Swainson's Warbler Research Articles

Catastrophic storms, forest disturbance, and the natural history of Swainson's warbler.

The core breeding range of Swainson's warbler (Limnothlypis swainsonii) overlaps a zone of exceptionally high tornado frequency in southeastern North America. The importance of tornadoes in creating breeding habitat for this globally rare warbler and other disturbance-dependent species has been largely overlooked. This paper estimates tornado frequency (1950-2021) and forest disturbance in the 240 counties and parishes in which breeding was documented from 1988 to 2014. The frequency of destructive tornadoes (EF1-EF5) varied 6-fold across the breeding range with a peak in the Gulf Coast states. Counties from east Texas to Alabama experienced the lowest median return interval of 5.4 years per 1000 km2, resulting in approximately 2477 ha of forest damage per 1000 km2 per century, based on current forestland cover. Tornadoes were significantly less frequent north and east of the core breeding range, with return intervals increasing to 9.1 years per 1000 km2 for breeding counties on the Atlantic coastal plain, 10.2 years per 1000 km2 in the Ozark Mountains, and 32.3 years per 1000 km2 in the Appalachian Mountains. Breeding counties within 150 km of the coastline from east Texas to North Carolina are also subjected to the highest frequency of hurricanes in the Western Hemisphere. Hurricanes often cause massive forest damage but archived meteorological and forestry data are insufficient to estimate the aggregate extent of forest disturbance in breeding counties. Nevertheless, the combined impact of tornadoes and hurricanes in the pre-Anthropogenic era was likely sufficient to produce a dynamic mosaic of early-successional forest crucial for the breeding ecology of Swainson's warbler. To ensure the long-term survival of this rare warbler, it is advisable to develop habitat management plans that incorporate remote sensing data on early-successional forest generated by catastrophic storms as well as anthropogenic activities.

Incidence of Invasive Privet (Ligustrum spp.) on Swainson's Warbler Breeding Territories in the Gulf Coastal States

Limnothlypis swainsonii (Swainson's Warbler) readily colonizes novel habitat formations that meet basic requirements for understory stem density and visual screening. Little is known about the warbler's behavioral response to naturalized populations of introduced plants. I surveyed the incidence of introduced invasive Ligustrum spp. (privet) and native Arundinaria spp. (cane) on breeding territories (n = 590) of Swainson's Warbler in the southeastern United States. Privet occurred frequently on territories in Mississippi (65.0%), Alabama (55.7%), Louisiana (52.8%), and Texas (39.5%). Territories with privet (49.6% of total) were observed in 90 counties and parishes. The survey revealed numerous instances of warbler territories located in near-monocultures of Ligustrum sinense (Chinese Privet). Territories with cane (33.1% of total) were observed in 72 counties and parishes. In a broader context, the survey data suggest that Swainson's Warbler populations have rapidly adapted to invasive privets, which are physiognomically similar to several native thicket-forming shrubs.

Swainson's Warbler Breeding Distribution and Habitat Characteristics in Bottomland Hardwood Forests of the Lower Piedmont in North Carolina: Importance of Chinese Privet

The Piedmont physiographic province in southeastern North America generally comprises a large gap between low montane and Coastal Plain breeding populations of Limnothlypis swainsonii (Swainson's Warbler). I conducted land-based surveys of singing males during spring 2017–2018 in late-successional bottomland hardwood forests along and adjacent to accessible areas of the Great Pee Dee River in the Lower Piedmont of Anson and Richmond counties, NC. I located 38–41 territorial males each year and in 2018 measured 12 territories where birds were present both years. Arundinaria spp. (cane) culm counts among the 4 shrub subplots across all plots were low (median = 694/ha), and cane was absent from half of the 12 plots. In contrast, the dense understory within bottomland hardwood forests occupied by Swainson's Warblers was generally dominated by Ligustrum sinense (Chinese Privet). Chinese Privet was present in all plots and had high understory-stem counts (median = 32,539/ha), 55% of the total of the median of all small woody stems and cane culm counts (59,325/ha). Despite the replacement of native shrubs and cane by the riparian invasive Chinese Privet, this latter median count exceeds the recommended minimum count of understory-stems and culms (∼40,000/ha) for Swainson's Warbler breeding territories. This is the first study to document anywhere within Swainson's Warbler breeding range that Chinese Privet constitutes the most important habitat characteristic in bottomland hardwood forests. This study is also the first anywhere within the Piedmont to document that Swainson's Warbler is locally fairly common at the edge of their range along the Great Pee Dee River of North Carolina.

