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2010 SOUTHEASTERN NATURALIST 9(3):563–572
Winter Roosting Ecology of Silver-haired Bats in an
Roger W. Perry1,*, David A. Saugey2, and Betty G. Crump3
Abstract - Although summer roosting by Lasionycteris noctivagans (Silverhaired
Bats) has been studied in various ecological regions of North America, no
quantitative studies have examined winter roost selection. We radiotracked 11 bats to
31 day-roosts during winter in forests of the Ouachita Mountains, AR. We quantified
roost structures and examined the association between roosts and forest stands. We
also examined effects of temperature on roost use. Ninety percent of roosts were in
trees (5 species): 55% of all roosts were under loose bark on the bole of live overstory
Pinus echinata (Shortleaf Pine), 3% of roosts were in a rock outcrop, and 6% were at
ground level (under a tree root or in a cavity at the base of a live pine). Bats selected
pine or pine-hardwoods stands >50 years old, and used forest stands 15–50 years of
age less than their availability. Roost locations were influenced by temperature and
solar radiation; most (90%) roosts were on southern topographic aspects, and bats
roosted in the rock outcrop or on the ground on colder days (<5 °C). Retaining open
pine and hardwood stands >50 years old on south slopes would likely maintain roosting
habitat for wintering Silver-haired Bats in the Ouachita Mountains.
For bats, roost sites are critical for hibernation, rearing young, protection
from predators, and thermoregulation (Kunz and Lumsden 2003). Because
roosts are critical to survival of forest bats (Vonhof and Barclay 1996), an
abundance of studies on roost selection in forested ecosystems has recently
emerged (e.g., Kalcounis-Rüppell et al. 2005). Despite recent gains in
understanding the summer roosting ecology by temperate forest-dwelling
bats, information on roosting ecology during winter is limited (Cryan and
Many bats common in forests during summer migrate to other areas or
move short distances to hibernate in caves, mines, or man-made structures
during winter (Kunz and Fenton 2003). Only a few bats that inhabit temperate
regions are known to roost in trees during winter (Cryan and Veilleux
2007), and the extent that these bats hibernate is unknown. Instead, these
bats arouse to forage during warm winter nights and undergo bouts of torpor
to limit energy expenditure during unfavorable conditions (i.e., below freezing
temperatures). This behavior is common in foliage-roosting bats (genus
Lasiurus), which often roost in leaf litter during winter (Hein et al. 2005,
1Southern Research Station, Forest Service, US Department of Agriculture, PO
Box 1270, Hot Springs, AR 71902. 2Forest Service, US Department of Agriculture,
Ouachita National Forest, Jessieville, AR 71949. 3Forest Service, US Department
of Agriculture, Ouachita National Forest, Hot Springs, AR 71902. *Corresponding
author – email@example.com.
564 Southeastern Naturalist Vol. 9, No. 3
Mormann and Robbins 2007, Saugey et al. 1998). During winter, roosting
bats experience different physiological and survival pressures from those of
summer. Furthermore, mortality among tree bats during winter may be high
(Cryan and Veilleux 2007). Thus, roost sites selected by tree bats during
winter may be especially important to their survival.
Lasionycteris noctivagans LeConte (Silver-haired Bat) occur throughout
North America, from southern Alaska and central Canada to northern
Mexico (Kunz 1982), and are considered a migratory species. During winter,
Silver-haired Bats migrate to the southern United States and generally reside
south of a line from Pennsylvania to Missouri to southern Arizona and
California (Izor 1979). They are generally absent in the southeastern United
States during summer (June–August; Cryan 2003). During migration and
summer residency, female Silver-haired Bats either roost alone or in maternity
colonies, whereas males typically roost alone (e.g., Barclay et al. 1988,
Betts 1998, Mattson et al. 1996). During winter, Silver-haired Bats roost in
mines, caves, houses, rock crevices, under loose bark, and in hollow trees
(Beer 1956, Frum 1953, Jackson 1961, Pearson 1962). Although Silverhaired
Bats may hibernate in caves in northern portions of their winter range,
they may remain active during winter in southern portions (Izor 1979). Other
than anecdotal accounts of winter roosts, little is known about their roosting
ecology in forests during winter. Therefore, our objective was to quantify
characteristics of roosts used by Silver-haired Bats during winter in forests
of Arkansas. Using radiotelemetry, we examined the types of roost structures
used, the types of forest habitats where roosts were located, and the effects
of ambient temperature on roost locations.
