Several studies of Swainson's warbler have been performed in "bottomland hardwood forests". These are forests near rivers and include a mosaic of swamps, floodplain forests, and riverfront vegetation [43]. Oaks (Quercus spp.), elms (Ulmus spp.), and sweetgum (Liquidambar styraciflua) are usually major components of the canopy in these habitats [3,43,61], but several other species may be present including bald cypress (Taxodium distichum) [61], sugarberry (Celtis laevigata), ashes (Fraxinus spp.), maples (Acer spp.), and yellow-poplar (Liriodendron tulipifera) [43]. The understory is variable but typically includes saplings of canopy and subcanopy trees, cane (Arundinaria gigantea), dwarf palmetto (Sabal minor), holly (Ilex spp.), and/or vines such as greenbrier (Smilax spp.) and peppervine (Ampelopsis arborea) [3,43,52,61].
Swainson's warblers often occur in areas with cane-dominated understories [4,45,52,61]. These habitats occur in several coniferous and deciduous forests of the southeast. Cane-dominated stands are sometimes referred to as canebrakes [43,69]. In southern Missouri, 75% of canopy trees in canebrakes occupied by Swainson's warbler were either boxelder (Acer negundo, 34%), American elm (Ulmus americana, 13%), hackberry (Celtis occidentalis, 12%), honey-locust (Gleditsia triacanthos, 10%), black walnut (Juglans nigra, 4%), or American hornbeam (Carpinus caroliniana, 3%) [69]. More information is available in the Distribution and Occurrence section of the FEIS review of cane.
In addition to the plant communities listed above, Swainson's warbler is associated with the rhododendron-mountain laurel-eastern hemlock-American holly (Rhododendron spp.-Kalmia latifolia-Tsuga canadensis-Ilex opaca) community type of the Appalachian Mountains [40].
Swainson's warblers require a well-developed understory. Four study areas in Arkansas comprised mainly of bottomland hardwood forest had significantly (P<0.001) greater understory density in occupied (10.9 stems/m²) than random (5.1 stems/m²) sites. Occupied sites also had greater (P=0.022) shrub cover (13.3%) than unoccupied sites (10%) [4]. There was significantly (P<0.01) greater cover of low vegetation (0-5 feet (0-1.5 m)) at nests sites (57%) than at random sites (40%) within Swainson's warbler territories in a managed hardwood forest in South Carolina [52]. The 2 most important variables in models predicting Swainson's warbler territories in Bond Swamp National Wildlife Refuge in Georgia were cane density and the combined density of shrubs, vines, and saplings [45]. On 5 sites in Arkansas, Louisiana, Mississippi, and Florida, Swainson's warblers were observed in the greatest abundance (10-20 territorial males/km²) on sites with many (620-820 stems/ha) small (<25 cm DBH) trees and thickets of understory vegetation. Median stem densities on these sites ranged from 31,592 stems/ha to 48,281 stems/ha. Researchers concluded that stem densities from 30,000 to 50,000 stems/ha provide the cover necessary for high-quality Swainson's warbler habitat [27]. In southern Illinois, the average stem density in Swainson's warbler territories was 26,390 stems/ha. No Swainson's warblers were detected in areas with <5,000 stems/ha, and in 40 of 53 plots stem densities were greater than 10,000 stems/ha [18].
J. Michael Meyers, USGS Patuxent Wildlife Research Center
Other understory vegetation has been associated with Swainson's warblers. Vines may be an important component of Swainson's warbler habitat. In a managed bottomland hardwood forest in South Carolina, success rate of Swainson's warbler nests located in vines was higher (63%, n=8) than success rate of nests in cane (20%, n=5). In addition, categorical abundance of vines was the variable that was most consistently included in models for predicting Swainson's warbler occurrence. When cane and vines were abundant there was a 59% probability of detecting a Swainson's warbler on a site. Vine species on these sites included greenbrier, grape (Vitis spp.), trumpet creeper (Campsis radicans), and peppervine [52]. In a microhabitat model for predicting Swainson's warbler occupancy of sites in South Carolina, number of vines was positively associated with Swainson's warbler presence [47]. Swainson's warbler territories in the Great Dismal Swamp had greenbrier (Smilax laurifolia and S. rotundifolia) tangles significantly (P<0.001) more often (x = 3,300 stems/ha; range = 0 to 41,000 stems/ha) than unoccupied areas (median=0; range = 0-6,000 stems/ha). Redbay (Persea borbonia) was also significantly (P<0.001) more dense in Swainson's warbler territories. Coastal sweetpepperbush (Clethra alnifolia) occurred at significantly (P<0.001) lower densities in Swainson's warbler territories [26]. Foraging sites in northeastern Louisiana had significantly (P≤0.05) greater dwarf palmetto density than random sites (4.0) [3].
