BFL: Species Account: Swainson's Warbler

SWAINSON'S WARBLER

Swainson's Wabler (Limnothlypis swainsonii)

Distribution

Breeding: Locally through the southeastern U.S., from eastern Texas and Swainson's Warbler range map Oklahoma through the southern portions of Missouri, Illinois, Indiana, Ohio, and West Virginia to southern Delaware and Virginia, south to the Gulf Coast in northern Florida.

Winter: Cuba, Jamaica, northern Bahamas, Cayman Islands, Isla Cozumel, south to Honduras and east to Puerto Rico and Virgin Islands; sparingly on Yucatan Peninsula, including Belize and northern Guatemala.

Breeding habitat

Lowland populations, those found on the coastal plain, use mature, rich, damp, deciduous floodplain and swamp forests, where they normally occur in the shadiest parts of the forest. Forest structure consists of dense upper and lower canopy and shrubs, with little herbaceous cover.

Highland populations, found in the mountains, utilize moist lower slopes of mountain ravines at elevations up to 2,800 feet (850 meters). The dominant shrub layer consists of rhododendron and laurel species. In the southern Appalachians, individuals primarily found in sawtimber and in pole stands of second-growth cove forests.

Conservation status

This warbler is of high conservation importance, because of its small breeding range, specialized habitat requirements, low overall densities, and even more restricted winter distribution. It is poorly monitored in many areas, but populations appear to be stable at present. Understanding this species’ specific habitat requirements, area sensitivity, response to forest fragmentation and other land-use practices will be very important in conserving future populations.

Description

Male: A large and heavy bodied warbler with a long, spike-like bill. Generally brown above, richer brown on hindneck and crown. A broad, pale-white eyebrow and dark eye-line are similar to Worm-eating Warbler and both waterthrushes. Underparts are white with varying yellowish tint, also a varying gray-brown wash on flanks and chest. The tail is noticeably short, and the legs and feet are pinkish.

Female: Same as adult male.

Juvenile: Same as adult male.

Vocalizations

Songs: The "advertising" or "primary song" consists of 2-4 down-slurred notes (reminiscent of the beginning of the Louisiana Waterthrush’s opening) followed by several rapid and descending notes: whee whee whee whip-poor-will. A "whisper song" is simply a softer version of the primary song, also heard throughout the breeding season. A territorial "flight song" is as loud as the primary song, but continuous and lasting as long as the flight.

Calls: A distinctive, loud, and sweet chip or a flatter tsup, a very high, thin, slightly buzzy, and often doubled swees heard in flight.

Foraging strategy

Forages primarily on the ground by probing bill under leaves, pushing leaves up, and examining ground or leaves underneath for prey; may also open curled leaves by inserting and opening bill. Occasionally forages on top of fallen logs and in low undergrowth foliage; sometimes hawks insects from low perches in trees or gleans insects from lower parts of tree trunks.

Diet

Insectivorous, primarily spiders, caterpillars, beetles, ants, spiders, ichnemons, platygasterids, crickets, grasshoppers, katydids, stink bugs, flies, and millipedes. Not known to eat fruit or nectar.

Behavior and displays

Unlike most warblers, flies directly from perch to perch instead of hopping through branches.

Vocal threats include chipping excitedly and singing songs during and after territorial encounters. Visual threats displayed by laterally spreading wing and tail feathers and vibrating tail, then walking sideways back and forth along a branch, frequently turning around.

Males chase each other during breeding season, and may flutter on the ground together, sometimes flying 3-7 feet (1-2 meters) up from ground while grasping at each other’s bill.

Paired males appear to be more aggressive than unpaired males at border disputes.

Copulation often involves the male flying to the female as she forages on the ground, he then pounces on her, pecks at her rump, and struggles with her on ground for a few meters with very rapid movements.

Courtship

During precopulatory display male perches 3 feet (1 meter) from ground, extends quivering wings and raises head tail and rump feathers. When the female approaches, the male vocalizes with quiet twee-twee-twee.

Courtship feeding not observed.

Nesting

Nest site: Female selects site, usually at the edge of a dense growth of cane, vines, or rhododendron; often situated near (sometimes over) water, but also on high, dry sites. The nest is placed in the predominant understory vegetation where it is suspended by several thin vines or supported by small trees or bushes, especially at intersections of branches or separate plants; rarely placed in the thickest part of the vegetation. Often located near edge of territory, or at least male usually spends most of time away from 1 side of nest, sometimes outside of male’s territory.

Height: Usually low, average nest height just over 3 feet (1 meter) above ground, but can range from 1.5-10 feet (0.5-3 meters).

Nest: Female builds the cup nest using materials gathered near the nest site. The outer layer is loosely constructed of dried leaves, sticks, vines, and tendrils, giving an appearance of an unorganized clump of leaves. The shell of the nest cup is compactly built using skeletonized leaves and may be covered by a layer of twigs or needles. Nest building takes 2-3 days and takes place before noon.

Eggs: 3 eggs (rarely 4 or even 5) are laid in late April or May through July. Usually white and unmarked, though occasionally bluish or pinkish and rarely faintly spotted with reddish brown.

Incubation period: 13-15 days, only by female.

Nestling period: Young are altricial and nidicolous, primarily naked with closed eyes. Both parents feed young; both parents remove fecal sacs. Young remain in the nest 10-12 days.

Fledgling period: Fledglings are unable to feed themselves, birds 2 days out of the nest are not able to fly far, mostly hopping along the ground. Parents feed young for 2-3 weeks after young fledge, Meanley (1968) reported that only female fed young that fledged from a nest in Virginia, although male was often nearby. Fledglings emit chip notes to inform feeding parents of their location, increasing intensity as parents get closer.

Broods: Occasionally double brooded, but whether two clutches per season is ordinary is controversial.

Cowbird Parasitism: Known host of the Brown-headed Cowbird, may be common in some areas.

Notes

One instance of apparent polygyny with aggregated nest placement reported. Knowledge of how common such behavior is would be useful in estimating population size from singing male counts.

Reproductive success at nests observed in coastal plain habitat was low. Cowbird parasitization was the cause of at least three failures, while Common Grackles, Blue Jays, and snakes depredated nests. One researcher concluded that the Swainson’s Warbler would probably have a difficult time maintaining its present population level in the Coastal Plain.