The Swainson's Warbler, one of North America's most secretive bird species, is a medium-sized wood-warbler that breeds in the southeastern United States and overwinters in the Caribbean, Yucatán Peninsula, and eastern Mexico. Known to most birders by its distinctive song and fleeting appearances in dense vegetation, this warbler is uncommon and patchily distributed across its range. It is a species dependent upon dense understory vegetation within forests and it typically forages for arthropods by flipping over dead leaves on the ground. Males defend usually large territories against conspecific males. Pairs are socially monogamous, and females build a large, bulky nest in forest understory that resembles a clump of hanging dead leaves.
Swainson's Warbler breeds in several types of habitat—in the bottomlands and upland ravines of the Mississippi Alluvial Valley and Coastal Plain regions; in early-seral pine plantations of the Coastal Plain regions; and in mixed mesophytic forests of the southern Appalachian Mountains. It is generally scarce in the Southern Piedmont and Interior Low Plateau regions. On its overwintering grounds, Swainson's Warbler is commonly reported to use montane forests, but also occupies swampy areas, dry forest, and mangrove habitat.
The first reports of Swainson's Warbler came from swamps and river floodplain forests, hence its generic name, Limnothlypis, meaning “marsh finch.” First described by John James Audubon in 1834 from a specimen obtained in South Carolina in 1833 by John Bachman of Charleston, the species is named after Audubon's friend and fellow ornithologist William Swainson. However, evidence suggests that the original discoverer of the species was Georgia artist-naturalist John Abbot, who made drawings and watercolors of the bird as early as 1801 (1, 2). After the initial discovery of the Swainson's Warbler, there were few reports of the species for the next 50 years, until 1885 when Arthur T. Wayne located the first reported nest near Charleston, South Carolina (3). The following year, Troup D. Perry (4) reported that he had actually found a nest 21 days earlier than Wayne, but did not report his discovery as quickly.
Many investigators have expressed the difficulty involved with studying the Swainson's Warbler. Specifically, this species' dense habitat, retreating habits, drab coloration, patchy distribution, and large home range can make it challenging to locate and observe. However, persistent field ornithologists have learned much about this bird in recent years. Significant contributions to understanding the species' natural history were made by Brooke Meanley, culminating with the publication of his Natural History of the Swainson's Warbler in the North American Fauna series (1). Since then, Gary Graves has published important articles on the species, including aspects of foraging behavior, breeding biology, and breeding and overwintering habitat use (e.g., 5, 6, 7, 8, 9, 10, 11). Recent research by graduate students in Arkansas, Georgia, Illinois, Louisiana, Missouri, North Carolina, and South Carolina, has also revealed much about the behavior, habitat use, breeding biology, and demography of the Swainson's Warbler. Relatively few studies have focused on the ecology of this species during the overwintering period (e.g., 6, 12, 13), and no studies have specifically examined stopover ecology.
Swainson's Warbler has a global population estimate of 140,000 individuals (14), which is low relative to many other wood-warbler species. Bottomland hardwood forest in the southeastern United States has declined 55–80% (15) and altered hydrology of these forests may limit the Swainson's Warbler to a few relatively high areas not affected by flooding (16, 17, 18, 19, 20, 21). Likewise, a drastic decline in dense stands of giant cane (Arundinaria gigantea), a native bamboo often associated with the Swainson's Warbler, has likely contributed to a decline in their range and numbers. Giant cane once covered extensive areas of the southeastern United States, but now covers only 2% of its historic distribution (15). Across the range, the species could benefit from an increase in large forest tracts with dense understory vegetation. In some areas, productivity may be limited by high rates of nest predation and brood parasitism by the Brown-headed Cowbird (Molothrus ater) (22, 23, 24). Finally, habitat destruction throughout the limited overwintering range of the Swainson's Warbler is a potential threat to the species.