Although several sources suggest that Swainson's warbler habitat has little herbaceous vegetation [18,40,43], the relationship between Swainson's warblers and herbaceous ground cover is uncertain. For example, southern Missouri canebrakes occupied by Swainson's warblers had significantly (P≤0.05) less herbaceous ground cover (24.4%) than unoccupied areas (34.6%) in 1992, but significantly (P≤0.05) more (35.6%) than unoccupied areas (20.8%) in 1993 [69]. Despite no significant differences in forb cover at 3 other Arkansas study areas, Swainson's warblers were detected on sites with significantly (P<0.001) less forb cover (19.6%) than random sites (49.7%) in bottomland hardwood forest in White River National Wildlife Refuge [4]. In contrast, herbaceous ground cover in Swainson's warbler territories and adjacent unoccupied areas was greater (55-90%) than in unoccupied control plots in the bottomland forest of Big Hammock Wildlife Management Area [61]. In bottomland hardwood forest in Louisiana, the density of ground foliage in plots centered on Swainson's warbler feeding locations were in accordance with availability [3].
Swainson's warbler habitat generally has a closed canopy. Sites in 4 Arkansas study areas where Swainson's warbler were detected had significantly (P=0.024) greater canopy cover (89.3%) than unoccupied sites [4]. In bottomland forest of southern Illinois, sites where male Swainson's warblers were observed singing had >55% canopy cover and typically canopy cover exceeded 75% [18]. Despite occupied and adjacent plots in Big Hammock Wildlife Management Area in Georgia having fewer trees than control plots, canopy closure was still high, ranging from 70% to 100% [61]. Although similar to canopy closure in unoccupied (x = 82.5%) canebrakes in Missouri, sites where Swainson's warblers were detected had high canopy coverage (x = 84.1%) [69]. However, Swainson's warblers can occur in or near areas with open canopies [11,27].
Species composition of the canopy does not have a major influence on Swainson's warblers. Floristics were relatively unimportant on 5 sites in Arkansas, Mississippi, Louisiana, and Florida [27], in lowlands of southern Illinois [18], and in the Great Dismal Swamp. Although black tupelo (Nyssa sylvatica) had a negative relationship with Swainson's warblers in several accurate habitat occupancy models and occurred at significantly (P<0.001) lower basal area in Swainson's warbler territories, it is possible this relationship is due to a significant (P≤0.002) positive correlation (rs= 0.47) between basal area of black gum and pooled water and not avoidance of the species itself [26].
Swainson's warbler's association with other canopy characteristics is more ambiguous. On 5 sites in Arkansas, Mississippi, Louisiana, and Florida canopy height was a relatively unimportant Swainson's warbler habitat feature [27], and in Missouri canebrakes significant (P<0.05) associations with canopy height and diameter at breast height were only observed in 1 of 2 years. The minimum canopy height of occupied canebrakes was 12 feet (3.6 m) [69]. In southern Illinois, Swainson's warblers were not observed in forests with tree heights less than 25 feet (7.6 m) [18]. In the Great Dismal Swamp Swainson's warbler territories had significantly (P=0.001) fewer medium-sized (10-16 inches (25-39.9 cm)) trees, while the number of larger trees (>16 inches (40 cm)) was similar on unoccupied and occupied sites [26].
Swainson's warblers occur in habitats at various stages of succession. In bottomland hardwood forests in North Carolina, Swainson's warblers were captured at a rate of 0.24 birds/100 mist net hours in young stands (6-8 years), 0.38 birds/100 mist net hours in intermediate-aged stands (17-21 years), and 0.71 birds/100 mist net hours in mature stands (>60 years) [15]. In South Carolina, Swainson's warblers occurred at a frequency of 0.04 in mature (≥40 years) upland hardwood forest and 0.11 in a young (≤17 years) white pine stand [37]. In the lowlands of southern Illinois, Swainson's warblers were found in late successional forests and in old fields surrounded by mature forest [18]. On 5 sites in Arkansas, Mississippi, Louisiana, and Florida, Swainson's warbler territories in mature forest were generally near disturbance gaps [27].
