Cerulean warbler  (Dendroica cerulea)

Named after the beautiful blue colour of its plumage, the cerulean warbler is a small canopy-dwelling wood-warbler. Adult male cerulean warblers have streaky bright sky-blue upperparts, white underparts and a narrow black band that falls, like a necklace, across the throat (2) (3). In contrast, the plumage of adult females is a shade of greenish-blue, becoming bluer at the crown and rump (3). The throat and breast are pale yellow and above the eye sits a prominent pale yellow ‘eyebrow' (3). While there is some seasonal variation in the plumage of both sexes (2), two wide white bars across the long, pointed wings and white spots on the short tail remain throughout the year (2) (3). As the cerulean warbler spends much of its time high in the canopy of forests, it can be difficult to spot, and so is best identified by the high-pitched song of the male: an accelerating series of short buzzes ending with a single longer, higher buzz (3).

The breeding range of the cerulean warbler extends from Quebec and Ontario, Canada, south to northern Texas, Louisiana, Mississippi, Alabama and Georgia in the USA. The cerulean warbler migrates south for winter where it can be found in Colombia and Venezuela, south to eastern Peru and northern Bolivia (3) (4).

During the breeding season the cerulean warbler inhabits mature deciduous forest, generally with open understorey, between 30 and 1,000 metres above sea level (2) (3). Over winter, the cerulean warbler can be found in broad-leaved, evergreen forests and woodland on the east slopes of the Andes and in the montane forests of Venezuela, at elevations of about 500 to 1,500 metres (2). It shows a preference for mature primary forest, although this warbler has also been found in mature coffee plantations (5).

The cerulean warbler undertakes an impressively long migration considering its small size, covering approximately 2,500 miles as it flies south between July and September (3) (5). The journey has to be completed once again at the end of winter, with the warbler reaching the breeding grounds in April and May (3).

Arriving at the breeding grounds a week earlier than the females, the males use this time to establish and maintain a territory. While this is achieved primarily by singing, aggressive physical attacks can occur (3), with males colliding in mid-air and grappling with bills and feet whilst they spiral towards the ground (2).

Cerulean warblers are apparently monogamous and so once the females arrive on the breeding grounds, pairs are formed, and the female sets about building a nest high in the canopy. This is generally a neat cup structure formed from shreds of bark, lichen, moss, and grass, bound together with spider webs and lined with hair and fine moss stems. Finally, the outside of the nest is often decorated with fragments of any grey or white material (2) (3). Usually each pair produces only one brood each year, laying an average of four eggs which are then incubated by the female for 11 to 13 days (3). The young are born with their eyes closed and are incapable of leaving the nest, and so are fed by the parents for the first 10 to 11 days of life (3). Cerulean warblers can first breed at the age of one, and may live for up to six years (2).

Cerulean warblers are insectivorous birds, feeding on a variety of insects as they hop along twigs examining the surfaces of leaves and bark for any potential prey (3) (2). They also sometimes consume small amounts of plant material (2).

Once a common species in the forest canopies of eastern North America, the cerulean warbler has since undergone a long-term, steep decline, and is now only frequently sighted in core parts of its breeding range (5). The primary force behind these dwindling populations is habitat loss and degradation, in both the warbler's breeding and non-breeding habitats (4) (5). While the breeding habitat is subject to forest loss, fragmentation, and a loss of suitable vegetation structure within mature deciduous forests, the winter forest habitat is suffering from clearance for the production of coffee, tea, coca and hill rice, and pasture for livestock. Around 60 percent of the cerulean warbler's wintering habitat may have already been lost to these activities (5).

In addition, the cerulean warbler faces the potential threats of, amongst others, an increased frequency of catastrophic weather events, and a shift in the location of suitable forest types as a result of climate change (5).

In 2001, the Cerulean Warbler Technical Group (CWTG) was formed (5), and together with various partners, initiated a number of conservation measures for this threatened bird. This has included undertaking research projects on the cerulean warbler's biology and ecology, conducting surveys, and beginning dialogues with representatives of the forestry and mining industries regarding development of recommended practices for Cerulean warbler conservation (5). The CWTG has identified many more actions that are required to ensure the long-term survival of this species, outlined in a Conservation Action Plan, particularly the need to prevent the further loss of large areas of forest (5).

For further information on the Cerulean warbler see:

• Cerulean Warbler Technical Group. (2007) A Conservation Action Plan for the Cerulean Warbler (Dendroica cerulea). USFWS Division of Migratory Bird Management Focal Species Program, U.S. Fish and Wildlife Service, Minnesota. Available at:
  http://www.fws.gov/midwest/Eco_Serv/soc/birds/cerw/cerw_actionplan_30june07.html

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