Swainson's Warbler

Limnothlypis swainsonii

Order:
Passeriformes
Family:
Parulidae
Sections

Demography and Populations

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Figure 6. Relative abundance of Swainson's Warbler during the breeding season.

Based on data from the North American Breeding Bird Survey, 2011–2015. See Sauer et al. (2017) for details.

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Figure 7. Regional trends in Swainson's Warbler breeding populations.

Based on data from the North American Breeding Bird Survey, 1966–2015 (Sauer et al. 2017). Data show estimates of annual population change over the range of the survey; areas of increase are shown in blue and declines are shown in red. See Sauer et al. (2017) for details.

Prior to 2000, little was known about the demography and population dynamics of this species because of difficulty finding nests and monitoring birds. Several recent studies have expanded our knowledge of these topics considerably (142, 145, 166, 195, 22, 147, 43, 24, 158).

Measures of Breeding Activity

Age at First Breeding; Intervals Between Breeding

First breed at age 1 year, and annually thereafter. Based on color-marked populations, ASY (“after-second-year”—adult, ≥ 2 yr old) birds reliably attempt breeding every year (TJB). Many new birds filling vacant territories are likely SY (“second-year”—1 yr old) birds and also attempt breeding, but because plumage differences are not consistently reliable for ageing SY and ASY birds, differences in breeding activity between these age classes are largely unknown. Returning males banded as nestlings attempt breeding, often successfully, but may end up in suboptimal territories and seem less likely to attract a female (TJB).

Clutch

Clutches are generally 3 or 4 eggs, although clutches of 1 to 7 eggs have been observed. Mean clutch sizes reported throughout the breeding range include 3.43 in Arkansas (range 2–4, n = 68) (22), 3.30 in Louisiana (range 2–4, n = 84) (158), 3.65 in Missouri (n = 17) (159), 3.19 in South Carolina (range 1–7, n = 69) (43), 2.98 in North Carolina (range 1–4) (24), and 3.3 from nest records and collections from throughout the range (range 2–5, n = 90) (Cornell Nest Record Card Program, WFVZ).

Some nests with only 1 or 2 eggs may indicate incomplete clutches or cases of predation of ≥ 1 egg, although partial predation often causes nest abandonment (23). Female generally builds a new nest and lays a replacement clutch if the a clutch is destroyed; females may lay up to 3–4 unsuccessful clutches in a season (1, 195, 24).

Annual and Lifetime Reproductive Success

Nest success varies among populations, with daily survival rates of nests varying from 0.949 to 0.980 which, assuming a 26-day nesting period, correspond to 26 to 59% nest success. Daily survival rates of nests were 0.949 and 0.956 in Louisiana pine plantations and bottomland hardwood forests, respectively (142), 0.950 in Arkansas bottomland and transitional bottomland–upland hardwood forests (22), 0.961 to 0.979 in intensively managed bottomland hardwood forests in South Carolina (145, 166, 43), 0.951 in bottomland forests in North Carolina (24), and 0.980 in riparian canebrakes in Missouri forests (159). Nest success may also vary within populations depending on landscape context, nest-site habitat, and temporal factors (22, 23, 43, 24).

Meanley (1) reported that only 3 of 16 nests for which he had reliable data were successful in fledging young. Of 13 known-fate nests reported to the Cornell Nest Record Card Program, 5 nests were destroyed in the egg stage, and 8 nests fledged young (1 fledged 4 young, 4 fledged 3 young, 2 fledged 2 young, and 1 fledged 1 young).

Adults are capable of raising complete broods to fledging if eggs or young are not destroyed by Brown-headed Cowbirds or predators, but have difficulty raising both warbler and cowbird young (see Breeding: Brood Parasitism). In Arkansas, parasitized nests that escaped predation or failure due to other causes produced 0.60 warbler young, whereas non-parasitized nests produced 2.75 young (22). This is similar to the 3.0 warbler fledglings per successful nest in Louisiana (158) and slightly greater than the 2.1, 2.5, and 2.1 in Missouri, South Carolina, and North Carolina, respectively (159, 43, 24).

