Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Distribution, Migration, and Habitat

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Figure 1. Distribution of Swainson's Warbler.

This species is quite local and sporadic toward the northern limit of its breeding range.

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eBird range map for Swainson's Warbler

Generated from eBird observations (Year-Round, 1900-present)

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Figure 2. Annual cycle of molt, breeding, and migration of Swainson's Warbler.

Data represent molt, breeding, and migration of Swainson’s Warbler in the southeastern United States; birds from higher latitudes and higher elevation begin breeding later in spring than southern birds do. Thick lines show peak activity, thin lines off-peak.

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Swainson's Warbler habitat near a nest in an area dominated by eastern hemlock (Tsuga canadensis).

Franklin Gravely Management Area, Pickens County, South Carolina. Photo by Stanlee M. Miller.

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Swainson's Warbler habitat with overstory of loblolly pine (Pinus taeda).

Washington Parish, Louisiana. Photo by Donata Henry.

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Young loblolly pine (Pinus taeda) plantations occupied by Swainson’s Warblers in Escambia County, Alabama, June 2012.

Mean canopy height is 9.5 m. Photo by Gary R. Graves.

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Young loblolly pine (Pinus taeda) plantations occupied by Swainson’s Warblers in Neshoba County, Mississippi, May 2011.

Mean canopy height is 6.5 m. Photo by Gary R. Graves.

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Swainson's Warbler habitat near a nest in an area dominated by rhododendron (Rhododendron maximum).

Franklin Gravely Management Area, Pickens County, South Carolina. Photo by Stanlee M. Miller.

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Swainson's Warbler habitat dominated by pawpaw (Asimina triloba).
© Nick Anich, Arkansas, United States, 3 May 2008
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Swainson's Warbler habitat dominated by vines (Smilax spp.).
© Nick Anich, Arkansas, United States, 3 May 2008
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Swainson's Warbler habitat dominated by giant cane (Arundinaria gigantea).
© Nick Anich, Arkansas, United States, 3 May 2008
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Overwintering habitat of Swainson's Warbler in Tikal National Biosphere Reserve, Petén, Guatemala.
© John Gerwin, Petén, Guatemala, 22 December 2008
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Overwintering habitat of Swainson's Warbler in Tikal National Biosphere Reserve, Petén, Guatemala.
© John Gerwin, Petén, Guatemala, 22 December 2008

Distribution in the Americas

Breeding Range

Breeds locally from east-central and mid-coastal Texas, southeastern Oklahoma, southern Missouri, and Arkansas to southwestern and eastern Kentucky, western and eastern Tennessee, Alabama, east to West Virginia, western Virginia, the Dismal Swamp area of southeastern Virginia, South Carolina, and south to the Panhandle region of western Florida and the Gulf Coast (Figure 1; 1, 53, 54, 55, 56, U.S.G.S. Bird Banding Lab data, and many personal communications—see Acknowledgments).

Northern Limits of Range

Missouri: Scattered populations occur in the south, with a sizable population along the Current River in the Ozark National Scenic Riverway (57; W. Eddleman, personal communication) and the Eleven Point River (57). Primarily associated with riparian cane in the 2 southernmost tiers of counties (57).

Illinois: Nearly extirpated, occurring mainly in extreme south. In 1976, Eddleman (58) documented 22 males along Cedar Creek and Cave Creek in Jackson County, a site that supported birds since at least 1951 (59). Though single birds occur with some regularity, no breeding was documented from 1992–2007 (G. Williamson, personal communication). In 2008, a pair was present during the breeding season in Alexander County and in 2009 a probable pair and single male were found in Jackson and Johnson counties, respectively (60).

Kentucky: Occurs locally along southeastern and southwestern borders of the state; the species has declined in southwest (61).

Pennsylvania: Nesting has not been confirmed, but there are numerous records of summering individuals in southwestern Pennsylvania, mainly Fayette County (62, 63). The most recent breeding bird atlas project (2004–2009) produced 4 reports of singing males, included one on territory for over a month, but no female was observed (64).

Ohio: Nesting has not been confirmed, but a summering male was collected in Lawrence County on 21 June 1947 (65). Summering birds have rarely, but regularly, been reported in the heavily forested southeastern portion of the state (66). The most recent breeding bird atlas project (2006–2011) produced 2 unsubstantiated reports of singing males in eastern Ohio (Columbiana County), an area with a history of Swainson's Warbler reports, near the Pennsylvania border (67).

