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Scientific Name: Dendroica cerula
Common Name: Cerulean Warbler

(Information for this species page was gathered in part by Ms. Paige Campbell for a Biology 220W class at Penn State New Kensington in Spring 2013)

Cerulean warbler - image by Mdf from Wikimedia CommonsThe cerulean warbler (Dendroica cerula) is a small (4.75 inches long and 0.3 ounces) song bird that is found in its summer range high up in the canopy of mature, northern deciduous forests. Its winter range is high in the evergreen forests in the sheltered valleys of the Andes Mountains in northwest South America. The male cerulean warbler is blue across his upper body and white below with prominent black side stripes and a “necklace” of a thin, black band across his chest. The female cerulean warbler is greenish gray across her back with a dull blue on her head. She also has a white belly but no black stripes. Both the male and the female have prominent black and white wing bars and thin, pointed bills.

Image by Mdf, Wikimedia Commons.

In its summer range it seldom comes down lower than the mid-canopy of its forest ecosystem. It particularly favors oaks, maples, sycamore, black locust, cottonwood, and elm forests. It forages for insects high in the canopy and makes its nests on horizontal branches in the mid-canopy layer. Nests are small, cup-shaped structures of dry grasses, bark, lichens, and mosses held together by spider webs. If a cerulean warbler has to re-build a damaged nest, it is particularly careful to recycle and reuse the spider web material from the first!

In the winter the cerulean warbler also inhabits high tree canopy habitats. These trees, though, are the tall evergreen trees of the humid forests located in the sheltered valleys on the eastern slopes of the Andes Mountains in northwestern South America. The cerulean warbler joins great mixed species flocks in these forest canopies (common mixed flock species include American redstarts, bluegray tanagers, Tennessee warblers, bay-breasted warblers, blackburnian warblers, and more than a dozen other migrant and native species). The female cerulean warblers tend to form distinct groups in these flocks while the male cerulean warblers tend to remain unattached to each other.

Loss of Habitat
A major concern for the cerulean warbler is the extensive loss of these Andean Mountain forests. The valleys in which the forests were formerly abundant are sites with rich agricultural potential. In particular coffee plantations have replaced many of these stratified, complex forest ecosystems. A number of conservation groups have been encouraging coffee planters to establish shade-grown coffee plantations so that an upper canopy of trees suitable not only to the cerulean warbler but also its dozens of mixed-flock cohabitants is maintained. The lower productivity of these shade-grown systems, though, and the economics of the coffee industry have made these changes difficult. The “Cerulean Warbler Bird Reserve” near Bucaramanga in central Columbia is a conservation foundation subsidized shade grown coffee plantation that is the winter preserve for 270 species of birds including the cerulean warbler.

Loss of their winter habitats is not only reason that the cerulean warbler has had the greatest, recent declines in its population among all North American songbirds. Loss and fragmentation of its northern forest summer habitats, increase nest parasitism by the brown-headed cowbird (which gains access to the warbler nests as forests become more fragmented), and climate change altering the delicate timing of insect emergences (and thus changing the timing of the abundance of food available to the warblers and their nestlings) have all negatively impacted upon this species.

Mating and Reproduction
Male cerulean warblers arrive in their northern breeding regions in late April or early May a week or so before the females. The males partition their upper canopy habitats into sufficiently large territories to support a nesting pair and the nestlings. Males utilize territorial songs and even physical aggression and contact to keep rival males from encroaching into the territories. When the females arrive they select a male based on the quality of the territory and the quality of their songs and displays. They then make their nest in the mid-canopy layer of the trees. Mating and nestling nurturing seems to be the result of a monogamous relationship between the male and the female.

The female lays three to five eggs and incubates them for twelve to thirteen days. Nestlings are fed by both parents and will fledge after ten more days. Usually there is one clutch per season, but if the initial nest is destroyed a second nest will be built and a replacement clutch generated.

The location of the nests high in the mid-canopy reduces the number of potential nest predators that can affect the cerulean warbler. Raccoons and squirrels and some snakes, though, may still raid and/or disrupt their nests.

The cerulean warblers leave their northern breeding regions in August to go back to their valley forest in the Andes. Their migration paths both north and south go directly over the Gulf of Mexico. Observations long the coast note that the cerulean warblers are seldom locally seen during their migration times indicating that this species takes great extended flights across the gulf that begin and end far from the coast edges.

The cerulean warbler is a charismatic symbol of the ongoing decline in song bird populations in the Americas. The watch groups and conservation groups that are dedicated to this species utilize this species’ “star power” to educate the public and to generate funds to study and to preserve not only this species but the dozens of similarly endangered species of birds.

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