No information on lifetime reproductive success.

Number of Broods Normally Reared per Season

Females may lay one clutch per season, but several clutches are generally laid if nests are destroyed, and second broods are often attempted after successful first nests early in the season (166, 195, 43, 24).

Proportion of Total Females that Rear at Least One Brood to Nest-Leaving or Independence

With renesting probabilities of 50, 75, or 100%, and up to 4 nesting attempts, 41, 53, or 70% of individuals, respectively, would produce at least one fledged brood given the estimated nest survival in Arkansas (195). Similarly, given the same renesting probabilities, 7, 14, or 25% of females, respectively, would successfully rear a second brood (195).

Life Span and Survivorship

The oldest age recorded is ≥ 10 yr for a male originally captured as AHY (≥ 1 year old) in South Carolina (J. Gerwin, personal communication); a male captured as AHY in Arkansas, and another in South Carolina, lived for at least 9 yr (JCB; J. Gerwin, personal communication), and males in Louisiana, South Carolina, and Virginia have been documented at ≥ 8 yr (S. Somershoe, J. Gerwin, G. Graves, personal communication). During the overwintering period, the oldest age recorded was 7 yr, 9 mo for an individual of unknown sex banded and recaptured near the same location in Jamaica (221).

In Arkansas, annual apparent survival of adult males was 58% at 2 mature forest sites, 47% at an intensively managed site, and 25% at a frequently flooded location (195). The lower survival estimates at 2 of the 4 sites were likely related to emigration caused by timber harvest and flooding, respectively. Indeed, an expanded dataset that incorporated additional years for the 2 mature-forest sites encompassed severe flooding events and estimate apparent survival as 60% during the pre-flood and 44% during the post-flood period (21). Other survival estimates for this species include 63% for males in intensively managed South Carolina forests (222) and 52 to 74% from the Monitoring Avian Productivity and Survival (MAPS) program (223).

Assuming a 50:50 sex ratio, annual apparent survival for first-year males (i.e., from fledging to first breeding season) was at least 23.8%, and likely greater given an unknown degree of natal dispersal away from study sites (224).

Disease and Body Parasites

The trematode Paratanaisia bragai (Digenia: Eucotylidae) has been found in the urinary tract of Swainson’s Warbler (225). Blow-fly pupae of Protocalliphora deceptor (Diptera: Calliphoridae) were observed in 8 of 12 nests collected in Oklahoma (226). Ticks (Ixodida: Ixodidae) have been reported on Swainson’s Warbler, including larval Amblyomma sp. on a Swainson’s Warbler in Jamaica (227). Three birds in Virginia were found with ticks (228), one of which was identified (via extracted DNA) as a juvenile Haemaphysalis leporispalustris (E. Heller, personal communication). Eleven percent of Swainson’s Warblers sampled in Arkansas were observed to carry 2 newly described species of chewing lice (Insecta: Phthiraptera): Myrsidea bensoni (Amblycera: Menoponidae) and Brueelia limnothlypiae (Ischnocera: Philopteridae), species that have only been reported on Swainson’s Warbler (229).

Causes of Mortality

Natural causes of mortality in adults are largely unknown, although there is video suggesting a brooding female was eaten by a ratsnake in North Carolina (N. Chartier, personal communication; see Behavior: Predation). Apparent survival of adult Swainson's Warblers in Arkansas was strongly related to body condition at capture (147), suggesting that habitat quality may affect true survival, emigration decisions, or both. Nests are destroyed during floods in some floodplain forests (1), and flooding may have significant impact on local populations in some years and affect long-term stability (16, 17, 19, 21). Moreover, some nests are lost because of heavy winds during storms or due to trampling by large mammals (142, 23).