West Virginia: Occurs in Appalachian Mountains (68).

Maryland: A small breeding population persisted into the early 2000s at several locations along the Pocomoke River. There have been no breeding season reports since 2004, but the remote interior swamps that may hold birds are rarely searched (69, 70, 71, eBird 2018; J. McCann, personal communication).

Delaware: Only one Swainson's Warbler has been documented since 1988, a male present for several days in May 2005 (eBird 2018). Heckscher and McCann (71) suggest the species may sporadically still occur in the Delaware portion of Great Cypress Swamp, or along the Pocomoke River, Nanticoke River, and Marshyhope Creek drainages, where little survey effort occurs.

Western Limits of Range

Oklahoma: Reported from at least 10 counties; formerly nested in Delaware and Washington counties (72). Breeds regularly at Little River National Wildlife Refuge in McCurtain County (73). Recent sightings also at Tishomingo National Wildlife Refuge, Johnston County (72). In 2007, species found along river drainages in Pushmataha and Delaware counties, and several counties not previously reported to hold birds (Cherokee, LeFlore, Okmulgee, and Muskogee) (M. Revels, personal communication).

Texas: Occurs locally in the eastern third of the state, west to Bastrop County (74, 75).

Southern Limits of Range

Texas: Local in the eastern third of the state; southernmost breeding population occurs in Aransas County (74, 75).

Florida: Uncommon breeder in the north, mainly in the panhandle (76, 77; B. H. Anderson, personal communication).

Other Records

Extralimital occurrences recorded in Manitoba, Ontario, Quebec, Nova Scotia, Wisconsin, Illinois, Indiana, Michigan, Ohio, Pennsylvania, New York, New Jersey, Maine, Massachusetts, Nebraska, Kansas, Colorado, New Mexico, and Arizona. (78, 79, 80, 81, 82, U.S.G.S. Bird Banding Lab data, eBird 2018).

Overwintering Range

Regularly overwinters in the Bahamas, Cuba, Jamaica, Hispaniola, Puerto Rico, and as far east (rarely) as Anguilla (Figure 1; 83, 84, 85, 86, U.S.G.S. Bird Banding Lab data). On the mainland, occurs on the Yucatán Peninsula (including Guatemala and Belize) and also to the west, but distribution from the Yucatán Peninsula to Veracruz is poorly understood (54, 87, U.S.G.S. Bird Banding Lab data). Increasingly, a few individuals are recorded overwintering in southern Florida (eBird 2018). Vagrant records as far south as Venezuela (88).

More study is needed on overwintering distribution. Species may be overlooked on the overwintering grounds because of its secretive nature, infrequent singing, preference for dense undergrowth, and lack of knowledgeable observers in many areas of the overwintering range. The high percentage of overwintering records that come from mist-nets suggests the species may often go undetected. Graves (6) used song playbacks, and detected many more birds on Jamaica than had previously been reported, suggesting use of this technique could improve our understanding of overwintering distribution. N. Chartier, A. Savage, and J. Gerwin (personal communication) used playbacks to capture birds on overwintering grounds in Guatemala. Mist-netting has revealed multiple birds on Útila Island off the coast of Honduras and San Andrés Island off the coast of Nicaragua, which suggests the species may occur farther south along coasts and islands than was previously recognized (89, 90; S. L. Glowinski Matamoros and F. Moore, unpublished data). The large number of overwintering records from islands may indicate the species has an affinity for islands in the Caribbean, or perhaps it is also that birders and banders that have the affinity for Caribbean islands, biasing coverage towards these areas.

United States: Rarely reported overwintering, but several documented records in recent years. Apparently increasing as a overwintering bird in southern Florida, where it is now annual since 2013. One bird present 19–30 January in coastal Louisiana was photographed (91). One Christmas Bird Count (CBC) report for Texas on 14 December 2009 was well described (eBird), although several prior CBC reports for Texas and Florida have insufficient evidence (92, K. Arnold, B. H. Anderson, personal communication). In the 5 years since the first well documented overwintering report from Long Key in southern Florida on 15 January 2013 (93), 9 birds have been well documented from southern Florida during the overwintering period (eBird 2018). There is only one Florida record north of Miami, a bird from the Florida panhandle photographed on 30 January 2017 (eBird).