Nesting: Nests are typically found at or below about 6 feet (2 m) in various vegetation. In the Great Dismal Swamp in Virginia, the average height of Swainson's warbler nests was 4 feet (1.3 m) and ranged from 18 inches (0.5 m) to 6 feet 3 inches (2 m). Nests were built in abundant understory species, including greenbrier, Japanese honeysuckle (Lonicera japonica), coastal sweetpepperbush, and cane [43]. In Franklin L. Gravely Wildlife Management Area, 2 nests were found in eastern hemlock at 6 feet (1.9 m) and just under 7 feet (2.1 m) [46]. A review notes occurrence of nests from just over 1 to 10 feet (0.4-3.1 m) in saplings, shrubs, and vines [7].
Nests are often placed near patches of dense vegetation, typically associated with canopy gaps. In a managed hardwood forest in South Carolina nests were significantly (P<0.05) closer to vine masses (11 feet (3.3 m)) than were random territory plots. They ranged from 10 to 16.4 feet (3-5 m) from these vine masses, which occurred in canopy gaps that averaged 22,600 feet² (2,100 m²) [52]. Meanley [43] found nests typically occurred along the edge of canebrakes. Similarly, a summary of a report from a hardwood forest in West Virginia states that Swainson's warblers' nests were typically placed within 50 feet (15 m) of, but not within, the densest vegetation [43]. In addition, Eddleman [18] notes a report of 2 nests occurring on the edge of canebrakes in southern Illinois.
Other vegetation characteristics at nest sites were reported by [52,69]. In a managed hardwood forest in South Carolina, cover of low vegetation (0-5 feet (0-1.5 m)) was significantly (P<0.01) greater at nest sites (57%) than at random sites within Swainson's warbler territories (40%). In contrast, vegetation cover at the highest stratum (5-8 feet (1.5-2.5 m)) was significantly (P<0.01) lower at nest sites (3%) compared to unused sites (13%). Overall, nest sites typically occurred in areas with 20% to 30% ground cover, 60% to 65% cover at 3 to 5 feet (1-1.5 m) above ground, and 65% to 75% cover at 5 to 8 feet (1.5-2.5 m) above ground [52]. In 1993, nest sites (n=17) were in areas with significantly (P=0.009) taller cane (10 feet (3.1 m)), lower (P=0.047) grass cover (6.9%), and higher (P=0.051) leaf litter coverage (58.4%) than random sites (cane height = 9 feet (2.7 m), grass cover = 15.4%, leaf litter cover = 49.2%) in canebrakes of southern Missouri [69].
Foraging: Swainson's warbler foraging habitat may be less dense than nesting habitat at and just above ground level [3,25,27,43]. Swainson's warblers in 70 locations throughout the southeast tended to forage in vine tents and shady glades less of than 430 feet² (40 m²) [25]. In addition, a review notes use of small <50 feet² (5 m²) areas with few obstructions near ground level by foraging male Swainson's warblers [43]. However, the ground foliage densities on foraging sites in northeastern Louisiana were in accordance with availability [3].
It is possible that foraging occurs in more interior locations within a stand. Swainson's warblers were not observed foraging on the forest edge in Jamaica [36].
According to reviews [7,43,63], Swainson's warbler breeds in the southeastern United States and winters on Caribbean Islands and portions of the Yucatan Peninsula. Swainson's warbler's breeding range extends along the Gulf Coast from northwestern Florida to eastern Texas, north through eastern Oklahoma, and east through southern Missouri, southern Illinois, and parts of Tennessee, Kentucky, Virginia, and West Virginia to the coast of North Carolina. Although rare, Swainson's warbler also breeds in Maryland and Delaware. Swainson's warblers migrate through southern Florida or along the Gulf Coast to wintering grounds on the Yucatan Peninsula including northern Guatemala, Belize, and the Mexican states of Quintana Roo and Campeche. Swainson's warblers also overwinter in Cuba, Jamaica, the Bahamas and other Caribbean islands [7,43,63]. A map of Swainson's warbler's range can be found at NatureServe.
The following lists are speculative and are based on Swainson's warbler's distribution information and the habitat characteristics and species composition of communities Swainson's warblers are known to occupy during migration and breeding. There is not conclusive evidence that Swainson's warblers occur in all the habitat types listed, and some community types may have been omitted. Swainson's warblers are observed rarely in some of the communities listed and more often in others. See the Preferred Habitat section for more detail.
