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Monotypic Nest Site Selection by Swainson’s Warbler in the Mountains of South Carolina
J. Drew Lanham and Stanlee M. Miller

Southeastern Naturalist, Volume 5, Number 2 (2006): 289–294

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2006 SOUTHEASTERN NATURALIST 5(2):289–294 Monotypic Nest Site Selection by Swainson’s Warbler in the Mountains of South Carolina J. Drew Lanham1,* and Stanlee M. Miller2 Abstract - Surveys for nesting Limnothlypis swainsonii (Swainson’s Warblers) were conducted in the Appalachian mountains of northwestern South Carolina (Pickens County) during the breeding seasons from 1999 to 2003. A total of 74 nests were located, of which 60 (81%) were found in young (small) Tsuga canadensis (eastern hemlock). This nest-site selection tendency in montane populations has not been described. Habitat data collected in 1999 revealed trends of nests placed low, supported by multiple stems, close to the main tree stem, well concealed from above with leaf litter, poorly concealed from below and relatively close to streams. We suggest that conservation of areas in the Southern Appalachians where eastern hemlock is a component of the forest may play an important role in Swainson’s Warbler conservation. Introduction Limnothlypis swainsonii Audubon (Swainson’s Warbler) is considered by professional and amateur ornithologists alike to be one of the most elusive and difficult passerines to observe in North America (Brown and Dickson 1994). A high-priority species for conservation in the Blue Ridge Physiographic Province (Hunter et al. 1993), the species occurs disjunctly in bottomlands of the southern and southeastern coastal plains and in southern Appalachian forests (Brown and Dickson 1994). The species’ choice of inaccessible habitats in thick canebrakes and “rhododendron hells” of mountain forests is well documented (Meanley 1966, 1971). Southern Appalachian populations, which were only discovered in the 1930s (Brown and Dickson 1994), are considered by some to represent a distinct subspecies (Limnothylpis swainsonii alta). Because of its unique disjunct distribution among southern lowlands and highlands, and due to its status as a species of special concern across much of the breeding range (Hunter et al. 1993, Smith et al. 1993, Thompson et al. 1993), it is important to understand the breeding ecology of this species. In the description of the Swainson’s Warbler (No. 126) in “The Birds of North America,” Brown and Dickson (1994) cite the need for defining macro- and microhabitat requirements. While breedinghabitat data for lowland coastal plain populations are most frequently described in the literature, relatively little has been published about the habitat requirements of montane populations. This paper includes results of initial nesting surveys and habitat analyses conducted in 1999 as well as nestingsubstrate data collected from 2000–2003. 1Department of Forestry and Natural Resources, Clemson University, 261 Lehotsky Hall, Box 340317, Clemson, SC 29634-0317. 2Department of Biological Sciences, Clemson University, 132 Long Hall, Clemson, SC 29634-0314. *Corresponding author - 290 Southeastern Naturalist Vol. 5, No. 2 Study Area and Methods Forested habitats in the Appalachian mountains of northwestern South Carolina (Pickens County) were surveyed for nesting sites during the breeding season (15 May–15 July 1999). Additional searches were conducted by Miller in subsequent breeding seasons and continue presently (2000–2005). Swainson’s Warbler nesting surveys were concentrated in the 11,800-ha (29,500-ac) Franklin Gravely Wildlife Management Area (36°02'N, 82°50'W) of the Jocassee Gorges, a public landholding on the Blue Ridge escarpment managed by the South Carolina Department of Natural Resources (SCDNR). This region of the escarpment is characterized