Swainson's Warbler (Limnothlypis swainsonii) in a Monoculture of Invasive Japanese Knotweed (Reynoutria japonica)

This paper reports the first record of a territorial Limnothlypis swainsonii (Swainson's Warbler) associated with the invasive Reynoutria japonica (Japanese Knotweed). The observation adds to a growing body of literature documenting rapid behavioral adaptation to novel habitats in this globally rare warbler.

Natal Philopatry and Juvenile Survival in Swainson's Warblers (Limnothlypis swainsonii)

ABSTRACT.— Understanding natal dispersal and rates of philopatry can help provide minimum estimates of juvenile survival, a vital demographic parameter for which little information is available. However, despite the large number of hatch-year migratory birds banded every year at their natal sites, few are resighted or recaptured. Here, we report on the return of 22 Swainson's Warblers (Limnothlypis swainsonii) banded as nestlings or fledglings between 1997–2008 at breeding sites in Arkansas, North Carolina, and South Carolina, USA. Of 284 nestling Swainson's Warblers banded, we found 13 (4.6%) exhibiting natal philopatry. Of 95 fledglings banded, we detected 9 (9.5%) in later years. The median distance moved from the original capture to an encounter location the subsequent breeding season for all second-year birds we detected was 1.5 km (range = 54 m to 15.4 km). The mark-recapture estimate for first-year apparent survival was 0.119 (SE = 0.032). Estimated recapture probability was between 0.55 and 0.65. ...

Does flooding effect the apparent survival and body condition of a ground foraging migrant passerine?

Natural disturbances play a fundamental role in maintaining habitat and landscape heterogeneity; however, these events can also have negative effects on some species. While we know that disturbances can reduce habitat quality for many species, leading to diminished populations and altered community structure, the effect of these events on individuals that continue to occupy affected areas remains unknown. We focused on understanding the impact of flood-mediated reduction of habitat quality on Swainson’s Warblers (Limnothlypis swainsonii). In 2008, a catastrophic flood event occurred on the Mississippi River and its tributaries, severely affecting one of two locations where we had studied territorial males since 2004. To determine the impact of flooding on this species, we evaluated how body condition and apparent survival of males differed between locations and in pre-flood (2004–2007) and post-flood (2008–2010) periods. Body condition did not differ between locations after the flood, suggesting that flooding did not cause food limitation for this obligate ground forager. Apparent survival in the post-flood period was lower at both locations and led to near population extirpation at the heavily flood-impacted site. Overall, this study demonstrates the vulnerability of species to extreme hydrological events, an increasing threat due to climate change. Future research should focus on identifying species that are vulnerable to these events and determining appropriate conservation strategies. Conservation for the Swainson’s Warbler should focus on identifying and conserving the highest elevation, least flood prone areas within bottomland hardwood forests and managing those areas for thick understory vegetation.

Common Denominators of Swainson's Warbler Breeding Habitat in Bottomland Hardwood Forest in the White River Watershed in Southeastern Arkansas

Abstract The most intensively studied breeding population of Limnothlypis swainsonii (Swainson's Warbler) is in the White River watershed of southeastern Arkansas. However, because vegetation sampling protocols employed at this site have been significantly different from those used elsewhere, it has been difficult for land managers to reconcile datasets across the species' range in order to construct consensus quantitative benchmarks for optimal breeding habitat in bottomland hardwoods. We used a standardized sampling protocol to compare the physiognomic and floristic characteristics of breeding territories at 2 sites in the White River watershed with comparable data from other populations in Arkansas, Mississippi, Louisiana, Florida, and Virginia. Breeding territories in the combined dataset for this rare migratory species varied substantially in successional stage, floristic diversity, hydrology, and management history. Visual screening provided by understory thickets of saplings, vine tangles, and shru...

The Chewing Lice (Insecta, Phthiraptera) Fauna of the Swainson's Warbler, Limnothlypis swainsonii (Aves, Parulidae).

We examined Swainson's warblers (Limnothlypis swainsonii (Audubon, 1834), Aves: Parulidae) for lice fauna during 2 yr at three study sites in Arkansas, USA. A total of 66 individuals were examined; eight birds (10.6%) were parasitized with 16 lice of two new species belonging to two genera Myrsidea Waterson, 1915 (Amblycera: Menoponidae) and Brueelia Kéler, 1936 (Ischnocera: Philopteridae). Parasitological parameter data are given on the prevalence of lice on Swainson's warblers. Species descriptions and illustrations are provided for Myrsidea bensoni sp. nov. and Brueelia limnothlypiae sp. nov.; including a key for females of the genus Myrsidea that parasitize Parulidae (Passeriformes).