Swainson's warblers forage on and near the ground. In northeastern Louisiana, Swainson's warblers had a mean foraging height of 1.3 feet (0.4 m). This was the lowest relative foraging height observed and included the most occurrences of a species foraging on fallen debris. Of 17 foraging observations, 71% were on the ground and the remainder were in shrubs [3]. In Jamaica, Swainson's warblers captured 95% of insects on the ground [36].
Swainson's warbler's diet is comprised almost entirely of arthropods. In the southeast, crickets (Gryllidae), ground beetles (Carabidae), ants (Formicidae), and spiders (Araneae) are the primary prey items of Swainson's warblers. Caterpillers (Lepidoptera), stinkbugs (Pentatomidae), and grasshoppers (Acrididae) have also been reported as major food items [43]. Less often Swainson's warblers eat millipedes (Diplopoda), beetle larvae, and moths (Lepidoptera) [25,43]. In Jamaica, Swainson's warblers ate insects in all (n=32) foraging observations [36]. Regurgiation samples from Swainson's warblers in Jamaica were comprised primarily of beetles (Coleoptera), spiders, and ants. In addition to the typical prey items, seeds and a species of gecko (Sphaerodactylus goniorhynchus) were also eaten. See the table below for occurrence and frequency of prey items.
Prey in regurgitation samples of 13 Swainson's warblers in Jamaica [64] Prey TypeNumber
(% of total)
Frequency
(% of birds)
Steve Nanz, http://www.stevenanz.com/
Swainson's warblers forage primarily by using their bills to search through and capture prey in loose litter. For example, in northeast Louisiana sweeping motions of the bill through ground leaf litter was used in 63% of foraging observations (n=41 observations, n=17 Swainson's warblers). Gleaning insects off of the ground and low shrubs occurred in 32% of observations. Only 5% of observations were of flying Swainson's warblers capturing prey off a substrate [3]. Of 399 individuals observed in over 70 locations only 2 used foraging methods other than the "leaf-lifting" technique. The author suggests observations of arboreal gleaning by Swainson's warblers are of agitated individuals [25]. A review of the foraging habits of the Swainson's warbler is included in [7].
Very little information is available on the effects of fire on Swainson's warblers. The available information is comprised mainly of Swainson's warbler abundances on sites with varying fire histories and does not include information at varying temporal and spatial scales. There is also a lack of information regarding the effects of various site and fire characteristics on Swainson's warblers' response to burning, and as of early 2007 there were no comparisons of Swainson's warbler demographic data from burned and unburned areas or experiments with replicates and/or controls. The majority of the information that follows is speculation based on Swainson's warblers' habitat and cover requirements.
Swainson's warblers have been observed in recently burned areas. Swainson's warblers were transients in fire-suppressed longleaf pine (Pinus palustris) stands, but did not occur in fire-maintained stands, which had significantly (P<0.05) lower percent canopy cover (62.78% vs. 77.29% on fire-suppressed sites), mean number of small trees (4.35 vs. 14.28 on fire-suppressed sites), and mean number of large saplings (2.56 vs. 4.71 on fire-suppressed sites) [34]. Swainson's warblers were observed in 4.3% of surveys performed on a grass-forb regeneration site in South Carolina that had been cut and burned the previous year, and they occurred in 15% of censuses in shrub-scrub regeneration sites that were cut and burned 2 to 5 years previously [37].
Several factors are likely to influence Swainson's warblers response to fire, including fire season, frequency, uniformity, severity and site characteristics.
Burning during the nonbreeding season will probably reduce impacts of fire on Swainson's warbler survival and reproduction. Recommendations regarding timing of silvicultural treatments include scheduling disturbances from October 1 to April 1 [4,18].
Frequent fires are likely to have a negative impact on Swainson's warbler. Swainson's warblers were observed on sites in North Carolina that were not burned in the growing season and had relatively few dormant season burns, but were not seen on sites maintained with at least 1 prescribed fire in the growing season and ≤5-year dormant season fire intervals [34]. A similar trend occurred in southwestern Mississippi [10]. In contrast, regular fires in canebrakes at intervals of about 4 to 10 years [30,38] may help maintain Swainson's warbler habitat.