Bahamas: Species has overwintered on northern and central islands including Abaco, Andros, Eleuthera, Grand Bahama, Great Exuma, New Providince, Little San Salvador, and San Salvador, (U.S.G.S. Bird Banding Lab data, 94, 95, eBird 2018; P. Sykes, T. White, unpublished data). Species known on Bimini, Cat Island, Cay Lobos, and Cay Sal Bank only in migration (U.S.G.S. Bird Banding Lab data, 96, 97, 98, 99; T. White, unpublished data). Like Florida, increasing reports of overwintering on the northern islands in recent years, however birds may be overlooked on many islands owing to lack of coverage and dense habitat (P. Sykes, T. White, personal communication).

Turks and Caicos: Reports of birds on 28 December and 16 March (eBird 2018).

Cuba: Kirkconnell et al. (100) reported 58 individuals during the overwintering periods of 1991–1994, suggesting the species is a regular overwintering resident.

Jamaica: Lack and Lack (101) used a walking survey method and found Swainson's Warblers using montane, mid-level, and lowland riverine forests in Jamaica. Graves (6) located 42 individuals in Jamaica using audio playbacks and estimated that the Blue and John Crow Mountains National Park could hold between 17,000 and 25,000 birds. Birds also occur in dry limestone forest, second growth scrub, and shade-coffee plantations (102, 12, 13, 103, 104).

Cayman Islands: Photos and recordings indicate an overwintering population at Mastic Trail on Grand Cayman (eBird 2018).

Hispaniola: Captures of 20 birds between 1997 and 2003, in both the Dominican Republic and Haiti, and the documentation of overwinter site fidelity suggest the species is an uncommon but regular overwintering resident in some areas (105, 106, 107, 85, 108).

Bermuda: A rare but probably annual overwintering resident (A. Dobson, personal communication).

Puerto Rico: Sporadic overwintering resident; Raffaele (83) lists one record and J. Faaborg (personal communication) had 8 captures in 432 net-days of mist-netting in dry subtropical forest, including capturing one bird in 2 different years (109). Norton et al. (110) suggested that the species may be an uncommon visitor in the higher karst mountains.

U.S. Virgin Islands: Three records from St. John (83).

Anguilla: One bird captured in mangrove habitat on 18 Jan was likely overwintering (86).

Mexico: The bulk of overwintering birds are reported from Quintana Roo (including Isla Cozumel), and Campeche, but have also been reported in Yucatan, Tabasco, Chiapas, Veracruz, and Oaxaca (111, 112, 87, 113, 114, 115, eBird 2018). One bird captured in a mist-net on 17 January 2008 at El Cielo Biosphere Reserve in Tamaulipas was north of the known overwintering range (R. Brito-Aguilar, personal communication; eBird). More study is needed to determine the extent of overwintering range west of the Yucatán Peninsula, but given the rest of their distribution and apparent northward overwintering trend, it would not be surprising if they occasionally overwintered up the gulf coast towards Texas.

Belize: An uncommon but expected overwintering visitor, most commonly seen on the cayes (116).

Guatemala: Overwinters in the north and along the coast (87, 117); sightings also documented at Tikal National Park, Petén (118). In December 2008, N. Chartier, A. Savage, and J. Gerwin (personal communication) spent approximately 60 hours conducting tape playback surveys and found 11 birds in Tikal National Park, Petén (capturing 5) and 4 birds in Uaxactun, Petén (capturing 3). Birds also captured along the coast of Izabal in February and March, and one banded bird was recaptured the following year (N. Komar, C. Robbins, personal communication; U.S.G.S. Bird Banding Lab data).

Honduras: In 2005, 43 birds were captured between mid-September and mid-November on Útila Island, and recaptures indicated 3 of those birds were present for at least 22, 34, and 47 days, which suggests overwintering (S. L. Glowinski Matamoros and F. Moore, unpublished data). Also apparently overwinters regularly on Islas de la Bahía (eBird 2018). Reports from 16 November 2013 (eBird) and 11 February 2014 (eBird) in Atlántida. One overwintering record for the Swan Islands, and another at Lake Yojoa (119).