Effects of prescribed burning on swainson's warbler home‐range size and habitat use

ABSTRACTPrescribed fire is an often‐used tool for managing or manipulating wildlife habitats. However, studies on the effects of prescribed fire on conservation‐priority songbirds are limited. We examined effects of low‐intensity prescribed burns on vegetation structure and composition, and on habitat use of 45 radiotagged male Swainson's warblers (Limnothlypis swainsonii) during the 2008 and 2009 breeding seasons in the St. Francis National Forest in eastern Arkansas, USA. We found that home ranges in areas with extensive burning (>50% burned) were significantly larger ( = 14.4 ha, SE = 3.5, n = 13) than home ranges in areas with partial burning (<50% burned; = 6.1 ha, SE = 1.3, n = 5) and unburned home ranges ( = 8.5 ha, SE = 1.1, n = 27). Burning decreased understory vegetation density, leaf‐litter depth, and cover, but had no effect on density of woody stems, including giant cane (Arundinaria gigantean; a native bamboo that is thought to benefit from fire). Swainson's warblers did not completely avoid burned regions of their home ranges, but individuals selected dense understory habitats in burned and unburned areas, and burning appeared to intensify selection for areas with less‐variable litter depth. Low‐intensity burning appeared to have minimal impacts on Swainson's warbler habitat use, but did result in increased home‐range size, possibly to compensate for burn‐impacted habitat patches and to enable warblers to acquire adequate food resources. Based on our findings, we suggest that high‐intensity fires or frequent burning could have significant negative impacts on Swainson's warbler habitat. Also, prescribed burning alone was not sufficient to restore remnant populations of cane or a dense understory. We suggest management involving canopy openings in combination with low‐intensity fire may more likely promote the development of dense understory habitat, including giant cane preferred by Swainson's warblers and other understory‐dependent birds. © 2015 The Wildlife Society.

A test of the Swainson's warbler habitat suitability index model

ABSTRACTWe tested a recent Habitat Suitability Index (HSI) model proposed for the Swainson's warbler (Limnothlypus swainsonii; a species of conservation concern in the southeastern United States) using data on habitat, bird occupancy, and abundance collected at White River National Wildlife Refuge, Saint Francis National Forest, and Big Island in Arkansas between 2005 and 2010. The Swainson's warbler HSI includes a combination of 6 variables: landform, landcover, successional age class, forest patch size, proportion forest in a 1‐km radius, and small‐stem density (<2.5 cm diam breast ht; these variables are combined to produce scores ranging 0–1, representing unsuitable to optimal habitat. Mean HSI scores were significantly lower ( = 0.65, n = 99) for sites occupied by Swainson's warblers compared with unoccupied locations ( = 0.78, n = 177) at White River National Wildlife Refuge and Big Island, contrary to our expectation for a useful model. We also found that Swainson's warbler abundances were not correlated with HSI scores both at Saint Francis National Forest and at White River National Wildlife Refuge. Our tests of the current Swainson's warbler HSI model demonstrate that it was not an effective predictor of Swainson's warbler occupancy and was not a reliable indicator of warbler habitat at the scale of our study areas. We propose a new suitability function called proportion elevation that takes relative elevation into account at potential sample locations that may improve the model. Specifically, this function should index the likelihood a site is infrequently flooded and should support the development of a thick understory, a habitat feature that has been demonstrated to be important to Swainson's warblers. © 2014 The Wildlife Society.

Mechanisms of Flood-Induced Territory Abandonment in an Obligate Ground-Foraging Bird

Abstract. Large-scale disturbances, by affecting habitat suitability, can influence bird populations and communities. Although we know that disturbances can lead to population decreases and shifts in community structure, our understanding of finer-scaled mechanisms leading to these patterns is limited. We focused on why some territories remain occupied by a species following a large-scale disturbance, while others are abandoned. The Swainson's Warbler (Limnothlypis swainsonii) is especially susceptible to flooding because of its preference for nesting in thick understory vegetation and foraging in leaf litter. In 2008, a major flood inundated a study site where we have studied Swainson's Warblers since 2004. In 2008 and 2009, we resurveyed 42 Swainson's Warbler territories that were occupied between 2005 and 2007 and examined changes in habitat structure and composition and prey abundance influencing territory abandonment. Occupancy declined between the preflood and postflood surveys. Territory abandonmen...

