SWAINSON’S WARBLER

Photo Credit: Chandler Robbins

SCIENTIFIC NAME: Limnothlypis swainsonii (Audubon)

OTHER NAMES: Swamp Worm-eater.

DESCRIPTION: A plain, medium-sized (14.0 cm [5.5 in.]) warbler with a relatively short tail and a noticeably long, pointed bill. Adults brown to olive brown above and off- white to yellowish white below; flanks grayish and crown reddish brown; distinct, dull white eyebrow line contrasts with a dark eye line. Bill dark above and pale below, and legs are flesh-colored. Juveniles similar to adults, but lack distinct eyebrow line. Typical song is loud and consists of three to four high pitched, clear, well-separated notes followed by three rapidly descending notes that sound like whee-whee-whee-whee-Whip-poor-Will (Brown and Dickson 1994). No subspecies currently recognized (Brown and Dickson 1994).

DISTRIBUTION: Breeds locally from southeastern Oklahoma, southern Missouri, and southern Illinois; east to western Tennessee, northern Alabama, and into southern Appalachian Mountains of northern Georgia, eastern Tennessee, and western North Carolina; north to eastern Kentucky and southern West Virginia; east to southeastern Maryland; and south throughout the Atlantic and Gulf Coastal Plains from Virginia to northern Florida to eastern Texas. Winters primarily in the West Indies and Central America (Brown and Dickson 1994). In Alabama, breeds statewide wherever suitable habitat exists except in Piedmont Region and southern portions of Mobile and Baldwin Counties (Imhof 1976).

HABITAT: Found in greatest densities in floodplain forests having extensive understory thickets. In Alabama, favors bottomland hardwood forests and other forested lowland areas where dense undergrowth of switch cane occurs, especially in the lower Coastal Plain Region, where it is most abundant. Also utilizes a variety of other habitat types consisting of dense stands of shrubs, vine tangles, and saplings where cane is sparse or absent (Graves 2001, 2002). Additional habitats include fragments of old-growth bottomland forests, early seral stages of deciduous bottomland forests, young pine plantations with deciduous components, second-growth bottomland forest with scrub palmetto undergrowth, dense thickets of rhododendron, mountain laurel in the Appalachian Mountains, and hardwood cove forests in the Appalachians (Graves 2001, 2002). Underlying theme suggests physical structure of understory may be more important in determining habitat selection than specific floral community compositions. Winters in montane forests, humid bottomland forests, and mangroves where dense undergrowth and extensive leaf litter exists (Dunn and Garrett 1997).

LIFE HISTORY AND ECOLOGY: In spring, arrive on northern Gulf Coast from late March through early May with peak migration occurring in April (Brown and Dickson 1994). Males arrive a week to 10 days earlier than females to establish territories. Breeding pairs form soon after females arrive and nesting begins shortly afterward (Meanley 1971). Nest built entirely by female and construction usually takes two to three days; typically placed in dense vegetation often supported by vines or small branches where limbs from more than one plant intersect (Brown and Dickson 1994). In Alabama, 30 of 31 nests were placed in, or associated with, canebrakes and placement varied with culm density. Seventeen nests (57 percent) were placed where culm density was moderate, nine (30 percent) were placed where culm density was thick, and four (13 percent) were placed where the cane was thin. Height of nests from ground ranged between 0.6 to 4.1 meters (1.8 to 13.5 feet), averaging 1.8 meters (6.1 feet). All 31 nests were in shady thickets in heavily forested bottoms interspersed with sloughs and ponds, but none placed over water (B. Summerour, unpubl. data). Nest is largest of all North American wood warblers (Meanley 1971). It is a bulky, loosely attached and loosely constructed deep cup of dead leaves with many ragged and tattered ends. In Alabama, nests built in cane have been constructed entirely, or nearly so, of cane leaves, but others may be built with dead hardwood leaves. Inner cup is thin and composed of skeletonized leaves and an assortment of stems; lined with various rootlets, coarse grass or dead Spanish moss (B. Summerour, unpubl. data). Clutch size varies between three to five eggs with three being most common. Seventeen of 31 nests found contained fertile eggs or young (15 with eggs and two with young), while other 14 contained one or two infertile eggs or were empty. Ten of 15 nests with eggs contained full sets, with seven holding three eggs and three containing four eggs, with an average of 3.3 eggs per clutch (B. Summerour, unpubl. data). Eggs are white or pinkish white. Only warbler, other than Bachman’s warbler, to lay all white eggs (Meanley 1971). Females usually lay first egg two to four days after nest completion, with one egg laid per day until clutch is completed. Incubation begins after final egg is laid and lasts 13 to 14 days. Young are altricial and are fed by both adults. Brooding done entirely by female. Young usually fledge after 10 to 12 days in nest. Adults feed fledglings two to three weeks after young have left nest (Brown and Dickson 1994). One brood raised each year; however, building dates extending into July may suggest more than one reared (B. Summerour, unpubl. data). Susceptible to brood parasitism by brown-headed cowbird (Brown and Dickson 1994). Two (12 percent) of 17 active nests in Alabama were parasitized (B. Summerour, unpubl. data). Breeding in Alabama ranges from third week in April through first week in August (Summerour 1979, unpubl. data). Often remain on nesting grounds following breeding and males continue to sing well into September. Most return to wintering grounds between mid-September to mid-October (Brown and Dickson 1994). A terrestrial dead-leaf specialist that lifts leaves up with its bill and vibrates its feet to find foods such as lepidopteran larvae, beetles, ants, spiders, ichneumons, crickets, grasshoppers, katydids, true bugs, flies, and millipedes (Brown and Dickson 1994, Graves 1998).

BASIS FOR STATUS CLASSIFICATION: As denizens of canebrakes and swampy tangles, remains among the most secretive and poorly known species of all North American songbirds (Brown and Dickson 1994). Habitat destruction resulting from extensive timber harvest, conversion of bottomland hardwood forests and canebrakes to agricultural fields, pine plantations, reservoirs, and housing developments has negatively impacted local populations (Graves 2001). Further, increased forest fragmentation resulting from clear-cutting, power and gas line right-of-ways, and creation of roads has probably increased the incidence of brood parasitism by the brown-headed cowbird (Brown and Dickson 1994). Currently, listed as a species of concern in most states throughout its breeding distribution (Brown and Dickson 1994) and is considered by some the second most endangered breeding songbird in the Southeast (Graves 2002). Priority designation in Alabama based on low relative abundance, limited breeding and wintering distribution, and current and future threats to breeding and wintering habitats (PIF 2002).

Author: Eric C. Soehren