Male: Blue upper parts with dark striping; bold white wing bars; underparts white; dark striping on the flanks and across chest. Female: green upper parts; pale eyebrow; bold white wing bars; yellow throat; white belly with pale striping on the flanks. 11.5 cm (4.5 in) in length. This species has a short tail and is one of the smallest Dedroica warblers. The song of the Cerulean Warbler is a series of buzzy notes.
The breeding season begins in mid-May, peaks later that month to early June, and extends until late June. Breeding habitat for the Cerulean Warbler includes large areas of old growth deciduous forest, usually near rivers, streams, or other wet areas. The nest is usually 30-50 feet (m) above the ground in the outer portions of a mature tree's canopy. The nest is cup-shaped and probably built mostly by the female out of bark strips, lichen, moss, and other plant material. The female lays 3-5 (usually 4) eggs that she incubates for 12-13 days. The young are altricial and fledge an unknown number of days after hatching. The young are cared for by both adults while they are in the nest.
During migration, the habitat this species uses is variable and includes any type of wooded area. The winter habitat is primarily mature evergreen forest. The Cerulean Warbler diet includes primarily insects and spiders. It usually hunts for food in the tree canopies. It moves around the tree searching for insects, usually on the underside of leaves. This species is a long-distance migrant.
Populations of Cerulean Warblers are mostly isolated and occur in the eastern half of the United States. Most breeding populations are in the northern states east of the Dakotas, Nebraska, Kansas, Oklahoma, and Texas. Most Cerulean Warblers spend their winters in northern South America, although some also winter in southern Central America. In Georgia, this species is found in the northwest corner and extreme northern portions of the state only during the breeding season, although it can be seen in other areas of the state during migration.
This species is much less common now than historical reports indicate, but it is not yet listed as Threatenedor Endangered in any portion of its southeastern range. The decline of the Cerulean Warbler is atrributed to changes of land use from forested habitat to farmlands. Along with habitat loss, Brown-headed Cowbird parasitism and forest fragmentation have attributed to its decline. Brown-headed Cowbirds lay their eggs in other species nest, leaving their young to be cared for instead of or in addition to the host young. For a small bird like the Cerulean Warbler, cowbird young are dominant over the host young and will push the warbler's young out of the nest.
The most similar species to first year birds and female Cerulean Warblers is the Blackburnian Warbler. The Blackburnian Warbler has a much longer tail, and is slightly larger than the Cerulean (13 cm). Viewed closely, the Blackburnian Warbler shows stripes on the back and a pale stripe on the middle of the crown. The adult male Cerulean Warbler would be difficult to mistake for any other species.