by complex relief, including steep ravines and narrow valleys created by the many first-order streams flowing through the area. The Jocassee Gorges region is a critical area for the preservation of biodiversity along the Blue Ridge Escarpment (Camp 2004, Rankin 1988) and therefore has a high priority for assessment and management. The area is characterized by a high diversity of flora and fauna as a result of its location at the interface between the Piedmont and the Blue Ridge Mountains (Abella 2002, Rankin 1998). Plant communities from both regions are represented along the escarpment, creating a transitional zone from the lower elevation, rolling hills of the piedmont to the higher elevation, rugged peaks of the mountains (Braun 1950). Key among the communities present are mixed mesophytic-cove segregate forests dominated by canopy species such as Liriodendron tulipifera L. (tulip poplar) and Tsuga canadensis [L.] Carr. (eastern hemlock) (Barry 1980). Temperatures in the region are moderate (annual average minimum–maximum: 7.6–21.7 °C), lacking the extremes of heat and drought, and rainfall in the area is high (Cooper and Hardin 1970). Annual precipitation ranges from 160 to 339 cm, averaging 173.72 cm, with the highest periods of rainfall coming in June to August and January to February (USACOE 1996). Weekly searches for nests were conducted from mid-May through mid- August 1999. Although surveys included locating singing males as potential indicators of nesting territories, most nests were located by intensive searches that occurred along abandoned logging roads and railroad beds, game trails, streams, and coves. Once found, nest locations were marked with colored flagging placed away from the nest site to preclude disturbance by predators or hikers. Each nest found in 1999 was checked once or twice a week to determine nest and nestling status. Conditions of the nests (construction, active, inactive) and nest fate were recorded at the time of discovery. We assumed nests were “successful” in fledging young unless eggs and/or nestlings disappeared prematurely or evidence of nest disturbance indicated potential predation. Nest success was simply calculated as the number of individuals “fledged”/total number of eggs. Habitat data in 1999 was collected after the young had fledged or nests were otherwise determined to be inactive. Habitat characteristics of 12 of the nests identified in 1999 were measured and included the following data: nest-tree species, nest-tree diameter (dbh-cm), number of supporting stems, nest 2006 J.D. Lanham and S.M. Miller 291 distance from tree stem (m), nest height from ground (m), nest orientation, litter depth below nest (cm), % cover below nest (ocular estimation), nest-tree leaf litter 0.5 m above nest (ocular estimation), % cover above nest (ocular estimation), nearest distance (m) and orientation respective to the closest stream, densiometer estimates of total plot canopy cover (0.04 hectare [0.10 acre]), aspect, and % slope (clinometer). In addition to the nesting-ecology data recorded in 1999, the following results also present nest substrate (tree species) recorded for all Swainson’s Warbler nests found from 2000–2003. Results Fourteen Swainson’s Warbler nests were found in 1999 with condition assessed as follows: four under construction, one active with an incomplete clutch (one egg), three actively incubated, one inactive (evidence of predation), and five complete nests with no eggs. The five nests without eggs were found late in the breeding season (after 18 July), most likely after breeding activity had ceased. A total of 29 eggs were found. We determined that four active nests fledged a total of 13 young (= 3.25/nest). We estimated a nest success rate (number fledged/ total number of eggs) of 44%. Thirteen of the 14 nests (93%) were found in small eastern hemlocks. Subsequent