El Salvador: One bird was captured 11 October at 2000 m in a cloud forest at Santa Ana Volcano, Los Volcanes National Park was presumably a vagrant (120; O. Komar, personal communication).

San Andrés Island, Colombia (east of Nicaragua): 13 birds were captured between 21 September and 18 March 2004–2007 (89), and Salaman et al. (90) describe it as regularly encountered there in the nonbreeding season.

Three reports in extreme south of the overwintering range. In Panama, one bird was captured 13 March near the Panama Canal (121), there was one sight record on 2 February from mangrove forest in northwestern Venezuela (88), and 2 individuals were reported on Aruba 8 September 2011 (eBird).

Distribution Outside the Americas

No records.

Nature of Migration

Medium-distance migrant moving between its breeding range in the southeastern United States and overwintering range in southeastern Mexico, Belize, Guatemala, and Caribbean islands. Essentially no overlap between breeding and overwintering ranges (Figure 1), although one bird was known to overwinter in southern Louisiana (91). Generally, our understanding of Swainson's Warbler biology during the migration period is lacking due to the secretive nature of the species; most observations are limited to occasional mist-net captures and night kills at communication towers.

Timing and Routes of Migration

Individuals apparently follow direct routes between overwintering and breeding grounds (1), which likely includes both trans-Gulf and cirum-Gulf migration (migration over and around the Gulf of Mexico, respectively). Nothing is known about migration routes taken by individual birds, as no banded Swainson's Warbler has ever been recovered > 16 km away from its original banding location (U.S.G.S. Bird Banding Lab data).

Lowery (122) and Stevenson (80) consider Swainson's Warbler a trans-Gulf migrant, and Meanley (1) noted that “the northern coast of Yucatan is a natural departure point for trans-Gulf flight to the Gulf Coast of the United States.” However, data from mist-net sampling to confirm these inferred migration routes are sparse. For example, spring capture rates at Johnson's Bayou on the Gulf Coast in western Louisiana were 0.44 Swainson's Warblers/1,000 net-hr (123) and Simons et al. (124) averaged 1.13/1,000 net-hr on a barrier island in Mississippi. Net captures were even less frequent in the fall at Fort Morgan along the Gulf Coast in Alabama (0.12 Swainson's Warblers/1,000 net-hr (123). Although data are limited, mist-net captures suggest the majority of birds reach the Gulf Coast east of Chenier Plain, Louisiana (W. C. Barrow, personal communication). Landfall locations may be dependent on wind patterns during spring.

Crawford (125) suggested the spatial distribution of carcasses recorded in fall beneath the Tall Timbers TV tower near Tallahassee Florida (20 southwest of tower and 16 southeast of tower) reflected both trans-Gulf migrants and migrants flying over the Florida peninsula, respectively. However, the effectiveness of determining migration route based on carcass position beneath a tower is uncertain (E. Macchia, personal communication). Two studies demonstrated movement along the Florida peninsula; Taylor and Anderson (126) recorded 48 birds killed during 3 autumns beneath a tower in central Florida, and Taylor and Kershner (127) found that a large building on the Atlantic coast caused kills of 39 birds in spring and 6 in fall. Birds from the West Indies likely reach the U.S. by island-hopping to southern Florida, as illustrated by March and April records from the Bahamas (1).

Limited data suggest only a few individuals overwintering in eastern Mexico migrate around the Gulf through northeastern Mexico and the Texas coast. Out of 5,174 spring migrants captured on the coasts of Texas and Louisiana in 1993, only 5 were Swainson's Warblers (W. C. Barrow, personal communication).

