Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Conservation and Management

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Effects of Human Activity

Shooting and Trapping

Swainson's Warbler was collected for millinery uses in the late 1800s (238), probably with little impact on the population (see Human/Research Impacts).

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 [10] 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).

Human/Research Impacts

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).

Management

Conservation Status

The global population of the Swainson's Warbler population is estimated at 140,000 individuals (14). The species has been listed as a species of concern in many parts of its range (260, 261, 262), and was listed as a species of conservation concern by the U.S. Fish and Wildlife Service in 8 Bird Conservation Regions, 4 U.S. Fish and Wildlife Service administrative regions, and nationally (263).

Measures Proposed and Taken

Measures may vary by region and specific habitat type but, in general, creating dense understory vegetation within forests should benefit Swainson's Warbler (see Habitat in the Breeding Range). Habitat characteristics needed to encourage nesting and feeding success regardless of forest age include a well-developed leaf litter layer with sparse ground-level herbaceous cover and well-developed forest midstory and/or overstory. The presence of “understory thickets”—areas with dense vine tangles, patches of giant cane, or shrubs with stem densities between 30,000–50,000 stems/ha—exemplifies the habitat requirements of the Swainson's Warbler (9, 165, 142, 166, 264, 11). Where available, palmetto fronds should be conserved as they are selected at the nest patch scale in both pine and hardwood forests (142).

Helpful management practices include conserving a mosaic of mature bottomland forests, especially relatively high-elevation sites not prone to frequent flooding, and also young forest stands (265). Protecting and restoring relatively large bottomland-hardwood forest tracts is also recommended for this species, as forests larger than 300 ha may be preferred (58, 266). As Swainson's Warbler is less abundant in wet forests prone to leaf-litter submergence, efforts should focus on the conservation of higher, drier areas within large bottomland forests and on providing habitat heterogeneity in these forests (237, 254, 267, 18, 20).

Many bottomland forests throughout the Southeast are relatively even-aged forests dominated by homogeneous, closed canopies, and have little vertical or horizontal diversity (268, 267). These forests provide relatively poor Swainson's Warbler habitat, and human disturbances, especially tree harvests, could mimic natural disturbances and provide the heterogeneity to these forests that Swainson's Warbler prefers (268, 267).

Pashley and Barrow (269) suggested selective cutting operations that mimic tree-falls and increase intensity of light reaching the forest floor while maintaining a relatively closed canopy would help the Swainson's Warbler, but thought that larger group-selection cuts might not be beneficial. However, group-selection and shelterwood cuts may provide suitable habitat; research in Louisiana has demonstrated that variable-retention, clustered-thinning methods with or without embedded patch cuts may benefit the Swainson's Warbler (267). Single-tree selection is likely not ideal for the Swainson's Warbler, as it may not create large-enough gaps to stimulate understory growth. (D. Twedt, personal communication).

As the Swainson's Warbler requires canopy or subcanopy cover, large clearcuts might not be ideal given that all vegetation is removed, which makes stands unsuitable for at least 5 years after the cut. Clearcuts may provide suitable habitat after trees begin to form a closed canopy and provide shade for a well-developed layer of leaf litter and dense understory, especially if other suitable forest stands are present on the landscape during this regeneration period. Indeed, the Swainson's Warbler can be abundant in intensively managed forested landscapes, including those managed primarily for timber production, provided that a sufficient range of stand ages is present and they are relatively free from the adverse effects of flooding. With all of these silvicultural options, allowing natural regeneration of the vegetation after cutting, rather than using treatments such as shearing (i.e., using tractor-mounted blade to cut vegetation at the ground level), is likely most beneficial (192).

To create future breeding habitat by promoting regeneration of giant cane, Eddleman et al. (164) proposed creating forest openings (maximum 4 ha) by clear-cutting areas without giant cane, but adjacent to existing cane stands. To manage current habitat, they suggested selective cutting of mature trees in males' territories during the non-breeding season to increase giant cane production and longevity. Additionally, to manage giant cane in bottomland hardwood forests, Gagnon (270) proposed a combination of disturbances including prescribed burns every 7 to 10 years and overstory thinning to maximize the vigor and density of cane stands.

Effectiveness of Measures: the Species' Response

No studies have been implemented specifically to assess management effects on the Swainson's Warbler, although several studies have demonstrated that this species routinely occupies intensively managed forests (145, 157, 195, 147, 43, 10, 158).

Everitts et al. (167) found short-term effects of low-intensity prescribed fire, including that Swainson’s Warbler used larger home ranges in areas with extensive burning compared to partially burned or unburned home ranges. This suggests that in the short term, fire may reduce territory quality, or at least make birds travel farther to reach good foraging patches. Birds using burned home ranges tended to select areas with uniform litter depth, more arthropod prey, and remaining patches of dense understory (231, 167). The burn described in Everitts et al. (167) did not produce significant regeneration of giant cane, and therefore, if prescribed fire is used to restore giant cane or other understory vegetation, it should be combined with creating openings in the forest canopy (271).

Swainson's Warblers may respond to forest restoration as soon as 5–9 years after restoration, depending on restoration method (272). However, responses to silvicultural treatments and restoration are likely most pronounced after 10 years, possibly peaking 15–20 years post-treatment, and then gradually declining until another treatment is applied (273, 268, 267). Indeed, Swainson's Warbler densities in Louisiana forests treated with variable-retention, clustered-thinning methods increased from 5 years post-treatment to the conclusion of the study at 13 years post-treatment (267 and selectively harvested stands that were 12–18 years post-harvest had much greater warbler densities than stands that were 1–5 or > 30 years post-harvest (273). In pine plantations, inclusion of some hardwood species could be beneficial as this provides a combination of pine and hardwood leaf-litter.

Graves (9) recommended forests should be managed every 15–25 years to maintain Swainson's Warbler populations. However, Twedt and Somershoe (267) suggest longer intervals (25–30 years) would likely be beneficial, as they found Swainson's Warbler densities remained high in 28-yr-old forests. Similarly, Roa Vásquez (274) found high Swainson's Warbler abundances in stands that were last harvested more than 40 years ago in southeastern Arkansas. Therefore, it would seem appropriate to define the frequency at which a forest should be managed based on the unique characteristics of each site and local Swainson's Warbler population estimates.

Although large-scale timber harvests could provide suitable habitat several years later, they should not take place in areas currently occupied by Swainson's Warblers; clear-cutting forest stands occupied by Swainson's Warblers caused the disappearance of local populations in Illinois (S. Bailey, personal communication). Additional controlled studies with long-term monitoring that evaluates multiple silvicultural treatments are ultimately needed to determine the best way to manage for Swainson's Warbler. Moreover, the effects of timber management on Swainson's Warbler reproductive success remain unknown, yet are necessary for determining successful long-term management for this species.

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