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Status: Population size: 3.1 million Photo by U.S. Forest Service.

Bay-breasted Warbler
(Dendroica castanea)

A neotropical migrant, the Bay-breasted Warbler lives in boreal coniferous forests in central and eastern Canada and winters in Panama and northern South America. This species' numbers increase and decrease in correspondence with spruce-budworm outbreaks; pesticides used to control the spruce-budworm may be affecting this bird's overall population. Habitat destruction including tropical deforestation is another concern.

Identification
A 5.25-inch long, chunky, short-tailed warbler, the adult spring male has a black face and rufous or bay-colored crown, throat, and flanks, contrasting with creamy-yellowish neck patch and belly. It also has two white wing bars. The spring female is similar, though duller. In non-breeding plumage, the Bay-breasted Warbler can easily be confused with non-breeding plumaged Blackpoll Warbler. Bay-breasteds in fall usually have some trace of rufous on their sides and not as much streaking as in Blackpoll. The legs and feet are also dark compared with orange-yellow on the Blackpoll.

Distribution and Population Trends
The Bay-breasted Warbler breeds across the vast boreal forests of Canada from Northwest Territories to Newfoundland and the Maritimes and northern New England. It migrates in fall to winter mostly from Costa Rica and Panama to northwestern South America. Populations rise and fall with outbreaks of spruce-budworm, its favorite prey. Data show that population densities increased during budworm outbreaks in New Brunswick in the 1950s and 1960s, compared with other years.

Breeding Bird Survey data from 1966 through 2001 showed a slight 2.7% annual decline in population; from 1980 through 2001, the increase was more substantial (6.6%). In New Brunswick, estimates at Fundy National Park show that 20,000 pairs bred there in 1979, but only 3,400 pairs bred there in 1992.

Ecology
The Bay-breasted Warbler chooses cool, dense coniferous forests interspersed with small openings such as bogs in which to breed. It prefers balsam fir and spruces mixed with tamaracks, white pines, birches, and aspens. It may nest in pure deciduous growth when caterpillar numbers soar.

The female typically lays eggs from middle June through early July. The nest, typically in the lower third of a dense conifer, is built with conifer twigs, bark shreds, lichen, dried grasses, and other vegetative material upon a horizontal branch. Incubation lasts 12 to 13 days. The male guards the nest while the female keeps the eggs warm. The female flaps her wings and spreads her tail to feign injury when humans approach the nest. The young leave the nest within 10 to 11 days after hatching. The Brown-headed Cowbird rarely parasitizes this species.

The Bay-breasted Warbler eats mostly moth and butterfly larvae, including spruce budworm, during the breeding season. In fall, Bay-breasted Warblers will eat berries including those from Virginia creeper as well as insects and caterpillars. The winter diet consists of insects as well as fruits from trees and shrubs, when insect populations are low.

In winter range occurs in forest edge and second-growth habitat. Birds begin leaving breeding territories in August and September to fly to their winter homes; the flight back to their breeding grounds begins in April. The Bay-breasted is one of the later warblers to migrate north in the spring.

Threats
Large-scale clearcutting of Canadian boreal forest is undoubtedly causing a decrease in available habitat for Bay-breasted Warblers. Use of pesticides to control spruce budworm infestations are, no doubt, contributing to higher-than-normal mortality rates. Birds collected in New Brunswick in May 1989 several days after spraying occurred, for example, had a life-threatening brain disorder.

TV tower kills is another possible concern. In northern Florida, for example, 466 Bay-breasted Warblers were killed at a TV tower between 1955 and 1980.

Conservation
Various groups have begun working together to achieve a vision of protection and sustainable use of the Canadian boreal forest. Fulfillment of this vision will be important in the long-term health of Bay-breasted Warbler populations.

Lower dosages of and less frequent applications of pesticides could decrease the mortality rate of affected Bay-breasted Warblers. Integrated pest management, which takes into account the fact that periodic spruce budworm outbreaks are part of the natural cycle of the maturing of balsam firs, should be considered.

What Can You Do?
Audubon's Important Bird Areas program is a vital tool for the conservation of Bay-breasted Warblers as well as other species. To learn more about the Important Bird Area programs and how you can help, visit: http://www.audubon.org/bird/iba/.

Information on where Bay-breasted Warblers occur and in what numbers is vital to conserving the species. Help in monitoring this and other species by reporting your sightings to eBird. A project of Audubon and the Cornell Lab of Ornithology, eBird is the world's first comprehensive on-line bird monitoring program: http://www.audubon.org/bird/ebird/index.html.

The Canadian Nature Federation and Bird Studies Canada, Audubon's BirdLife partners in Canada, jointly administer an Important Bird Areas program that is working to identify and protect habitat for Bay-breasted Warblers and many other species. To learn more visit: http://www.ibacanada.com/.

References
Curson, J., D. Quinn, and D. Beadle. 1994. Warblers of the Americas: an Identification Guide. Houghton Mifflin, New York.

Dunn, J. and K. Garrett. 1997. Peterson Field Guide: Warblers. Houghton Mifflin. New York.

Pearson, S. F. 1996. Bay-breasted Warbler (Dendroica castanea). In The Birds of North America, No. 206 (A. Poole and F. Gill, eds.). The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologists' Union, Washington, D. C.