Swainson's Warbler is typically the last of the southern-breeding warbler species to reach the breeding grounds in spring (128), although this species migrates earlier than many of the more northern-breeding species. On overwintering grounds in Jamaica, residual body mass of Swainson's Warblers showed a significant increase from 20 February to 7 April, as birds increased fat reserves for migration (13). Spring migration occurs from mid-March through May (see Figure 2), with birds at the northern end of the range arriving on the breeding grounds later than their southern counterparts. Swainson's Warblers arrive in late March along the Gulf Coast and as late as late April in the northern part of the range and in the mountains (27, 1, 58, 129). Migrants reported in Florida as early as 15 March and as late as 19 May (77). Extreme dates on the Louisiana coast are 27 March and 16 May, with peak arrival between 8–18 April (J. V. Remsen, personal communication). First arrival at a breeding site in Desha County, Arkansas was regularly 8–9 April (16), and in Kanawha County, West Virginia, 15–23 April (130). Peak arrival of males in eastern Arkansas is approximately April 13–24 (NMA, TJB). Individuals have been reported in Veracruz, Mexico, as late as 7 April (131), and in Cuba as late as 14 April (100). Data from birds marked with radio-transmitters indicated that birds left Jamaica between March 26 and April 20 (A. Brunner, personal communication).

Fall migration route essentially reverses that of spring (1), but the Atlantic coasts of Georgia and Florida apparently funnel birds migrating south to the Bahamas and other Caribbean islands. Fall migration occurs mid-August to mid-December, with most birds migrating mid-September to mid-October. Migrants have been recorded in southern Florida as early as 6 August and as late as 6 December (dead on road; 77). A tower in central Florida killed 27 Swainson's Warblers on 29 and 30 September (126). A single bird discovered under a TV tower in central Kentucky on 17 October was thought to have been killed 12 October (132). Extreme dates for fall migration in Louisiana are 24 August and 27 October (J. V. Remsen, personal communication), plus one overwintering record (91). Earliest reported dates for fall arrival, which may represent birds overwintering or passing through, include 12 September on Útila Island, Honduras (S. L. Glowinski Matamoros and F. Moore, personal communication), 15 September in Cuba (100), and 20 September in Veracruz, Mexico (131). Bradley (133) reported 3 birds on the Cayman Islands on 16–17 December.

Migratory Behavior

Migration appears to be primarily nocturnal, based on birds colliding with towers and objects at night (134, 135, 136, 137). Swainson’s Warblers typically depart Jamaica during spring migration one hour after sunset (B. Dossman and A. Brunner, unpublished data). After crossing large bodies of water during spring migration, individuals have remained in the area of first landing for ≥ 3 days to renew energy reserves (A. Stedman, personal communication).

May migrate in small groups, based on tower kills (135, 137). B. Sargent (personal communication) reported good numbers of migrant Swainson's Warblers (up to ~30 birds in one day) on the Alabama coast in early April, and a foraging flock of 18 migrants in Belize on 12 March.

Males arrive on nesting areas several days to 2 weeks prior to females, (138, 139, NMA, TJB). Taylor and Anderson (126) reported that 27 birds killed on 29 and 30 September at a tower in Florida included both sexes, and both immatures and adults.

Control and Physiology of Migration

No information on control of migration.

In coastal Louisiana, Swainson's Warblers (n = 6) had a stopover duration of 1.67 d ± 0.82 SD, with an average mass gain of 0.58 g ± 0.49 SD (0.41 g/day ± 0.46 SD), after a northward trans-Gulf flight (140). This gain equaled 4.3% (± 3.9 SD) of a bird's body mass, or a 3.0% (± 0.8 SD) increase/day. Five of 6 Swainson's Warblers in this study gained weight during their brief stopover; one that lost weight was in good condition initially.

Habitat in Breeding Range

Swainson's Warbler uses a variety of habitats during the breeding period, including bottomland hardwood forests, mixed-mesophytic montane forests, and early-seral pine stands. Species typically found in areas with shaded and dense understory, abundant leaf litter, and little herbaceous ground cover. Generally found in large contiguous forests, but within these can occupy different age classes of habitat provided the appropriate structure exists (141).

In bottomland forest, uses moist but not inundated areas, but may occasionally forage along wet areas (139). Currently and historically, most Swainson's Warbler habitat has existed in relatively high-elevation and infrequently flooded bottomland hardwood forests. Species can withstand short-duration floods (16), but significant flooding can decrease habitat suitability and lead to local population declines (17, 18, 19, 21). Stands of giant cane, dwarf palmetto (Sabal minor), sweet pepperbush (Clethra alnifolia), or spicebush (Lindera benzoin) are commonly occupied vegetation types, and these areas often have a significant vine component, especially greenbrier (Smilax spp.) and grape (Vitis spp.; 139, 8, 9, 142, 143, 144, 11). Similarly, stands with understories dominated by vines, particularly greenbrier, are commonly occupied in some areas (145, 146, 147, 11). Territories in the eastern Piedmont of North Carolina (Richmond and Anson counties along the Great Pee Dee River) can have understories dominated by Chinese privet (Ligustrum sinense; D. McNair, personal communication) Seedling plots of the rare and endangered pondberry (L. melissifolia) were used by Swainson’s Warbler in Mississippi (148).