searches by S.M. Miller during the breeding seasons from 2000– 2003 resulted in the location of 60 additional nests with 47 (78%) constructed in small eastern hemlocks (Table 1). From 1999 to 2003, a total of 74 Swainson’s Warbler nests were located with 60 (81%) built in small eastern hemlocks. Eleven of 12 nests located in 1999 were built in small (mean dbh = 6.8 cm [2.7 in], SE = 0.10 ) eastern hemlocks. Two additional nests were also found in small hemlocks in 1999, but due to inaccessibility, nest-site data were not obtained. In addition to the nearly exclusive occurrence of Swainson’s Warbler nests in small eastern hemlocks, average values for nest-site habitats revealed a trend toward nests with relatively low placement (mean = 2.0 m [6.6 ft], SE = 8.1), supported by multiple branches (mean = 2.6 stems; SE = 0.70), located on southeastern aspects (mean = 158o, SE = 0.10), placed close to the main tree stem (mean = 0.70 m [2.3 ft], SE = 2.1) from tree bole, built “close” to streams (mean = 16.0 m [52.5 ft], SE = 0.10), well concealed from above with leaf litter (mean percent leaf drape = 81.3%, SE = 1.9), and poorly concealed from below (mean percent = 7.5%, SE = 0.50). Sites where nests were found tended to have moderate canopy coverage (mean percent coverage = 64.6%; SE = 9.2), located on/near steep terrain (mean slope = 29.6o, SE = 2.2). Table 2 summarizes nest site data for Swainson’s Warbler nesting in the study area. Table 1. Nest tree substrates of Swainson’s Warbler nests, Jocassee Gorges, SC, 1999–2003. Tree species Common name Frequency (%)* Tsuga canadensis (L.) Carr Eastern hemlock 81 Kalmia latifolia L. Mountain laurel 5 Leucothoe fontanesiana (Steud.) Sleumer Highland doghobble 5 Rhododendron maximum L. Rhododendron 4 Cornus florida L. Flowering dogwood 4 *n = 74 292 Southeastern Naturalist Vol. 5, No. 2 Discussion and Conservation Implications Although Swainson’s Warblers build the largest nest of any North American warbler (Kaufmann 1996), the secretive nature of the species and its inaccessible habitat conditions have heretofore inhibited intensive study in the southern Appalachians. As a result, data describing nest locations and habitatselection criteria are limited. Published descriptions of Swainson’s Warbler breeding ecology almost always include dense, impenetrable vegetation as requisites for habitation (Brown and Dickson 1994, Eddleman et al. 1980, Meanley 1966). Arundinaria spp. (cane) and Rhododendron spp. (rhododendron) thickets are typically the two plant species most closely associated with nesting sites in the coastal plain and southern Appalachians, respectively (Kaufmann 1996, Meanley 1971). Brown and Dickson (1994) list several species as nest sites including cane, Smilax spp. (greenbrier), Vitis spp. (grape), Lonicera japonica Thunb. (Japanese honeysuckle), Sabal spp. (palmetto), Table 2. Characteristics of twelve Swainson’s warbler nests found in the Jocassee Gorges region of Pickens County, SC, May–June, 1999. Nests Nest attributes 1 2 3 4 5 6 7 Nest tree (DBH) 2.8 14.0 9.5 3.5 0.5 18.0 7.2 Number of supporting branches 4.0 1.0 3.0 4.0 3.0 2.0 3.0 Distance from stream (m) 17.0 4.3 4.3 5.5 5.9 6.3 46.9 Nest height (m) 2.4 2.0 1.6 2.0 1.3 1.9 2.9 Nest orient. to bole (degrees) 210.0 50.0 90.0 140.0 220.0 160.0 320.0 Litter depth under nest (cm) 8.0 2.0 4.0 9.0 1.0 8.00 15.0 % cover below nest 5.0 0.0 0.0 0.0 5.0 10.0 0.0 % leaf cover above nest 20.0 10.0 5.0 20.0 0.0 10.0 0.0 % cover above nest 95.0 90.0 100.0 90.0 85.0 100.0 75.0 Nest cup to tree bole (m) 0.0 1.5 0.7 0.3 0.0 1.7 1.5 Nest orient. to stream (degrees) 0.0 180.0 180.0 0.0 0.0 0.0 0.0 % canopy cover in plot 100.0 75.0 50.0 50.0 40.0 80.0 50.0 Aspect (degrees) 235.0 235.0 270.0 45.0 270.0 235.0 45.0 Slope (degrees) 40.0 40.0 0.0 47.0 45.0 38.0 50.0 Nests Nest attributes 8 9 10 11 12 Mean S.D. S.E Nest tree (DBH) 0.3 0.8 15.0 7.3 2.5 6.8 6.2 0.01 Number of supporting branches 2.0 3.0 4.0 1.0 1.0 2.6 1.2 0.7 Distance from stream (m) 17.0 1.6 7.5 28.5 47.7 16.0 16.5 0.1 Nest height (m) 1.7 1.2 1.4 2.1 3.6 2.0 0.7 8.1 Nest orient. to bole (degrees) 140.0 180.0 * 98.0 130.0 158.0 73.9 0.1 Litter depth under nest (cm) 2.0 10.0 1.0 5.0 5.0 5.8 4.3 1.8 % cover below nest 0.0 0.0 10.0 0.0 60.0 7.5 17.0 0.5 % leaf cover above nest 0.0 10.0 0.0 70.0 10.0 12.9 19.4 2.1 % cover above nest 40.0 80.0 60.0 60.0 100.0 81.3 19.2 1.9 Nest cup to tree bole (m) 0.0 0.4 0.0 0.7 1.8 0.7 0.7 2.1 Nest orient. to stream (degrees) 180.0 0.0 * 0.0 0.0 49.1 84.1 2.1 % canopy cover in plot 80.0 50.0 60.0 50.0 90.0 64.6 19.5 9.2 Aspect (degrees) 0.0 270.0 235.0 135.0 135.0 176.0 99.6 10.9 Slope (degrees) 0.0 0.0 10.0 42.0 43.0 29.6 20.4 2..2 *Data not available 2006 J.D. Lanham and S.M. Miller 293 rhododendron, and Kalmia spp. (laurel). While eastern hemlock has not been cited as a recurrent nest site, it is cited in several references as a typical component of habitats where nesting is likely to occur (Legg 1946). The large size of Swainson’s Warbler nests and similarity in appearance to large leaf accumulations found in hemlocks lead us to speculate that the tendency of this tree species to “capture” large clumps of dead leaves and other detritus may serve as an impetus for nest-site selection. This tendency for large accumulations of “leaf drape” to collect in hemlocks may provide a pre-existing platform for nest construction and could facilitate rapid nest construction and/or the leaf-drape clumps may serve as effective nest decoys since they closely resemble Swainson’s Warbler nests. Because the Swainson’s Warbler is a species of high conservation concern in portions of the southern Appalachians (Hunter et al. 1993), it has drawn a great deal of attention from natural resource managers who need to understand the habitat requirements of the species in order to implement proper conservation and management activities. The Swainson’s Warbler’s nearly exclusive use of eastern hemlock nesting sites has not been reported in the literature. This discovery coupled with the species frequent choice of cane nesting substrates in many southern lowlands provides additional evidence that the species may exhibit relatively narrow habitat-selection preferences for either nesting-substrate type (species) or structure (dense shrub/midstory) in portions of its range. Species that exhibit such strong ties to a particular habitat type or structure may be important barometers of ecosystem health. As such, understanding the relationships between habitat condition and various aspects of their ecology are critical for developing conservation strategies to maintain population viability over space and time. While these efforts are often given highest priority in areas where anthropogenic disturbances (e.g., logging) have great potential to shape habitat conditions, it is important to also consider the impact of natural disturbances such as pest infestation. If the information presented herein regarding nesting-site selection of eastern hemlock is valid, then current concerns about the status of eastern hemlock and impacts from the invasive, destructive Adelges tsugae Annand (hemlock wooly adelgid) have the potential to dramatically affect Swainson’s Warblers and other wildlife populations in the region. Heretofore, it was generally accepted that Swainson’s Warbler nesting in the southern Appalachians was closely allied with rhododendron thickets (Meanley 1966, 1971). Our work shows that at least in the Jocassee Gorges region of the Blue Ridge escarpment in South Carolina, these assumptions may not portray a complete picture of the species’ ecology. While we often observed singing males in or near thickets of rhododendron, we never found nests there. Thus we suggest that