Many predators of eggs and young have been recorded; for details see Behavior: Predation and Table 3. Across the range, ratsnakes (Elaphe spp.) are among the most common predators (23, 43, 24; M. Revels, personal communication). Brown-headed Cowbird females have often been recorded removing eggs and young from Swainson's Warbler nests (23, 24), and may be responsible for significant reduction in annual recruitment (195; see Breeding: Brood Parasitism).

Range

Initial Dispersal from Natal Site

Of 284 nestling Swainson’s Warblers banded in Arkansas, North Carolina, and South Carolina, 13 (4.6%) were found exhibiting natal philopatry, and 9 of 95 birds banded as fledglings (9.5%) also returned (224). Only 1 of 22 returning juveniles detected was female, suggesting that natal philopatry appears to be more common in males. Given that in many regions the available Swainson’s Warbler habitat is limited by land use, understory vegetation quality, and hydrology, the species probably benefits by having some level of natal philopatry to sustain local populations.

From 13 nestlings banded and recaptured as breeding adults in Arkansas, North Carolina, and South Carolina, the median distance between banding and recapture locations was 3.4 km ± 1.0 SE (range 81 m–15.4 km) (224). Of 9 birds banded as fledglings in North Carolina and South Carolina and later recaptured at the study site, the median distance between banding and recapture locations was 267 m ± 234 SE (range 54 m–2.0 km) (224). There was no consistent pattern in direction of dispersal. Presumably, some percentage of birds are more distant dispersers, but we are not aware of any recaptures of birds banded as juveniles that were recaptured at other breeding locations (U.S.G.S. Bird Banding Lab data).

Fidelity to Breeding Site and Overwintering Home Range

Individuals generally return to breed in the same location for several years and occupy territories that overlap their territory of the preceding year(s) (1, 195, 147). Site fidelity has been observed throughout the species' range. Because Swainson's Warblers often have large home ranges (191, 167), widely separated observations of an individual may sometimes be confused with dispersal. The longest axis of Swainson's Warbler home range in Arkansas ranged from 229–1,648 m (mean 570 m) (153).

Breeding males rarely relocate to different territories; of 191 males banded from 2004 to 2006 in Arkansas, in the absence of flooding or timber harvest, only 7 returning individuals were documented to shift to a new territory (defined as traversing unsuitable habitat or ≥ 1 territory of a conspecific) (195). The dispersal distances for these 7 males ranged from 1.0 to 4.6 km (mean 2.3 km). Unpaired males seem most likely to disperse between years; of at least 4 documented cases of unpaired males abandoning a territory late in the breeding period, 2 males were resighted 1.2 and 1.8 km away from their original location, and 2 were not seen again within the breeding period, although 1 of these males was resighted the subsequent year 1.8 km from his original location (TJB). Less information is available for females, but site or territory fidelity seems much lower than for males (TJB); recapture probability for females in South Carolina (27%) was considerably lower than for males (89%) (222).

Site fidelity to overwintering locations is evidenced by individuals being captured at the same locations in successive years. Diamond and Smith (230) reported that 4 individuals (17% of their banded Swainson's Warblers) returned to the same overwintering areas in Jamaica for up to 7 years. Also in Jamaica (1995–1997), at least 4 of 21 banded birds returned between 2 successive overwintering periods (A. Strong, personal communication). Similarly, others have reported that banded individuals have returned to the same sites in Jamaica, Puerto Rico, Guatemala, and the Dominican Republic (P. Marra, J. Faaborg, A. Brunner, C. Robbins, personal communication; 230, 106).

Home Range

Based on studies that used radio-telemetry, Swainson's Warblers in Arkansas have large home ranges (mean 9.4 ha, range 1.6–30.8 [191]; mean 9.9 ha, range 2.1–53.0 [231]), and large territories (mean 6.5 ha, range 1.0–28.1 [191]; see Behavior: Spacing: Territoriality). Home ranges of males appear to be largest during the incubation/nestling phase (146). Although a combination of factors likely affects home-range size, Anich et al. (146) found the largest home ranges in patchy vine habitats and suggested home ranges are likely smallest in areas with dense, uniform understory vegetation. Everitts et al. (167) found that birds occupying areas that had recently burned used larger home ranges.