Nesting Ecology of Swainson's Warblers in a South Carolina Bottomland Forest

Abstract We studied the nesting ecology of Swainson's Warblers (Limnothylpis swainsonii) in Britton's Neck, South Carolina, USA and found 144 nests of which 78 were active. Nest initiations followed a bimodal distribution. Clutches averaged (± SE) 3.19 ± 0.20 eggs and 2.50 ± 0.33 fledglings per nest. The Mayfield nest success estimate for a 23-day cycle was 50%. Logistic exposure models indicate nest success to be most impacted by Brown-headed Cowbird (Molothrus ater) nest parasitism, nest age, and distance to the nearest swamp. Unparasitized nests that were younger in age and further from a swamp had the highest daily survival rates. Ten percent of nests were parasitized by Brown-headed Cowbirds with a 26% reduction in Swainson's Warbler hatching and an 89% reduction in fledgling production. Multiple brooding was observed in 21% of 2000 and 2001 Swainson's Warbler nests. Vines, such as greenbrier (Smilax spp.), were the most common substrate used for nesting, although two-thirds of the nests contained ca...

What factors explain differential use within Swainson's Warbler (Limnothlypis swainsonii) home ranges?

ABSTRACT. Why individuals use some areas within a home range more than others is poorly understood. One factor that may drive differential use is habitat heterogeneity. We radiotracked 37 male Swainson's Warblers (Limnothlypis swainsonii) at two sites in eastern Arkansas, St. Francis National Forest and White River National Wildlife Refuge. We calculated 95% kernel home ranges and designated areas within and outside of the 55% isopleth as the core area and low-use area for each bird, respectively. We then compared habitat characteristics among used points in core areas, used points in low-use areas, and randomly selected points within home ranges. We predicted that core-area points would have the highest-quality habitat, low-use points would have intermediate-quality habitat, and random points would have the poorest-quality habitat. We found that, in most cases, there were no habitat differences between core-area and low-use points, but used areas differed from randomly selected points. Core-area and low-...

Arthropod Communities Associated with Habitats Occupied by Breeding Swainson's Warblers

Abstract. The Swainson's Warbler (Limnothlypis swainsonii) is an insectivorous songbird and a species of conservation concern in the southeastern U.S. Because it is specialized for foraging in the litter of the forest floor, it is dependent on a well-developed leaf-litter layer. However, this reliance on the litter layer likely reflects selection of habitats with abundant arthropods for food. To investigate this possibility, we examined relationships between Swainson's Warbler presence and both abundance and diversity of ground-dwelling and litter arthropods. In 2004 and 2005, using song playbacks, we surveyed 1453 sites and collected arthropods in litter samples and pitfall traps at 45 randomly selected occupied and unoccupied sites. In litter samples, total mean arthropod abundance, abundance of arthropods ≥5 mm in length, total abundance of adults, and taxonomic richness were significantly greater at sites occupied by Swainson's Warblers than at unoccupied sites. In pitfall data, however, we detected n...

Hurricane Katrina impacts the breeding bird community in a bottomland hardwood forest of the Pearl River basin, Louisiana

We monitored breeding bird communities and vegetation both before and after Hurricane Katrina category 2 winds severely damaged extensive bottomland hardwood forest of the Pearl River basin, south Louisiana. Many trees were felled by wind, most others were stripped of leaves and branches, and the canopy opened considerably (57%). Blackberry thickets sprouted and expanded to cover almost all of what was previously a patchily open forest understory. The bird community changed distinctively following the hurricane, driven primarily by increased density of species that prefer dense understory (regenerating) habitat. Individual species that increased significantly in density included one year-round resident, Carolina wren, and five breeding migrants, white-eyed vireo, Swainson's warbler, Kentucky warbler, hooded warbler, and yellow-breasted chat. These patterns were predictable responses to the opened canopy and increased density of understory vegetation. However, over three years following the storm, most species, especially canopy breeders, showed no distinct numerical response to the hurricane, which suggests that the initial bird community was resistant to hurricane disturbance. Only one species, Acadian flycatcher, declined significantly after the hurricane, presumably because of loss of its preferred open understory breeding and feeding habitat. Our results thus document and reinforce the important role hurricanes play along the Gulf coast in structuring forest bird communities by altering understory habitat. We expect habitat changes will continue as invasive plant species further change forest community structure, and as large storms increase in frequency in relation to global climate change. Thus, we also expect continued changes to the bird community, which may include additional future declines.