Given the possibility of a preference for patchy habitats (see Landscape factors), Swainson's warbler may have a better response to small and/or patchy fires compared to large and/or uniform burns. Small (0.6 acre (0.25 ha)), patchy clearcuts (25/km²) were recommended as a way to promote establishment in marginal habitat in Virginia [27] and small (<1.2 acres (<0.5 ha)) group selection cuts were recommended on sites in Arkansas [4].
The influence of fire severity would depend in part on the time required for necessary habitat components to recover. Given their occurrence on sites with thick cover, Swainson's warbler would not be expected to occur in areas after severe fires. However, in some cases it is possible that stand-replacing fire could benefit Swainson's warbler by opening the canopy and allowing the development of a dense understory. Mixed-severity fires could potentially create a patchy mosaic of open and closed habitats, which may benefit Swainson's warblers (see Landscape factors), as well as thin out dense canopy cover. Less severe fires could improve understory cover by simulating sprouting of rhizomatous understory species, such as cane [4,31,58]. However, even low-severity surface fires could consume leaf litter [48] and nesting habitat. The impact this would have on Swainson's warbler is unknown, but it would likely be influenced by the availability of these features in neighboring areas and the time necessary for vegetation regeneration, leaf litter accumulation, and prey species recovery. Depending on fire characteristics, understory vegetation could recover quickly. For instance, cane can reach preburn heights within 2 growing seasons of burning [31,58]. Several other species Swainson's warblers use for nesting and/or cover sprout following fire, including mountain laurel, coastal sweetpepperbush and common and laurelleaf greenbrier. In some circumstances, prey species may also be able to recover in a relatively short time. In an oak-pine (Quercus-Pinus) forest in Kentucky, arthropod communities began recovering 2 years after prescribed burning [14], and some beetles were not substantially affected by prescribed burning in mixed-oak forests of Ohio [60,62]. However, fires can cause large [6,14], potentially long-term [9] changes in the forest floor arthropod community.
Fire Ecology: Swainson's warblers occur in habitats with a wide range of FIRE REGIMES. Canebrakes can be maintained with low severity fires about every 4 to 10 years [30,38]. See the FEIS review of cane for more details regarding FIRE REGIMES of forests where cane occurs and the effects of fire on cane. Although little is known about presettlement FIRE REGIMES in many Swainson's warbler habitats including oak-hickory and bottomland forests, a literature review [70] summarizes the information available and suggests that fire-return intervals in bottomland forests were from <35 to 200 years and that fires were typically of low severity due to rapid decomposition on these moist sites resulting in low fuel loadings. Stand replacing fires at intervals exceeding 1,000 years occur in most maple-beech-birch (Acer-Fagus-Betula spp.) communities [70].
The following table provides fire-return intervals for plant communities and ecosystems where Swainson's warbler is likely to occur. Find fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
The Swainson's warbler is considered vulnerable in many portions of its range. Partners in Flight consider Swainson's warbler a priority species [21,49,71]. Their range has apparently receded in Oklahoma [28] and Delaware [7]. In addition, Swainson's warbler populations in the Blue Ridge mountains may be declining. Populations may be increasing in some areas, such as the South Atlantic Coastal Plain [32,57]. However these estimates are limited by small sample size, low power to detect population trends, and/or low abundance [7,57].
Playback of Swainson's warbler songs is an effective censusing technique [23,24].
Methods of establishing and/or maintaining cane are addressed in [18,53].
Although demographic analysis in varying habitats and landscapes has not been performed [4], Swainson's warblers may benefit from some timber harvesting. Habitat characteristics of foraging sites in Tensas River National Wildlife Refuge in Louisiana led to the conclusion that selective cutting with rotations ≥100 years would have a positive effect on Swainson's warblers [3]. Small (0.6 acre (0.25 ha)), patchy clearcuts (25/km²) were recommended as a way to promote Swainson's warbler establishment in marginal habitat in Virginia [27]. In South Carolina, a viable Swainson's warbler population occurred in large (741-1,730 acres (300-700 ha)), approximately 15-year-old clearcuts where some lowland areas were left intact [52]. The degree of fragmentation and size of clearcuts at which impacts to Swainson's warbler become detrimental are uncertain [4]. Restricting harvesting to the nonbreeding season (1 October to 1 April) is likely to minimize negative impacts to Swainson's warblers [4,18]. A more detailed review of the impact of various management techniques on Swainson's warblers is provided by [7] and water management issues are discussed in [4,27].
