BIRD OF THE WEEK: April 17, 2020 SCIENTIFIC NAME: Limnothlypis swainsonii
POPULATION: 140,000
TREND: Increasing
HABITAT: Breeds in swamps and canebrakes, pine plantations, or rhododendron thickets. Winters in lowland forests.

Swainson's Warbler Range Map_Cornell

Swainson's Warbler range map courtesy Birds of North America, https://birdsna.org maintained by the Cornell Lab of Ornithology

The Swainson's Warbler is an extremely secretive bird, living most of its life concealed in thick undergrowth, where it is more often heard than seen. Lucky viewers may glimpse a brown-backed bird with a touch of rust on its crown walking on the forest floor on long, pink legs.

Like the Swainson's Thrush and Swainson's Hawk, this warbler was named for William Swainson, an English ornithologist and artist.

The "Beater Effect"

The Swainson's Warbler, like the Kentucky Warbler, requires a thick, flourishing understory with a dense layer of leaf litter for feeding. It forages by walking along the ground, picking up insects, spiders, larvae, and tiny lizards with its large (for a warbler) bill. The Swainson's Warbler may also glean prey from low shrubs. Unlike other warblers, it does not consume berries or nectar.

Biologists studying wintering warblers in Cuba in the 1990s noted that Swainson's join with other ground-foraging species, including Kentucky, Hooded, and Worm-eating Warblers, and Ovenbirds. The researchers noted that Ovenbirds seem to create foraging opportunities for Swainson's Warblers through the "beater effect” — the Ovenbirds walk steadily through the forest, cocking their tails and vigorously flipping leaves, while Swainson's Warblers seize flushed insects and other prey.

Which Warbler?

The Swainson's Warbler's rich, musical song belies its plain appearance. Some listeners compare it to the song of another warbler, the Louisiana Waterthrush, which often shares its habitat. See if you can tell the difference!

(Audio of Swainson's Warbler by Mike Nelson, XC102034. Accessible at www.xeno-canto.org/102034; Audio of Louisiana Waterthrush by Jorge de Leon Cardozo, XC176360. Accessible at www.xeno-canto.org/176360)

Dual Habitats

The Swainson's Warbler breeds in much of the southeastern United States, from humid eastern Texas to southern West Virginia (and, rarely, in Maryland and Delaware). The species winters on islands in the northern Caribbean, including Cuba and Jamaica, and in portions of the Yucatán Peninsula, such as in Mexico, Belize, and far-northern Guatemala.

Although it has no recognized subspecies, the Swainson's Warbler occurs in two separate populations, which breed in different habitats. The southern Appalachian population favors deep ravines choked with Mountain Laurel and rhododendrons. But the majority of Swainson's Warblers breed in lowland swamps, wet pine plantations — and, importantly, canebrakes. These dense stands of native bamboo were once prevalent in the understory of wet southeastern forests. Following European settlement and widespread forest clearing, many areas of canebrake vanished.

As canebrakes disappeared, important nesting habitat for Swainson's, Kentucky, and Hooded Warblers was also lost. The Bachman's Warbler, a canebrake specialist not seen in decades, is almost certainly extinct.

Indefatigable Breeder

Most wood-warblers such as the Black-throated Blue Warbler and the American Redstart are sexually dimorphic, with males and females sporting very different plumage colors and patterns. The Swainson's Warbler and the Ovenbird are among the exceptions: Males and females look alike.

The female Swainson's Warbler builds a large, bulky structure of dead leaves, sticks, and vines with a cup-shaped nest, lining it with pine needles, moss, hair, and grass. Nests are typically built one to ten feet above the ground in a shrub, palmetto, or vine. In particularly good habitat, Swainson's Warbler pairs may nest in fairly close proximity.

Swainson's Warbler flipping leaves in search of food. Photo by Ken Schmidt

Swainson's Warbler flipping leaves in search of food. Photo by Ken Schmidt

Clutches often fall prey to a variety of nest predators and are frequently parasitized by Brown-headed Cowbirds. The female Swainson's Warbler will respond to these incursions by building a new nest and re-laying, sometimes three or four times during a breeding season! Her unmarked white eggs differ from those of most other warbler species. In fact, the only other North American warbler known to lay plain white eggs was the canebrake-nesting Bachman's Warbler.

Partners for Swainson's Warblers (and Other Birds)

Although Swainson's Warbler populations appear to be increasing, the species continues to face threats from habitat loss. In addition to taking away nesting areas, habitat loss and fragmentation increases "edge" habitat favored by nest-parasitizing cowbirds.

ABC is a partner in the Lower Mississippi Valley Joint Venture, which prioritizes that region's conservation, restoration, and management for migratory birds. Its activities benefit bird species ranging from the Swainson's Warbler and Wood Duck to the Swallow-tailed Kite and Prothonotary Warbler.

We also work with the Sustainable Forestry Initiative in the southeastern United States to maintain important habitat needed by the Swainson's Warbler and many other birds.


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