Fact Sheet
Cerulean Warbler

Basic Facts About Cerulean Warblers

The Cerulean warbler (Dendroica cerulea) is a small, sky-blue bird most often seen flitting around the upper canopy of mature deciduous eastern forests.

As with most warblers, males and females look quite different from each other. Male cerulean warblers are bright blue above and white below. They have black streaks on their sides and back, two white wing bars and a black line, or “necklace”, across their front. Females are dull turquoise above with a pale blue crown and yellowish-white below. They also have white wing bars but do not have the necklace or streaking that males do.

Diet

Cerulean warblers eat insects.

Population

The cerulean warbler’s population is dropping faster than any other warbler species in the United States. Between 1966 and 1999, it declined an average of 4% per year throughout its eastern US breeding range for a total population loss of 70%. Current estimates are at around 560,000 birds.

Range

Breeding Range: Southeastern North America from the lower Great Lakes region, southern Quebec and New England down to northern Louisiana and northwestern Georgia. It is especially prevalent in southern Missouri and Wisconsin, eastern Kentucky, eastern Ohio and West Virginia.

Winter Range: The cerulean warbler spends most of the year in northern South America from northern Columbia and Venezuela down to southern Peru and northwestern Bolivia. 

Behavior

Song: Only the male warbler sings and his vocalization is quite distinctive – rapid buzz-like notes on one pitch followed by a short series of rising and accelerating notes, ending with a high buzz-like trill. ZHEE ZHEE ZIZIZIZI zzzzeeet.

Migration: They are nocturnal migrants and usually arrive in their breeding range in late April or early May. They leave in August and migrate to forested mountain regions of western South America, where they can often be found feeding with flocks of local birds like the tropical tanager.

Reproduction
Mating Season: April to May.
Clutch size: 3-4 eggs.
They usually have only one brood per breeding season. The eggs are gray to greenish white with brown specks. When they hatch, both parents feed the young.

Nest: The nest is cup-shaped and made of grass, bark fragments and hair. These are all bound together with a spider web. If the female has to abandon a nest and begin a new one, she will leave behind the grass, bark and hair, but will take the spider web with her for the new nest.

Threats

Habitat destruction due to agriculture and development is a major threat. Habitat fragmentation also causes egg parasitism by brown-headed cowbirds: When cerulean warblers have nests on the edge of a fragmented area, brown headed cowbirds lay their eggs in warblers’ nest (as well as in those of other species). The cowbird eggs hatch before cerulean warbler eggs and the young cowbird will often push the unhatched warbler eggs out of the nest. The adult cerulean warblers cannot tell the difference between the brown-headed cowbird and their own young, so they raise them as their own.

Climate change may also threaten cerulean warbler and their habitat by altering forest types, and by changing the delicately timed balance of insect emergence that the birds rely on for food.

Defenders believes the cerulean warbler should be listed as threatened under the Endangered Species Act. Unfortunately the U.S. Fish and Wildlife Service didn’t agree and in December 2006 officially declined to list the species. Without federal protection, there is little that can be done to stop the destruction of important cerulean warbler habitat.