There are few confirmed Swainson's warbler predators. No predation of adults has been reported. Nests are likely predated by several snakes, birds, and mammals [7,43,63,66]. Given their occurrence near the ground, nests would be susceptible to both mammal and snake predation [43,63]. Predation of a nest near Macon, Georgia was attributed to a mouse [43]. A review lists nest predation by the common milksnake (Lampropeltis triangulum) and possible predation by common grackles (Quiscalus quiscula) and blue jays (Cyanocitta cristata) [7].
According to a review, Swainson's warbler nests are parasitized by brown-headed cowbirds (Molothrus ater) throughout their breeding range [7]. The extent to which brown-headed cowbird parasitism affects Swainson's warbler is uncertain, with both rather high [43] and low rates [52] of parasitism reported. It has been suggested that Swainson's warbler's habit of nesting comparatively late in the spring prevents much brown-headed cowbird parasitism [44].
Swainson's warbler breeding habitat is generally similar to migratory and wintering habitats. Although Swainson's warblers are most commonly associated with floodplain forests and mountain ravines during the breeding season [7,18,36,63], they also occur in heavily disturbed habitats such as old fields and loblolly pine (Pinus taeda) plantations [11,18,68]. Swainson's warbler wintering habitat in Jamaica and Cuba includes scrub and lowland and montane forests, including "dry limestone" and second growth forests [33,36,65].
Swainson's warblers are found at low and mid-elevation sites. Reported elevations of breeding areas range from 220 feet (67.3 m) [4] to 2,800 feet (850 m) [43]. A review notes occurrences to 4,000 feet (1,220 m) in the Appalachians [63]. Swainson's warblers were reported from sea level [36] to 4,350 feet (1,325 m) on wintering sites in Jamaica [24] and on sites up to 4,900 (1,500 m) during migration [56].
Swainson's warbler habitats have large amounts of cover especially in the understory. For more information on the vegetative characteristics of Swainson's warbler habitat see Cover Requirements.
Due to its use as a foraging substrate [3,25,36] (see Food Habits), litter is an important component of Swainson's warbler habitat. In Arkansas, sites occupied by Swainson's warbler had significantly (P<0.001) greater litter cover (x = 59.5%) than random sites (x = 41.1%) in a study area comprised of oak-hickory (Quercus-Carya) forest and box canyons. There was also significantly (P=0.003) more litter cover on occupied sites (70%) compared to unoccupied sites (50.4%) in a bottomland hardwood forest study area. Data pooled from these and 2 other Arkansas study areas showed significantly (P=0.005) greater litter cover (x = 69.5%) at sites with Swainson's warblers than random sites (x = 51.0%). In addition, sites where Swainson's warblers were detected had insignificantly (0.298≥P>0.05) deeper litter than random sites [4]. Litter was the third most important variable in models generated from data collected at Bond Swamp National Wildlife Refuge in Georgia to predict occupancy of habitat patches by Swainson's warblers. On average, litter depth on plots in Swainson's warbler territories was 50% greater than on unoccupied plots [45]. The average litter cover at foraging locations in bottomland hardwood forest in Louisiana (98%) was significantly (P≤0.05) greater than at random sites (~91%) [3]. However, leaf litter cover was significantly (P=0.011) lower in areas of Missouri canebrakes where Swainson's warblers were present (38.5%) than in areas where Swainson's warblers were not detected (55.6%) in one year of a two-year study [69]. Importance of litter in wintering habitat in Jamaica is addressed by [65].
Although Swainson's warblers are typically located near water, standing water within their territories is rare. In lowlands of southern Illinois, all observations of male Swainson's warblers were within 660 feet (200 m) of open water [18]. In a model for predicting their presence on sites in South Carolina, Swainson's warblers were more likely to occur in areas close to water [47]. However, territories usually contain little or no open water. In the Great Dismal Swamp of Virginia and North Carolina, water was not recorded at any of the 600 reference points within Swainson's warbler territories, while standing water was observed at 43% of 880 reference points in unoccupied habitat [26]. A model for predicting occupancy of habitat patches by Swainson's warbler based on data collected in Bond Swamp, Georgia included a negative relationship with standing water [45]. In Tensas River National Wildlife Refuge, Louisiana, Swainson's warblers occurred most often in the second terrace floodplain forest, which does not seasonally flood [3]. On average only 5.1% of 1-acre (0.04 ha) plots within Swainson's warbler territories in southern Illinois lowlands were completely covered by water [18]. The proximity of territories to water and the primarily terrestrial habits of the Swainson's warbler leave the birds vulnerable to flooding [26,40,43].