We conducted the study on the Jessieville Ranger District of the Ouachita
National Forest (ONF), in southern Perry and northern Garland counties
(34°45'N, 93°15'W), within the Ouachita Mountains of central Arkansas.
The Ouachita Mountains consist of east–west oriented mountains and
valleys that extend from central Arkansas into east-central Oklahoma. Elevations
in the region range from 100 m to 800 m, mean annual precipitation
ranged from 112 cm to 142 cm, and the growing season was 200–240 days
(McNab and Avers 1994). Winters are generally mild. During our study,
average maximum temperature from December to March in the area was
12.4 °C and average minimum temperature was -2.1 °C, although the temperature
occasionally fell below -12.0 °C; average monthly precipitation was
10.0 cm, with 1.3 cm of this being frozen precipitation (NOAA 2009).
The 9619-ha study area was >99% forested, with <1% agricultural lands.
The most abundant forest type (43%) in the study area was mixed Pinus
echinata P. Mill (Shortleaf Pine)-hardwood forests managed by ONF. The
hardwood component in these forests was diverse (>32 species) and was primarily
Quercus spp. (oaks), Carya spp. (hickories), and Acer rubrum L. (Red
Maple). The study area also contained forests of mostly pure Shortleaf Pine
2010 R.W. Perry, D.A. Saugey, and B.G. Crump 565
(35%) and oak-hickory forest (14%). Nine percent of the area was intensively
managed, private timberlands that consisted mostly of closed-canopy P. taeda
L. (Loblolly Pine) plantations of approximately 20 years of age.
We captured bats between 1900 and 2200 CST with mist nets placed
across the North Fork of the Ouachita River, from 1993 to 1996. We determined
bat sex and weighed and banded captured bats, but did not determine
the ages of bats (juvenile or adult) because bats were captured after ossification of metacarpal-phalanx joints (Racey 1974). We attached 0.71-g
radiotransmitters (Holohil Systems Limited, Carp, ON, Canada) to the midscapular
region with Skin Bond® surgical adhesive (Smith and Nephew, Inc.,
Largo, fl). In addition, we instrumented 2 bats from a colony we found in a
rock outcrop via radiotelemetry. Mass of all captured males averaged 11.3 g,
and mass of females averaged 12.2 g (range = 10.3–13.8 g); transmitter mass
was 5.1–6.9% of bat mass and averaged 6.0 ± 0.2%. From 11 December to 19
March, 1993–1996, we tracked instrumented bats to their roosts daily (with
a few exceptions) until transmitter batteries failed or the radio signal could
no longer be located. Because of the relatively low heights of roosts in trees,
we visually located bats from the ground in all but 1 roost. Our methods of
capture, handling, and care of bats adhered to guidelines of the American
Society of Mammalogists (Gannon et al. 2007).
At each tree roost, we recorded tree species, diameter at breast height
(dbh; cm), aspect (N, S, E, or W) of roost on tree, and height (m) of the
bat in the roost using a clinometer. We also recorded topographic aspect
(N, S, E, or W) of each site using a compass. Our primary interest was in
responses by the species, and not sex-specific differences in roost selection.
Furthermore, we found no difference between males and females in
diameter of trees used (t26 = 0.90, P = 0.377), heights of roosts in trees
(t29 = –2.00, P = 0.060), or proportions of habitats used (χ2 = 3.85, df =
2, P = 0.146). Therefore, we combined data for males and females for all
analysis. We present all means ± SE.