Prey Selection by Swainson's Warblers on the Breeding Grounds

Abstract. Swainson's Warbler (Limnothlypis swainsonii) breeds in bottomland hardwood forests across the southeastern United States, where it is believed to be one of the rarest breeding songbirds. Although information on its nest-site habitat is considerable, little is known about its foraging habitat except that the species is insectivorous, with a large bill used to flip fallen leaves on the forest floor. We captured Swainson's Warblers and flushed their crops to determine their diet and sampled leaf-litter arthropods and vegetation at each location of capture. We compared the proportion of arthropod orders in the crop samples to the proportion of arthropods collected in the leaf litter to determine the warbler's prey in proportion to its availability. Although Acari (mites and ticks) and Chilopoda (centipedes) were the most abundant arthropods in the leaf-litter samples (51% and 18%, respectively), these orders rarely occurred in the warblers' crops. Conversely, Araneae (spiders) and Coleoptera (beetle...

Harvestmen as predators of bird nestlings

Abstract We report the first confirmed cases of harvestmen (Opiliones) feeding upon live vertebrates. In June and July of 2007, in two independent studies of the ecology of Swainson's warblers (Limnothlypis swainsonii Audubon) in Arkansas and North Carolina, we recorded predation of nestlings at two separate nests by harvestmen. Both events were nocturnal, involved relatively young and dependent nestlings, and contributed to or resulted in the death of one or more nestlings. One event involved a group of at least four harvestmen with one to four individuals feeding upon a brown-headed cowbird (Molothrus ater) nestling at one time, and the other involved up to two harvestmen present at the nest, but only one individual fed upon the two Swainson's warbler nestlings.

Effects of a Flood on Foraging Ecology and Population Dynamics of Swainson's Warblers

Abstract Swainson's Warblers (Limnothlypis swainsonii) have been described as terrestrial leaf litter foraging specialists, and are thought to vacate flooded areas entirely. We report Swainson's Warblers occupying a flooded area in Arkansas and foraging on unusual substrates. We observed Swainson's Warblers feeding in novel ways in the tree canopy and on floating debris. This is the first report of Swainson's Warblers occupying and foraging in flooded habitat, and suggests this species may have more short-term flexibility in foraging substrate than previously thought. Return rates to flooded territories were reduced compared to years without floods and several marked individuals switched to an unflooded area protected by man-made levees. The altered hydrology of rivers constrained by levees may reduce habitat quality for this species.

Effects of Brown-Headed Cowbird Parasitism on Provisioning Rates of Swainson's Warblers

Abstract We studied the effects of brood parasitism by Brown-headed Cowbirds (Molothrus ater) and other factors on food provisioning rates of Swainson's Warblers (Limnothlypis swainsonii), a secretive and poorly understood species of conservation concern. We used time-lapse video systems to collect provisioning data at 25 nests, nine of which were parasitized by Brown-headed Cowbirds. We found strong relationships between feeding rate and brood size with increases from 2.0 feeding visits/hr for nests with a single Swainson's Warbler nestling to 3.0/hr for broods of four. We also found an effect of nestling age with 1.9 visits/hr early and 3.2 visits/hr late in the nestling period. The relationships between cowbird parasitism and provisioning were complex. Nests with brood size of one or three that contained a single cowbird nestling had greater provisioning rates (2.5 and 3.3 visits/hr, respectively) than non-parasitized nests (2.0 and 2.6 visits/hr, respectively). Nests with two cowbirds and no Swainson'...

Relationships Among Survival, Body Condition, and Habitat of Breeding Swainson's Warblers

Abstract. For migratory birds, the selection of high-quality breeding and nonbreeding habitats has significant implications for fitness. One potential reflection of habitat quality is body condition. Condition may influence adults' survival, a key demographic variable for population growth. Access to abundant resources should lead to good condition and birds maintaining or improving their condition over time. As many species use multiple habitats, recognizing the relative quality of these alternative environments is important for understanding population dynamics and aiding conservation. One species that uses a range of habitats is Swainson's Warbler (Limnothlypis swainsonii), a species of conservation concern that breeds in the southeastern U.S. From 2004 through 2007, we studied survival and body condition of Swainson's Warblers at three sites in eastern Arkansas, two dominated by mature forest, and one in an industrial forest managed for timber production. Birds in better condition at initial capture s...