Shooting and Trapping
Pesticides and Other Contaminants/Toxics
Swainson's Warblers disappeared from breeding territories after treatment with nifluridide, an organophosphorus insecticide (239).
Collisions with Stationary/Moving Structures or Objects
Most collisions occur at night during migration along major migration routes, and many collisions appear to be associated with bright lights and poor visibility. Weather conditions linked to cold fronts (e.g., increased wind, fog, and lowered cloud ceilings) also have been associated with mass kills (240, 135, 137, 132).
Longcore et al. (241) examined avian mortality at central and eastern North American communications towers, and estimated that Swainson’s Warbler ranked second in the percentage of the overall population lost to communication towers, with 8.9% annual mortality. Given that this threat seems to be proportionately greater to Swainson's Warbler (perhaps mainly because they have a lower overall population, rather than a particular susceptibility to tower collisions), efforts to reduce tower kills (e.g., 242) may be especially important.
At a TV tower in Leon County Florida, 80 Swainson's Warbler casualties were reported during fall migration from 1956 to 1978. Between March and April 1960–1961, 96 Swainson's Warbler casualties were recorded at the same tower, and one was reported during the summer of 1966 (243). Several other Swainson's Warbler casualties were reported in October from a tower in West Virginia (244). Swainson's Warblers have collided with lighthouses during spring migration at Port Bolivar, Texas and Cay Sal, Bahamas. Several Swainson's Warblers struck the lighthouse in fall at Sombrero Key, Florida (245). Instances of birds colliding with towers, buildings, and other structures at night during migration from early to mid-October have been reported from Tennessee (134, 135, 136, 137) and Campbellsville, Kentucky (132). Taylor and Kershner (127) reported that 45 Swainson's Warblers collided with the Vehicle Assembly Building at the Kennedy Space Center in Florida between 1970 and 1981.
Natural phenomena have also caused mass kills; Wiedenfeld and Wiedenfeld (246) found 5 dead Swainson's Warblers along the beach after a tornado hit Louisiana on 8 April 1993 and estimated that 214 Swainson's Warblers were killed during this event.
Degradation of Habitat: Breeding and Overwintering
Habitat destruction, both on the breeding and overwintering grounds, is likely the greatest threat to the future of this species (247). Bottomland hardwood forests have declined dramatically because of conversion to cropland and pasture, creation of reservoirs, and urbanization. In the Lower Mississippi Valley, the 24% of the forest cover that remains has been highly fragmented and altered (236, 237).
Barrow (176) found that Swainson's Warbler had the smallest mean overall niche breadth of 19 forest species in northeastern Louisiana, indicating the vulnerability of this species to habitat alteration. However, studies in the last 20 years have shown that Swainson's Warblers inhabit and breed in managed forests, including 20–25 year old regeneration stands after clear-cuts (192, 142, 166, 10, 11). Carrie (154) and Bassett-Touchell and Stouffer (157) found Swainson's Warblers breeding in even-age pine stands between 7 and 20 years old. An increasing number of Swainson’s Warblers may be moving into pine habitats, however the relatively short period of suitability of a pine stand (Graves  estimated 8-15 years after planting as the prime window) necessitates stands of various ages be present on the landscape to sustain populations over time.
The demise of once vast canebrakes has undoubtedly harmed this species. Cane removal in a Missouri forest that previously supported several breeding pairs eliminated all but 1 territorial male (248). Residential development in the Appalachians of West Virginia has rendered formerly prime mountain habitat unsuitable for Swainson's Warblers (244). Fragmentation of bottomland hardwood forests and elimination of canebrakes have been proposed as possible causes for the demise of Bachman's Warbler (Vermivora bachmanii) (163, 249), and these factors may also prove detrimental to Swainson's Warbler, which is generally found in heavily forested landscapes (141). Additionally, fragmentation of bottomland forests, and cane habitat specifically, may attract predators, lead to decreased reproductive success, and therefore decrease habitat quality for Swainson's Warblers (22, 23).
Hurricanes are likely to affect both breeding and overwintering grounds for this species. While hurricanes may have short-term negative effects, a site in Louisiana was found to have greater Swainson’s Warbler density in the 3 years following Hurricane Katrina, presumably due to increased openings in the canopy that fostered growth of understory vegetation (250).
Degradation of overwintering habitat may also have harmed this species (251). For example, a correlation between habitat reduction and declining bird populations has been documented in southern Mexico (252). The overwintering distribution of this species is not well known and more research is required. Petit et al. (253) classified Swainson's Warbler as “highly vulnerable” to human alteration of tropical broadleaf forests.
Most remaining bottomland forests are at relatively low elevations and therefore prone to frequent and prolonged flooding, whereas higher elevation sites on which optimal Swainson's Warbler habitat may exist have been converted to agriculture or are highly fragmented (236, 237, 18).
Additional habitat degradation has occurred through alteration of natural flooding regimes; channelization of rivers and the construction of levee systems have influenced the species composition of forests towards a more hydric composition (236, 237, 254). Consequently, much of the remaining Swainson's Warbler habitat may be in marginal areas that are relatively flood prone and subject to significant year-to-year variability (8, 17, 18, 19, 20, 21).
Several authors have documented territory abandonment as a consequence of flooding (8, 255, 16, 17, 19, 21). Post-flooding territory abandonment is associated with changes in habitat structure, specifically, reductions in the leaf-litter cover and shrub layer on which this species depends for nesting and foraging (17). The flood-altered variables that appear to have the largest influence on territory abandonment are decreases in leaf-litter cover, arthropod abundance, and woody stem cover, and increases in cover of herbaceous vegetation (19). The lower-elevation areas that are more flood-prone are the most affected, and flood-induced changes in habitat occupancy can lead to local population declines (195, 16, 19, 21). Although little is known about how flooding impacts reproduction, significant flooding decreases apparent survival of adults, likely through increasing emigration (21).
The most significant human impact on this species has been destruction of breeding and overwintering habitat (see Degradation of Habitat). Research impacts may have been of local importance. Collectors of adults and eggs have been sufficiently efficient at times to eliminate local populations. For example, William Brewster and Arthur Wayne, early Swainson's Warbler researchers, collected 50 specimens in an area near Charleston, South Carolina, in 1884; in 1885 they rarely encountered any individuals, “perhaps thinned by our merciless collecting of the preceding season” (25). Perry (256) recorded that he and George Noble collected 24 clutches of eggs in one season. Winker and coauthors (257, 46, 47) collected 231 individuals from several states in the U.S. (mean = 19 individuals from 11 different breeding locations) and Mexico to conduct genetic studies. The effects of this collection on populations are unknown.
Playbacks are beneficial for research purposes and, in some areas (e.g., Pocomoke Swamp of southern Delaware and northern Maryland), birders have used loud recordings of the species' song to help locate and coax Swainson's Warblers out of thick cover for observation. Partly by disrupting breeding activities, the use of such recordings may have contributed to the almost complete disappearance of the species from this area during the 1970s and 1980s (258).
Negative effects of harness radio-transmitter attachment have been documented, although no nest abandonment occurred when capturing brooding females with 3–10 day-old nestlings in mist nets near the nest and fitting them with transmitters (N. Chartier and J. Gerwin, personal communication). Anich et al. (259) used a glue-on technique to attach radio-transmitters to 40 male Swainson's Warblers and found no detrimental effects based on return rates.
Targeted mist-netting within 5 m of incubating females may cause them to abandon nests (N. Chartier and J. Gerwin, personal communication).