To determine habitat selection among types of forests stands, we delineated
6 forest classes using digital maps and forest-stand information
obtained from ONF; these maps included forest types, stand ages, and management
history. We determined locations of bat roosts across the study area
either from global positioning system coordinates or estimated locations
on topographic maps. We overlaid roost locations on vegetation maps in a
geographic information system and determined proportion of roosts in each
forest class. To determine available habitat, we created a 1169-m-radius
buffer around each roost. The radius of these buffers was based on mean
distance from capture site to roost locations for radiotracked bats (1169
± 259 m). We combined all buffers into a single polygon and designated
this area the available habitat; we then determined the percent of available
habitat comprised by each forest class. We compared proportion of used
566 Southeastern Naturalist Vol. 9, No. 3
habitat types to proportion of available habitat types with multiple binomial
tests. We used the Benjamini-Hochberg method to control the positive false
discovery rate (FDR) for the overall experiment at 0.05 (Benjamini and
Hochberg 1995, Waite and Campbell 2006). We included each roost only
once, regardless of the number of times the roost was used, in analyses of
spatial location (e.g., forest-stand use) and roost characterizations (e.g.,
roost aspect, roost height).
To determine if temperature affected roost placement, we obtained regional
weather data (minimum and maximum daily temperatures) from the
National Weather Service (NOAA 2009). We used weather data from the
closest weather station (Blakely Mountain Dam), which was approximately
21 km from the study site. We used a t-test to compare mean minimum daily
temperature when roosts were located in trees with the mean temperature
when roosts were located in rock crevices or at ground-level.
We captured and instrumented 13 bats (10 females and 3 males); however,
2 females were not relocated. Maximum temperature of days when bats were
captured using mist nets averaged 15.6 ± 1.4 °C (range = 6.1–20.0 °C), and
minimum daily temperature averaged -1.6 ± 1.7 °C (range = -6.1–8.9 °C).
Generally, bats were captured on days when the average temperature was
For the 11 bats we located via radiotelemetry, each bat was found an
average of 14.4 days (range = 5–26 days). We documented 31 roosts (22 for
females and 9 for males). The mean number of roosts located for each bat
was 3.0 ± 0.6 (range = 1–6), and the mean number of days we found a bat
in a particular roost was 5.1 ± 0.9 (range = 1–20). One bat was tracked to a
Shortleaf Pine, but its location in that tree was not determined, and the location
within the study area of another roost was not recorded.
Twenty-eight roosts were in trees, 1 roost was in a crack of a rock
outcrop, and 2 roosts were on the ground. The rock outcrop was used by
3 instrumented female bats during different tracking periods. Other nonbanded
bats were periodically observed in the rock crevice, indicating the
outcrop was used by multiple bats over multiple years and occasionally
contained small colonies of 2–3 bats. Of the 2 roosts on the ground, one
was located in a hole at the base of a tree, and one was located under an
Most (74%, n = 23) roosts were in Shortleaf Pine. Other tree species used
for roosting were Nyssa sylvatica L. (Blackgum; 3%, n = 1), Quercus rubra
L. (Northern Red Oak 3%, n = 1), Quercus alba L. (White Oak; 10%, n = 3),
and under the roots of a Red Maple (3%, n = 1). Average diameter at breast
height (dbh) of all roost trees was 33.1 ± 2.0 cm (range = 10.2–50.8 cm). In
trees, the average height to roost was 5.1 ± 0.5 m (range = 1.0–9.1 m). Among
all roosts that were visually confirmed, 57% (n = 17) were under loose bark
on the bole of live overstory Shortleaf Pines (mean tree dbh = 31.8 ± 2.5,
2010 R.W. Perry, D.A. Saugey, and B.G. Crump 567
range = 10.2–49.8 cm). Five roosts (17%) were in cavities of live hardwoods
(mean tree dbh = 41.3 ± 3.3, range = 31.2–50.8 cm), including 1 roost in a
basal cavity of a large (43.9 cm dbh) Blackgum. One roost (3%) was in a
crevice of a Shortleaf Pine that was created by a lightening strike. Four roosts
(13%) were on the boles of Shortleaf Pines, where the bat was found in sunny
spots without cover. Most roosts located under loose bark of tree boles were
on southerly sides of trees; aspect of bats on the boles were 14% north facing
(n = 3), 5% east facing (n = 1), 5% west facing (n = 1), and 76% (n = 16)
southerly (90°–270°). Bats disproportionally located their roosts on southfacing
aspects; 3% were east facing (n = 1), 7% were north facing(n = 2), and
90% (n = 27) were on southerly aspects (90°–270°; Fig. 1).