Swainson's warbler may avoid areas where the soil is comprised of gravel [18,27]. In southern Illinois, only 1 of 42 males occurred on sites with gravel dominated soils despite thorough searches in such areas [18].
Density/Territory: Reported territory sizes vary widely across habitats and throughout Swainson's warbler's range. In a review, Meanley [43] summarizes territory sizes observed over several years of investigation. The smallest territory was 0.3 acres (0.1 ha), while the largest territories were over 4 acres (1.6 ha). This included a 6-acre (2.4 ha) territory in the Great Dismal Swamp in Virginia and another that expanded from 4.1 acres (1.7 ha) earlier in the breeding season to 8 acres (3.2 ha) in July. Size and arrangement of habitat were suggested as possible factors influencing Swainson's warbler territory size and distribution [43]. In a second-growth forest comprised of yellow-poplar, red maple (Acer rubrum), and eastern hemlock (Tsuga canadensis) a male Swainson's warbler had a territory with a diameter of approximately 820 feet (250 m) [46]. In a managed hardwood forest in South Carolina territories averaged 4.5 acres (1.84 ha) on a sheared site (saplings and stumps removed) and 5.7 acres (2.32 ha) on an unsheared site [52]. A Swainson's warbler territory in a lowland hardwood forest of northeastern Alabama was 1.7 acres (0.69 ha) [66]. According to a review, territories in southern Illinois ranged from 0.3 to 8 acres (0.12-3.2 ha) with a mean of 2.2 acres (0.9 ha) [7].
Population densities are similarly variable. High density estimates include 25 male Swainson's warblers/km² in oak-gum (Quercus-Nyssa) forest in Louisiana [16] and 20.5 males/km² on a mixed-aged second-growth floodplain forest site in Louisiana. One of the lowest reported densities of Swainson's warbler was 3.8 males/km² in second growth floodplain forest in Arkansas [27]. A fairly comprehensive summary of population densities is included in [7].
Landscape factors: It is generally accepted that Swainson's warblers prefer large areas of habitat [18,26,27]. In southern Illinois, the smallest lowland forest stand with a Swainson's warbler territory was about 865 acres (350 ha) [18]. Using typical densities of Swainson's warblers, Graves [27] suggested an area of 15,440 to 24,710 acres (6,250-10,000 ha) would be needed to support 500 pairs of Swainson's warblers.
Scale of investigation was important for detecting effects of various habitat characteristics on presence of Swainson's warbler in South Carolina [47]. A model for predicting Swainson's warbler presence included opposite relationships to the same variables at differing scales. For instance, at fine scales Swainson's warblers were less likely to occur in older stands (5-acre (2 ha) scale) of relatively uniform age (49-acre (20 ha) scale), while at large scales Swainson's warbler was positively associated with mean stand age (6,990-acre (2,827 ha) scale ) and "continuity of age" (1,210-acre (491 ha) scale) [47].
At a small scale, patchy habitat may be important to Swainson's warblers. In addition to the finding noted above regarding "continuity of age" at the 49-acre scale [47], it is possible that in some areas Swainson's warbler's require territories that have closed canopies and litter-covered, relatively open areas at ground level as well as dense understory thickets typically associated with canopy gaps [27] (see Cover Requirements). More research is needed on the relative importance of each of these features for nesting, perching, and/or foraging and the impact of their size and arrangement in several landscape contexts [4,52,61].
Migration: According to literature reviews, Swainson's warblers migrate into the southeastern United States from late March to late April. Males typically arrive several days before females. During fall migration birds generally leave sometime from August to October, although Swainson's warblers have been recorded in southern Florida in November. More detailed information regarding migration routes and timing is available [7,43,63].
Breeding: Nesting begins in late April and early May [7,41,43,46]. Females build bulky nests in 2 to 9 days using materials near the nest site [41,42,66]. Swainson's warbler clutches typically contain 3 or 4 eggs [7,43,63]. A literature review found a mean clutch size of 3.3 eggs (n=90), with 68% of nests containing 3 eggs [7]. In canebrakes of southern Missouri, the average clutch size was 3.65 eggs [69]. Eggs are incubated for 13 to 15 days [42,43,66] and the young fledge in 10 to 12 days [7,43]. According to a review, Swainson's warblers may begin breeding at 1 year, although data supporting this are lacking [7]. A male Swainson's warbler returned to a territory in the Great Dismal Swamp in Virginia 3 years in a row [43], and a review notes site fidelity to wintering areas in Jamaica [7]. For information on breeding behaviors such as courtship, brooding, and parental care, see [7,42].