Habitat in Aransas County, Texas, consists of dense stands of live oak (Quercus virginiana), with dense understory of greenbrier, American beautyberry (Callicarpa americana), and yaupon holly (Ilex vomitoria), with little herbaceous ground cover (W. C. Barrow, personal communication).

Species occurs in at least 2 habitat types in the Appalachian Mountains: (1) communities dominated by rhododendron, mountain laurel (Kalmia latifolia), eastern hemlock (Tsuga canadensis), and American holly (Ilex opaca), and (2) mature mountain cove hardwoods, including yellow poplar (Liriodendron tulipifera), oak (Quercus spp.), and maple (Acer spp.) associations with understories of spicebush and greenbrier (149, 130, 150, 151). One territorial male was occupying a patch of invasive Japanese Knotweed (Reynoutria japonica) in West Virginia (152). Upland forests occupied by Swainson's Warblers on Crowley's Ridge in eastern Arkansas resemble mature mountain cove hardwoods and generally have understories dominated by spicebush, pawpaw (Asimina triloba), and greenbrier, with giant cane occurring along streams (153, 143).

Swainson's Warbler reported in 3–20 yr-old loblolly pine plantations (154, 155, 142). Wilson and Watts (156) reported use of 19–22 yr-old loblolly pine (Pinus taeda) stands 5 years after the first thinning and 28–29 yr-old stands 3 yr after the second thinning. Breeding also reported in 7–24 yr-old, unthinned, even-aged pine stands (142, 157, 158). Graves (10) examined 297 territories with pine across the breeding range, and found pines in most occupied stands to be between 6–12 m tall (8–15 years after planting), with no territories in pine less than 4 m tall and very few in pines over 14 m tall. Graves (10) found most occupied pine stands to consist of only loblolly pine, but some stands had a mix of loblolly pine with slash pine (Pinus elliotii) or longleaf pine (P. palustris). Understory plants in pine plantations consist of yaupon holly, Chinese privet, huckleberry (Gaylussacia spp.), greenbriar, Viburnum spp., and wax myrtle (Morella cerifera) and often the edges of pine stands feature a large amount of broadleaf vegetation (154, 142, 157, 10).

Overall, key components of Swainson's Warbler breeding habitat include dense canopy cover with occasional disturbance gaps that function to maintain dense shrub-level vegetation for nesting, abundant leaf litter, sparse herbaceous vegetation, moist soils, appropriate hydrologic regimes, and substantial forest cover at the landscape scale (1, 58, 159, 7, 8, 9, 160, 161, 143, 144, 146, 11).

Historically, Brewster (25) and Meanley (69) noticed a close association between the presence of giant cane and of Swainson's Warbler; Brewster even suggested that cane may be required by this species. However, recent studies (e.g., 8, 9, 161, 10, 11) have found that cane is not required, although it may be preferred over other habitat types, when present (143, 144, 162). Although Swainson's Warbler uses areas with cane stem densities as high as 25,000–80,000 cane stems/ha (1, 163, 164, 159, 143, 144, 146) the species also commonly uses areas with as few as 0–5,000 cane stems/ha (8, 9, 165, 166).

In a study of breeding habitat in the core of the breeding range (9, 11), canopy height, basal area, and floristics appeared to be relatively unimportant provided that suitable understory was present, perhaps explaining why Swainson's Warbler uses a wide variety of habitats. Whether cane is present or not, a high density of woody stems that provide a dense understory structure seems to be required in all habitat types (1, 9, 165, 142, 145, 157, 144). Graves (9) suggested that 30,000–50,000 small woody stems/ha, may provide high quality cover for nesting and foraging throughout the breeding range, although Graves and Tedford (11) revised this number to > 45,000 small woody stems/ha (4.5/m2).

