulean warblers molt into an adult plumage prior to the breeding season following their hatching year. Because cerulean warblers often remain concealed in foliage high in the forest canopy, they are usually best identified by their buzzy calls, which rise in pitch at the end. Birds of North America.