Bats switched between tree roosts and roosts located in the rock outcrop
or on the ground (in holes or under tree roots). Use of these rock and
ground roosts appeared to be influenced by minimum ambient temperatures.
The mean minimum temperature of days when instrumented bats roosted
in the rock outcrop or on the ground (-5.6 ± 1.1 °C) was significantly less
Figure 1. Locations of winter roosts (n = 30) used by Silver-haired Bats in relation to
topography and aspect in the Ouachita Mountains of Arkansas. Dark areas represent
areas of southern and southeastern aspect; lighter areas are northern and western
aspects. Roost locations are represented by white circles.
568 Southeastern Naturalist Vol. 9, No. 3
(t171 = -3.82, P = 0.002) than days when roosts were located in trees (-0.5 ±
0.4 °C). In general, bats tended to use the rock and ground roosts more often
when temperatures fell below -4 °C (Fig. 2).
Silver-haired Bats roosted in 4 forest classes (Table 1). Among the 6 forest
classes for which we compared use with availability, bats selected pine
or pine-hardwood stands >50 years old. They also selected pine or pinehardwood
stands >50 years old that had recently been partially harvested
Figure 2. Minimum daily temperature (°C) and total number of days Silver-haired
Bats roosted in 2 types of roosts during winter in the Ouachita Mountains of Arkansas.
Tree roosts were located under bark, in cavities, or in crevices above ground in
standing trees. Rock and ground roosts were either in the crevice of a rock outcrop
or on the ground (in holes or under tree roots).
Table 1. Proportions of available forest classes (derived from merged 1169-m radii around
roosts) compared with proportions of habitats used for roosting by Silver-haired Bats (n = 30
roosts) during winter in the Ouachita Mountains of Arkansas.
Available Used % Select or
Forest class % (no. roosts) Z PA avoid
<15 years old, PB or P/H 9.6 3.3 (1) –1.17 0.244
15–50 years old, P, H, or P/HC 20.4 0.0 (0) –2.77 0.006* A
>50 years old, P or P/H 52.2 80.0 (24) 3.05 0.002* S
>50 years old, H 14.0 3.3 (1) –1.68 0.092
>50 years old, P or P/H, partial harvest, burn 3.4 13.3 (4) 3.00 0.003* S
Others 0.4 0.0 (0) –0.35 0.728
ACompared using multiple binomial tests and a Z approximation.
BFor forest types, P = pine, H = hardwood, and P/H = mixed pine and hardwood forests.
CIncluded closed-canopy Loblolly Pine plantations (5.5 % of available).
*Significant difference between used and available; controlled for experiment-wise error rate
using Benjamini-Hochberg control of the false discovery rate (FDR).
2010 R.W. Perry, D.A. Saugey, and B.G. Crump 569
via single-tree selection and subjected to recent controlled burning. Stands
15–50 years old were used less than their availability, and other habitat types
were selected in proportion to their availability. The single roost located in a
forest stand <15 years old was the rock outcrop. This outcrop was located in
a young clearcut where the rocks were exposed to full sun during the day.
We found Silver-haired Bats roosted mostly under loose bark of live,
mature (>50 years old) Shortleaf Pines during winter, but they also roosted
in other living trees, including cavities of hardwoods. We found no roosts in
snags. Similarly, Silver-haired Bats during migration roosted primarily under
bark of live Salix amygdaloides Anderss (Peach-leaved Willow) in Manitoba
(Barclay et al. 1988). We found bats occasionally roosted in cavities that
were on the ground, and they roosted relatively low in trees (average height
= 5.1 m) compared with studies of summer roost selection (e.g., Campbell
et al. 1996, Mattson et al. 1996). In other regions of North America, female
Silver-haired Bats (and many other cavity-roosting bats) typically roost in
tall snags during summer, which are generally taller than surrounding trees
or have less canopy cover than random trees (e.g., Betts 1998, Campbell et
al. 1996, Mattson et al. 1996). Roosting in taller, more-exposed trees may
increase solar radiation (i.e., heating) that may increase juvenile growth,
make roosts more easily accessible, and provide clear areas for navigation
by newly volant young. However, roost height of solitary Silver-haired Bats
(mostly males) may be lower in height than females in summer (Mattson et
In our study, bats roosted at relatively low heights that provided potential
solar exposure. Because deciduous trees comprised ≈40% of all
overstory trees and nearly all midstory trees in most stands where bats
roosted, abundant sunlight likely reached the forest understory in winter.