Sites occupied by Swainson's Warblers, used areas within home ranges, and nest sites all have dense understory structure relative to randomly selected or unused locations (145, 157, 143, 144, 162, 167). Moreover, areas that are uniformly dense rather than consisting of isolated patches of dense vegetation appear to be higher quality habitat (143, 144, 146, 147, 17, 162).

A well-developed canopy structure also appears to be essential across all habitat types (164, 159, 141, 165, 142, 157, 143, 144, 162). In many cases, this well-developed canopy may result from overstory or midstory trees filling canopy gaps over the dense understory vegetation that results from disturbances. However, this structure can also be achieved > 7 yr following a timber harvest when relatively small trees form a dense, short, canopy that shades the understory and leaf litter (147, 11). Graves (8) suggested that this warbler is currently associated with early-successional forest, but was historically associated with disturbance gaps in mature forests. Mosaics of regenerating clearcut or selectively harvested stands of various ages often support relatively large breeding populations of Swainson's Warbler (145, 166, 147, 11). McClure and Hill (168) found that Swainson’s Warblers were more likely to abandon sites near human development later in the season, and hypothesized this was due to greater nest failure or parasitism by Brown-headed Cowbirds at these sites.

Abundant leaf litter is an important habitat component across all habitat types; indeed, both cover and depth of leaf litter are associated with habitat use, likely because Swainson's Warblers forage for arthropods in litter (see Diet and Foraging: Feeding); associated with this well-developed layer of leaf litter, key habitat generally has few herbaceous plants, possibly because these obstruct foraging (7, 8, 9, 165, 142, 157, 143, 144, 162, 167). Indeed, following large flooding events, Reiley et al. (19) found that territories that were most likely to become abandoned experienced the greatest decreases in litter cover and increases in herbaceous vegetation. Brown et al. (169) found that total abundance and taxonomic richness of arthropods in the leaf litter were associated with presence of Swainson’s Warbler.

See also Breeding: Nest Site.

Habitat in Migration

Little information; probably uses habitat similar to that used during the breeding season (170).

Habitat in the Overwintering Range

Wide spectrum of habitats used on overwintering range; more study needed regarding preferences. Species does not appear to be restricted to particular elevations during winter, but is probably restricted to forests with a well-developed leaf litter layer and abundant, diverse litter fauna (6, 13).

In Jamaica, recorded in mangrove (Rhizophora), lowland riverine, middle elevation, and montane natural habitats, but not in lowland arid habitats, gardens, or parklands (101, 94). Graves (6) reported 42 Swainson's Warblers in undisturbed and slightly modified montane forest in the Hardwar Gap region of Jamaica's Blue Mountains, of which six were in dry lowland woods and 36 were in damp forest. Also in Jamaica, Swainson's Warblers inhabited dry limestone forests, second-growth scrub, and shade-coffee (Coffea sp.) plantations (12, 13). The dry limestone forest had canopies dominated by burn wood (Metopium brownii), gumbo-limbo (Bursera simaruba), and broom thatch (Thrinax parvifolia) while the subcanopy was dominated by crab wood (Ateramnus lucidus) and lance wood (Oxandra lanceolata). Strong (12) reported that second-growth scrub habitat used by Swainson's Warblers was dominated by logwood (Haematoxylum campechianum) while shade-coffee plots (0–1 per 5 ha) had well-developed overstories consisting primarily of moruro blanco (Pseudalbizzia berteroana) or guamo (Inga vera).

On Jewfish Cay, Bahamas, Swainson's Warblers were netted primarily in mixed wet cattail (Typha sp.) and shrub habitat (M. Case, personal communication). On Andros Island, species captured in low numbers in dry scrub habitat (94), but no netting was done in moister habitats. On the Yucatán Peninsula, Swainson's Warblers are thought to occur only in moist areas with medium-height vegetation (171). In Chiapas, Mexico, seven Swainson's Warblers were reported in karstic tropical evergreen forest (115). Records in Cuba indicate this species occurs in the lowlands, montane region, and swampy areas, while they apparently prefer semideciduous forest with high shrub and tree stem density, complete or nearly complete canopy cover, and abundant dry leaf litter near streams or water holes (100).