Brunner (104) estimated similarly large overwintering home ranges in Jamaica (range 1.9–23.7 ha).

Population Status

Numbers

Using data from the North American Breeding Bird Survey (BBS), the Swainson's Warbler population was estimated at 140,000 individuals for the United States (global population) from 2005–2014 (14).

Reported density estimates are usually extrapolations from surveys in small study plots in prime habitat, and therefore are usually inflated. Suitable habitat usually occurs in discontinuous patches interspersed within larger areas of unsuitable habitat. In many cases, the suitable habitat consists of relatively high-elevation ridges surrounded by lower elevation and more flood-prone areas (144, 18). Similarly, even on higher elevation sites that are potentially suitable for Swainson's Warblers, succession may lead to patchiness of suitably dense understory thickets. For example, at White River National Wildlife Refuge in Arkansas, only 4.8% of 1,453 relatively high-elevation sample points were determined to be occupied by Swainson's Warblers (144). Depending on size and quality, a patch of suitable habitat may contain a cluster of several territories in a relatively small area; a survey of that patch would indicate high breeding densities, but a survey of the entire area would show much lower overall density. Therefore, reported density estimates are dependent on the scale at which surveys were conducted and are usually not comparable.

Reported densities are as follows. An oak–gum stand and a swamp in Louisiana each had an average density of 10 territorial males/40 ha (1, 232). Norris (188) reported an average of 7 adults/40 ha in floodplain forests and 4 adults/40 ha in broadleaf deciduous forest in South Carolina. Hamel et al. (233) estimated the average number of territorial males per 40 ha in different habitats in the Southeast: 3 males/40 ha in sawtimber oak–gum–cypress stands, 8.8 males/40 ha in sawtimber cove hardwoods, and 17 males/40 ha in sapling–poletimber oak–gum–cypress stands. Stewart and Robbins (234) found 2 territorial males in 7.6 ha (11 males/40 ha) in second-growth swamp in Maryland. In Georgia, Wright (160) estimated a density of 3.8 males/40 ha. Thompson (166) estimated 6.1–7.5 males/40 ha in intensively managed bottomland forests in South Carolina. Henry et al. (158) estimated 1.2–4 males/40 ha and 1.6–4.8 males/40 ha in Louisiana pine plantation and bottomland areas, respectively. In a range-wide study, Graves (9) estimated 1.5 males/40 ha in eastern Arkansas, 1.9–3.7 males/40 ha in western Mississippi, 4.5–5.7 males/40 ha in southern Louisiana, 8.2 males/40 ha in southeastern Louisiana, and 2.2 males/40 ha in the panhandle of Florida. P. Marra (personal communication) estimated an average of 3–4 individuals/5 ha (24–32 individuals/40 ha) on the overwintering grounds in dry coastal scrub and dry limestone forest in Jamaica.

Transects through various habitats reported the following numbers of territorial males: 19 males in 3.2 km for canebrakes in Georgia, and 8 males in 0.8 km for a swamp in Virginia (1); 10–11 males in 2.4 km for rhododendron–laurel thickets in West Virginia (149). In the Blue Mountains of Jamaica, Graves (6) estimated 2.1–7.6 individuals/km of transect.

Trends

Based on early accounts and his own observations, Meanley (1) reported that Swainson's Warbler appeared to be more abundant in the Southeastern Coastal Plain and Gulf Coastal Plain in the late 1800s and early 1900s than in the 1960s. Breeding Bird Survey data for the survey-wide region showed the overall population increasing by 1.2% per year from 1966–2015, although this result was based on low sample size (n = 268 routes) and the 95% credibility interval (95% CI: –0.12, –2.31) included zero (235). Over a 45-year period (1970–2014), BBS data indicated that the survey-wide population has increased by an estimated 67% (14).

