Breeds mainly in northern hardwood or mixed coniferous forests dominated by maples, birches, beech, and other northern hardwoods, with varying amounts of eastern hemlock, spruce, and fir. A key component of this species' habitat is a dense shrubby understory, especially consisting of hobblebush and other viburnum species, as well as striped maple, rhododendron, or regenerating conifers. This species occurs mostly at higher elevations 2,600-5,200 feet (800-1,600 meters) in the southern Appalachians; in New York, occurs mainly above 950 feet (300 meters), absent at lower elevations including the major river valleys and coastal lowlands. In Maine and the Adirondack Mountains, this warbler has been found to be tolerant, or even benefiting, from silvicultural practices that favor a dense regenerating understory.
Although populations of this warbler appear to be stable or even increasing, it is of conservation importance because of its small overall range, low densities even in suitable habitat, and even more restricted winter range in the forests of the Greater Antilles. Associations with forest structure, especially in relation to silviculture and other land uses, are key to conserving future populations. Birds in the southern Appalachians are restricted to sensitive or threatened spruce habitats at high elevations and are of particular conservation concern.
Male: Deep blue upperparts with black face, chin, throat, and sides, and otherwise white underparts. A conspicuous white patch evident on the blue wings. Southern Appalachian population has a variable amount of black mottling on the back and black streaks on the otherwise blue upperparts.
Female: Olive-brown upperparts and buffy below, a thin white eyebrow; may show a blueish tinge in the crown. White wing patch (as in male) usually apparent.
Juvenile: Brownish above and dull, whitish-buff below with brownish-olive mottling on the breast. The head shows a suggestion of a buff eyebrow and a dark cheek patch.
Song: Singing is almost exclusively by the male, although females sing occasionally. Of the several song types, which vary considerably within and among individuals in speed, overall pitch, and number of notes, the most frequent consists of 3 to 7 notes, with the last commonly slurred upward: zee-zee-zee-zreeeee.
Calls: A flat sounding ctuk given frequently throughout the breeding period by both sexes, but especially by females. Unlike the chips of most warblers, this call closely resembles call note of Dark-eyed Junco.
Very active forager, with frequent fly-catching behaviors. Individuals primarily forage alone, although members of a pair sometimes forage near one another. By rapidly moving through dense foliage, both sexes visually search upper and lower leaf surfaces, branches and twigs, tree boles, and surrounding air spaces. The most frequent capture method is to snatch prey from a substrate while hovering or flying past, but individuals also glean prey from nearby substrates while standing on the vegetation.
Primary food items are butterfly larvae and adults, crane flies and other adult flies, spiders and other arthropods.
Behavior and displays
Nest site: The nest is usually located in the dense shrub layer of either deciduous or mixed coniferous/ deciduous forest, frequently in broad-leaved evergreen shrubs (e.g., Mountain laurel, rhododendron, and viburnum), conifer saplings, deciduous shrubs, or saplings.
Height: Nest placement is generally low, usually within 3-5 feet (1-1.5 meters) of the ground.
Nest: Constructed of thin strips of bark, often obtained from white or yellow birch, and occasionally pieces of rotten wood, glued together by cobwebs and apparently saliva. Built by female in 3-5 days, the male may contribute nest materials and may help shape the nest occasionally.
Eggs: Typically 4 eggs, clutch size can range from 2 to 5. Eggs are ivory or creamy white, speckled, blotched or clouded with tones of brown, chestnut, or gray. The female is rarely seen near the nest during the laying period.
Incubation period: 12-13 days, incubation solely by female.
Nestling period: Hatchlings are altricial: naked, except for downy tufts on head, neck, and dorsum, with eyes closed. Female broods recent hatchlings and also when the weather is cool or rainy. Both males and females feed the nestlings at about equal rates, and both appear to increase their feeding rate as fledging approaches. Departure from the nest ranges from day 8 to day 10.
Fledgling period: All young usually leave within a period of 30-50 minutes of one another. If late in the day, they may return to the nest for that night. At departure, young are close to adult weight, though tail and flight feathers are only about half grown. Fledglings can fly weakly, usually fluttering from one understory branch to another. The young usually remain with at least one parent who provides food and gives alarm calls. In a few cases, the parents split the brood.
Broods: Double brooded.
Cowbird Parasitism: Infrequently parasitized.