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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.
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 - firstname.lastname@example.org.
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.
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.
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
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.
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