Editor's Note: Phylogenetic analyses of sequences of mitochondrial and nuclear DNA indicate that all species formerly placed in Dendroica, one species formerly placed in Wilsonia (citrina), and two species formerly placed in Parula (americana and pitiayumi) form a clade with the single species traditionally placed in Setophaga (ruticilla). The generic name Setophaga has priority for this clade. See the 52nd Supplement to the AOU Checklist of North American Birds for details. Future revisions of this account will reflect these changes.
The Cape May Warbler breeds in boreal coniferous forests, where it sings, feeds, and nests high in the spruce canopy. During winter it is confined almost exclusively to islands of the West Indies, where it occurs in a variety of habitats. One of the most striking Dendroica warblers, the Cape May male in Alternate plumage has a yellow breast streaked with black, a yellow rump, white wing-patches, and chestnut ear-patches. Females and immatures are less boldly colored. Although the first illustrations of this species were based on birds taken in Canada, its English name refers to the locality from which Alexander Wilson first described the species—Cape May, New Jersey—where it was not recorded again for more than 100 years (Bent 1953a).
The Cape May Warbler is perhaps best known among ornithologists as a “spruce budworm specialist.” Its populations expand during outbreaks of the defoliating spruce budworm (Choristoneura fumiferana), reaching high densities in such areas and then sometimes disappearing between outbreaks. The Cape May Warbler has a larger average clutch size than most wood-warblers, which may enable it to increase rapidly during such short-term resource bonanzas. By dampening the boom-and-bust cycles of the spruce budworm, spraying to control this insect may stabilize local populations of the birds that specialize on it.
Few observers have studied this warbler on its breeding grounds. Its nest was not accurately described until the early twentieth century, and many of the basic details of its breeding biology, including reproductive success, remain unknown. The few accounts with information on breeding are generally anecdotal and/or dated. Some important information on the behavior of the Cape May Warbler during the breeding season is available from studies in Ontario (Kendeigh 1947) and Maine (Macarthur 1958), but these studies represent only a small portion of the breeding range of the species. This species' populations are apparently regulated largely by budworm availability, although there are few data on how spraying for the budworm affects breeding numbers.
The ecology of the Cape May Warbler is better known for migration and winter. Nonbreeders are often easily observed and are known to feed on nectar, among other things, taken up by means of a semitubular tongue. Much remains to be learned, however, about the demographics of wintering populations, age and sex ratios, site fidelity, and annual and overwinter survival. Most information on habitat use and spacing of wintering populations is qualitative. Additional studies are needed to determine the degree to which populations may be affected by habitat alterations on both breeding and wintering grounds.