Investigation of population trends in distinct regions of the breeding range is challenging because of the small number of routes with Swainson's Warbler in these regions and the overall low abundance of the species when present. The trends for several regions generally suggest population stability or increases. Further, BBS trends for species with specialized habitat requirements, such as Swainson’s Warbler, may have limitations given that BBS routes are limited to roads; consequently, reported trends should be used with caution. The estimated annual trends were 1.48% for West Gulf Coastal Plain/Ouachita (n = 53 routes), 1.91% for Southeastern Coastal Plain (n = 152 routes), and -0.06% for Appalachian Mountains (n = 32 routes) (235), although only the Southeastern Coastal Plain had an appreciable sample size and a 95% credibility interval that did not include zero.

Other habitat specialists often have large sample sizes relative to those for Swainson's Warbler. For example, population trends for Prothonotary Warbler, a species with specialized habitat requirements and a similar breeding range to that of Swainson's Warbler, are based on ~3 times as many BBS routes and > 8 times the relative abundance compared to those for Swainson's Warbler; even Cerulean Warbler (Setophaga cerulea) and Golden-winged Warbler (Vermivora chrysoptera) have sample sizes of > 100 more routes and relative abundances > 2 times those of Swainson's Warbler (235). These limitations of BBS data for the Swainson's Warbler may make extrapolation of these results to population sizes problematic.

Population Regulation

Few data, needs study. Numerous factors likely limit Swainson's Warbler populations. The dependence of this species on infrequently flooded bottomland forest areas, most of which have been converted to agriculture (236, 237), likely limits habitat (18). The presence of unpaired males in suboptimal habitat and the persistence of small populations in low-quality areas prone to flooding support this idea (17, 19). Consistent flooding can lead to both short-term and long-term decreases in Swainson's Warbler populations (195, 16, 17, 19, 21). Similarly, succession of suitable habitat likely leads to disappearance of the understory thickets upon which Swainson's Warbler depends; the absence of natural disturbance regimes or appropriate management to restore the dense understory structure preferred by Swainson's Warbler may lead to decreased local populations.

In contrast, many existing populations persist in privately owned forests managed for timber production (10, 11). Although little is known about the compatibility of commonly used harvest cycles with maintenance of Swainson's Warbler populations, timber harvest within and between breeding seasons causes some individuals to emigrate (195). Presumably, populations can remain stable at larger scales if timber harvest is implemented to provide a mosaic of stands in different successional stages, some of which are suitable for the species.

Fragmentation of higher-elevation forests and of patches of dense understory within forests is also problematic for Swainson's Warbler populations, given the large space-use requirements of individuals (191; see Behavior: Spacing: Territoriality). With the eastward expansion of the Brown-headed Cowbird, brood parasitism has become an increasingly important limiting factor on some Swainson's Warbler populations (22; see Breeding: Brood Parasitism). Elevated levels of nest predation in some populations may inhibit population growth (195), and both nest predation and brood parasitism may be increased by habitat fragmentation and the increased proximity of anthropogenic edges (22, 23). In Arkansas, the best predictor of the number of young fledged from successful nests was brood parasitism status (22; see Breeding: Brood Parasitism), and this variable also influenced nest survival in North Carolina and South Carolina where brood parasitism is less common (24, 43).

Habitat structure and composition affect adult body condition which, in turn, affects adult survival in the Swainson's Warbler (147). As survival is the demographic parameter that most influences population growth for this species (195), habitat quality and other factors that influence adult survival likely have large consequences for population stability.

Similarly, both habitat structure and food availability affect habitat use by the Swainson's Warbler (144, 169), but the effects of these components of habitat quality on reproduction, including the number and quality of young fledged from successful nests and the probabilities of renesting after failure or double-brooding after a successful first nesting attempt, remain largely unexplored. Renesting and double-brooding have potentially large consequences for population growth (195).

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