Swainson's warblers will renest, if initial nesting attempts fail. A review summarizes accounts of renesting, including a female that had 3 failed nests in 1 breeding season [7]. Although it appears fairly rare, double brooding has also been reported [43,46]. In Franklin L. Gravely Wildlife Management Area in South Carolina, the second nest of a pair of Swainson's warblers with young that fledged in early June was found on June 12, and nestlings from this nest were banded on July 8 [46].
Values of nest success for Swainson's warblers range from low to average. For example, Meanley [43] observed only 3 successful nests out of 16, and a literature review states that nest success "seems remarkably low" [7]. However, in canebrakes of Missouri the daily clutch survival (x = 0.9675) and survival during the 15-day incubation period (x = 0.6095) were within the range of other forest song birds. On average each Swainson's warbler nest produced 2.12 fledglings [69]. In a managed hardwood forest in South Carolina, 8 of 15 nests with known outcomes were successful. The estimated average daily nest survival rate was 0.961 which resulted in a 46% probability of nest success [52].
Lifespan: According to a review, the oldest documented Swainson's warbler was recaptured in Jamaica when it was 7 years and 9 months old [7].
Behavior: Descriptions of behaviors such as vocalizations, territory defense, courtship, and preening are reviewed by [7,41,43].
Swainson's warbler (Limnothlypis swainsonii) is a small species of New World warbler. It is monotypic, the only member of the genus Limnothlypis. Swainson's warbler was named after William Swainson, an English ornithologist.
Swainson's warblers are a small and rather nondescript songbird, though are fairly large for a New World warbler. Adults grow to 12.5–16 cm (4.9–6.3 in) in length and 11–20.5 g (0.39–0.72 oz) in weight. The wingspan averages 23 cm (9.1 in).[3][4] They are a plain olive-brown above and pale yellow-white below. They have a whitish eyebrow stripe that runs above their eye, and the top of their head is a rusty brown. Unlike most other New World warblers that are mostly dimorphic, there is no difference in appearance between a male or female Swainson's warbler.
Swainson's warblers are uncommon, mostly found in flooded swamplands and canebrakes of the south-eastern United States. More rarely, they will also occur in rhododendron thickets in the southern Appalachian Mountains. They are a migratory species, with part of the population migrating southeastwards to the Greater Antilles (where it overwinters in the Blue Mountains of Jamaica for example[5]) and the other southwestwards to the Yucatán Peninsula region in winter.
This species begins breeding at about 10 months of age.[6] Pairs form, and stake out and defend a territory for nesting. Nests are fairly large and bulky, constructed from moss, grass, and small leaves situated above ground in a tangle of tall reeds or vines. The female will lay between three and five eggs. The eggs are white and sometimes, but rarely, speckled with brown. Incubation is done by the female only and lasts for about 14 days, after which the eggs will hatch. The young leave the nest about 12 days later. It is not known how long pairs stay together, although once a pair-bond has been established they do not usually mate with other birds at least in the current nesting season. These birds live to as old as eight years.[6]
No subspecies are recognized. There appears to be some divergence between populations from Arkansas and others of the coastal plains. This does fit a pattern one would expect from genetic drift, but there seem to be no geographical or ecological barriers restricting gene flow. Even during the last ice age, when average temperatures, precipitation and sea levels were lower, there seems to have been ample contiguous habitat. Clearly, some factor restricting gene flow is at work, but it is not presently known what it is. It is possible that the subpopulations conform to the different wintering areas.[7]
In some migrant birds it is known that the initial direction of the migration is set by fairly simple hereditary mechanisms. Offspring of pairs comprising birds of different subpopulations will, in such species, attempt to migrate into an intermediate direction. Such a course would lead a Swainson's warbler deep into the Caribbean where there are no wintering or even stopover points, and the bird would almost certainly perish. More research such as analyzing bird banding data is needed to determine whether this mechanism applies in Swainson's warbler.[7]
Swainson's warbler (Limnothlypis swainsonii) is a small species of New World warbler. It is monotypic, the only member of the genus Limnothlypis. Swainson's warbler was named after William Swainson, an English ornithologist.
