Swainson's Warbler, one of North America's most secretive avian species, is a medium-sized wood-warbler that breeds in the southeastern United States and winters on Caribbean islands, the Yucatán Peninsula, and in eastern Mexico. Known to most birders by its distinctive song and fleeting appearances between patches of 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 often forages for arthropods by flipping over dead leaves on the ground. Males typically defend large territories against conspecific males. Pairs are socially monogamous, and females build a large, bulky nest that resembles a clump of dead leaves.
Swainson's Warbler breeds in several types of habitats: in the bottomlands and upland ravines of the Mississippi Alluvial Valley and across the Coastal Plains regions, in the mixed mesophytic forests of the southern Appalachian Mountains, and in early-seral pine plantations of the Coastal Plains. It is generally scarce in the Southern Piedmont and Interior Low Plateau regions. On its wintering grounds, Swainson's Warbler is commonly reported to use montane forests, but is also known from 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 his colleague John Bachman of Charleston, this species was named by Audubon after his friend and fellow ornithologist William Swainson, but 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 (Meanley 1971c, Simpson 1984b). After the initial discovery of the species, there were few reports of the bird for the next 50 years, until 1885 when Arthur T. Wayne located the first reported nest of the species near Charleston, South Carolina (Brewster 1886c). The following year, Troup D. Perry (Perry 1886) 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 Swainson's Warblers. Specifically, this bird's 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 the understanding of this species have been made by Brooke Meanley, culminating with the publication of his Natural History of the Swainson's Warbler in the North American Fauna Series (Meanley 1971c). Since then, Gary Graves has published a number of important articles on the species (e.g., Graves 1992a, Graves 1996a, Graves 1998, Graves 2001, Graves 2002). Recent research by graduate students throughout the breeding range, including studies in Arkansas, Georgia, Illinois, Louisiana, Missouri, North Carolina, and South Carolina, has also revealed much about Swainson's Warbler. Although some research has focused on this warbler during the non-breeding season (e.g., Graves Graves 1996a, Strong 2000, Strong and Sherry 2001), more work during the winter and migration periods is needed.
Swainson's Warbler is a high priority species of conservation concern (Flight 2007), and has a relatively low estimated global population size of 84,000 individuals (Rich et al. 2004). Bottomland hardwood forest in the southeastern United States has declined 55%–80% (Noss et al. 1995) and altered hydrology of these forests may limit Swainson's Warblers to a few relatively high areas not affected by flooding (Anich and Reiley 2010, Benson and Bednarz 2010b). Likewise, a drastic decline in dense stands of giant cane (Arundinaria gigantea; hereafter: cane), a native bamboo often associated with Swainson's Warblers, has likely contributed to a decline in their range and numbers. Cane once covered extensive areas of the southeastern U.S., but now only 2% of historical canebrakes remain (Noss et al. 1995). 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 Brown-headed Cowbird (Molothrus ater) parasitism (Benson et al. 2010a, Benson et al. 2010b). Finally, habitat destruction throughout the species' limited wintering range is a potential threat and Swainson's Warbler ecology and conservation during winter needs further study (Rappole et al. 1983).