investigators exercise caution in making broad assumptions of species ecology as different aspects of the life cycle may entail very different habitat requirements. As the southern Appalachians have historic precedence for dramatic losses of important ecosystem components—e.g., Cryphonectria parasitica (Murrill) Barr (chestnut blight), it is important that conservation efforts begin with persistent investigations of basic natural history and species 294 Southeastern Naturalist Vol. 5, No. 2 ecology in addition to applied research of human impacts. These multifaceted efforts will likely be more effective in helping to understand unique ecological relationships and conserving the rich biodiversity of the southern Appalachian’s Blue Ridge escarpment. Literature Cited Abella, S.A. 2002. Landscape ecosystem classification of forest ecosystems of Jocassee Gorges, South Carolina. M.Sc. Thesis. Clemson University. Clemson, SC. 132 pp. Barry, J.M. 1980. Natural Vegetation of South Carolina. University of South Carolina Press, Columbia, SC. 214 pp. Braun, E.L. 1950. Deciduous Forests of Eastern North America. The Blakiston Company, Philadelphia, PA. 596 pp. Brown, R.E., and J.G. Dickson. 1994. Swainson’s Warbler (Limnothlypis swainsonii). Pp. 1–19, In A. Poole and F. Gill (Eds.). The Birds of North America, No. 126. The Academy of Natural Sciences, Philadelphia, PA, and The American Ornithologist’s Union, Washington, DC. 20 pp. Camp, J.L. 2004. Use of a landscape ecosystem model to determine avian-habitat relationships in the mountains of South Carolina. M.Sc. Thesis. Clemson University, Clemson, SC. 130 pp. Cooper, A.W., and J.W. Hardin. 1970. Floristics and vegetation of the gorges on the southern Blue Ridge Escarpment. Pp. 291–330, In P.C. Holt (Ed.). The Distribution and History of the Biota of the Southern Appalachians. Part II: Flora. Virginia Polytechnic Institute and State University. Blacksburg, VA. Eddleman, W.R., K.E. Evans, and W.H. Elder. 1980. Habitat characteristics and management of Swainson’s Warbler in southern Illinois. Wildlife Society Bulletin 8(3):228–233. Hunter, W.C., D.N. Pashley, and R.E.F. Escano. 1993. Neotropical migratory landbird species and their habitats of special concern within the Southeast region. Pp. 159–171, In D.M. Finch and P.W. Stangel (Eds.). Status and Management of Neotropical Migratory Birds. General Technical Report RM-229. US Forest Service, Fort Collins, CO. 422 pp. Kaufmann, K. 1996. Lives of North American Birds. Houghton Mifflin, New York, NY. 675 pp. Legg, W.C. 1946. Swainson’s Warbler nests in Nicholas County. Redstart 13:24–25. Meanley, B. 1966. Some observations on habitats of the Swainson’s Warbler. Living Bird 5:151–165. Meanley, B. 1971. Natural History of the Swainson’s Warbler. North American Fauna No. 69. US Department of the Interior, Washington, DC. 98 pp. Rankin, D.M. (Ed.). 1998. Draft resource management plan for the Jocassee Gorges property, Oconee and Pickens County, South Carolina. South Carolina Department of Natural Resources. Columbia, SC. 97 pp. Smith, C.R., D.M. Pence, and R.J. O’Conner. 1993. Status of neotropical migratory birds in the northeast: A preliminary assessment. Pp. 172–188, In D.M. Finch and P.W. Stangel (Eds.). Status and Management of Neotropical Migratory Birds. General Technical Report RM-229. US Forest Service, Fort Collins, CO. 422 pp. Thompson, F.R., S.J. Lewis, J. Green, and D. Ewert. 1993. Status of neotropical migrant landbirds in the Midwest: Identifying species of management concern. Pp. 145–158, In D.M. Finch and P.W. Stangel (Eds.). Status and Management of Neotropical Migratory Birds. General Technical Report RM-229. US Forest Service, Fort Collins, CO. 422 pp. United States Army Corps of Engineers (USACOE). 1996. Water Control Manual: Savannah River Basin Multipurpose Projects, Georgia and South Carolina. Savannah, GA. 71 pp.