On Hispaniola, 19 of 20 birds captured were in montane wet broadleaf forest at 1,000–1,800 m above sea level (108). Estimated stem densities ranged from 5,700 to 13,400 stems/ha. The single outlier was in lowland rainforest (50 m above sea level, 2,000 stems/ha) (108).

S. G. Matamoros and F. Moore (unpublished data) captured 43 Swainson's Warblers in remnant patches of tropical dry forest at Útila, Bay Islands, Honduras. Mature subtropical dry forest also supports the species in southwestern Puerto Rico (109). In San Andrés Island, Colombia, birds were mist-netted mainly in shrubland, but also in young second-growth forest and mangroves (89). In Venezuela, a vagrant Swainson's Warbler was found in undergrowth beneath 7-m-tall white mangrove (Laguncularia racemosa) forest (88). In El Salvador, also south of the typical range, one bird was reported in 2,000 m cloud forest (O. Komar, personal communication).

Historical Changes to the Distribution

Swainson's Warbler was once thought to breed only in bottomland hardwood forests. However, by the 1930s the species was also discovered breeding locally at elevations of 300–900 m in the Appalachian Mountains. This was thought by some to be a recent range expansion (172), but it was more likely a previously undiscovered population of this inconspicuous bird. Recently, the species has been found breeding in loblolly pine habitat as well (154, 142, 157). The use of such early-successional pine stands appears to be a recent phenomenon, and birds are increasingly being found using pine (10).

Range and distribution remain imperfectly known, and the Swainson's Warbler is still being discovered as a breeding bird in areas where it was not previously recorded, probably because the dense understory habitat the species occupies can be somewhat dynamic, and changing management practices or successional changes can impact habitat suitability. In Texas, individuals were reported throughout the breeding season during the 1980s and early 1990s as far west as Bastrop County (G. Lasley, personal communication) and as far south as Aransas County (W. C. Barrow, personal communication), when they were previously thought to be confined to the eastern portion of the state. In 2007, individuals were found along river drainages in 4 Oklahoma counties where the species had not been previously reported (M. Revels, personal communication).

In other areas, the breeding range has contracted. Conversion of bottomland forest to other land uses, loss of dense understory and/or giant cane within forests, and changing hydrological regimes are apparent causes of loss of Swainson's Warbler populations at such sites. Many of the range reductions are along the northern periphery, where suitable forest habitat may be less contiguous or persistent, or subject to habitat change for a variety of reasons. In West Virginia, shade-tolerant rhododendron (Rhododendron spp.) is not as susceptible to the negative effects of canopy closure, but large populations of white-tailed deer (Odocoileus virginianus) may be overbrowsing the rhododendron and contributing to loss of understory habitat (J. Buckelew, personal communication). Within recent decades, Swainson's Warbler has likely been extirpated from southern Delaware (71), and has also declined in northeastern Florida (173), West Virginia (J. Buckelew, personal communication), and western Kentucky (61). The species is now absent from poorly-known areas of its former range in central Kentucky (61). A few individuals likely breed in southern Illinois (60), but this area no longer holds robust populations.

Data are insufficient to document broad historical changes to distribution on the overwintering grounds. The recent instances of birds beginning to overwinter in southern Florida indicates the overwintering range is creeping northward (eBird 2018).

Fossil History

There are no positive identifications for this species in the fossil or prehistoric record, although Brodkorb (174: 196) lists a record by Bernstein (175) under Limnothlypis swainsonii without any explanation. Bernstein (175: 281) referred the anterior portion of a rostrum from the late Pleistocene (< 400,000 years before present) of Dominican Republic: Cerro de San Francisco to “Parulidae, gen. et sp. indetermined.” This rostrum, discovered in a cave along with numerous other vertebrate remains in Dominican Republic in a deposit thought to be from the late Pleistocene, most closely resembles that of Swainson's Warbler in size and shape but has some subtle differences with modern specimens, including the shape of depression in the palate and the robustness of the maxillary nasal process (175).

Recommended Citation

Anich, N. M., T. J. Benson, J. D. Brown, C. Roa, J. C. Bednarz, R. E. Brown, and J. G. Dickson (2019). Swainson's Warbler (Limnothlypis swainsonii), version 3.0. In The Birds of North America (P. G. Rodewald, Editor). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bna.swawar.03