Roosting low in trees or on the ground during winter appears to be common
among some temperate forest bats that roost in forests during winter,
including Lasiurus borealis Müller (Eastern Red Bats; Mormann and Robbins
2007) and L. seminolus Rhoads (Seminole Bats; Hein et al. 2005,
2008). These species often roost in leaf litter when temperatures are coldest
(Hein et al. 2005, Mormann and Robbins 2007, Saugey et al. 1998).
Although we found Silver-haired Bats did not roost in leaf litter, they occasionally
roosted on the ground or in a rock crevice when temperatures were
lowest. Roosts also were located primarily on the southern sides of trees
that were situated on southern aspects, similar to winter roosts of Eastern
Red Bats (Mormann and Robbins 2007). Furthermore, other studies have
shown that switching to roosts close to or on the ground during colder periods
also may be common among other forest-dwelling bats that remain
active during winter (e.g., Boyles et al. 2005, Hein et al. 2008). Roosting
on or near the ground during winter likely provides thermal advantages because
soil and rocks retain heat during colder days, and ground-level roosts
570 Southeastern Naturalist Vol. 9, No. 3
may provide a more thermally stable environment during winter (Boyles et
al. 2005, Hein et al. 2008). Thus, ambient temperature and solar radiation
appear to be important to the location of roosts during winter, at both the
microhabitat scale and topographic landscape scale.
With the exception of one roost located in a rock outcrop in a young
clearcut, all roosts were located in pine or pine-hardwood stands that were
>50 years old, and bats used stands 15–50 years of age less than available.
Older stands provided abundant structure in the form of large, hollow trees
and older trees with exfoliating bark. Because young stands typically are
more dense (higher stem density) than older stands, these stands were likely
more cluttered, which presumably makes aerial navigation more difficult for
bats. Furthermore, the dense, closed-canopies of young pine stands likely
allow less solar radiation to reach the understory compared with more open,
older stands. The extensive use of mature pines we found may have been
an artifact related to the topoedaphic response of Shortleaf Pines in the
Ouachita Mountains. Shortleaf Pines are shade intolerant and grow primarily
on ridge tops and southern slopes in this region where direct sunlight is most
available; they are generally not abundant on north slopes. Nevertheless,
older stands, with their abundance of roosting sites (i.e., holes or peeling
bark on large trees) and more open vegetation structure appeared to be
important to the winter roosting ecology of Silver-haired Bats in our study
area. Similarly, forests containing mature (>50 years old) trees and less clutter,
including thinned and prescribe-burned forests, are important roosting
habitats for many species of forest bats during summer, whereas immature
forests with dense clutter are generally avoided (e.g., Kalcounis-Rüppell et
al. 2005, Perry and Thill 2008, Perry et al. 2007).
Our relatively low sample size and limited geographic spatial scale may
have reduced the precision of our inferences. However, this is the first quantitative,
telemetry-based study of winter roosting by Silver-haired Bats in the
southeastern US. Consequently, we believe our data provides useful insight
into the ecology of this species during winter in North America. Because
Silver-haired Bats roosted low in overstory trees, mostly on south slopes,
and mostly in stands dominated by mature (>50 years old) trees, retaining
open pine and hardwood stands >50 years old on south slopes would likely
provide suitable roosting habitat for wintering Silver-haired Bats in the
We thank R.L. Vaughn for Field Assistance and D. Robertson for providing GIS
data. This study was funded by the Ouachita National Forest. Earlier drafts were reviewed
by D.B. Sasse, S.B. Castleberry, N.E. Koerth, D.A. Miller, T.C. Carter, S.C.
Loeb, and guest editor C.D. Hein. The use of trade or firm names in this publication
is for reader information and does not imply endorsement of any product or service
by the US Department of Agriculture.
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