J. Zool., Lond. (2004) 262, 197–206
C 2004 The Zoological Society of London
Printed in the United Kingdom
DOI:10.1017/S0952836903004503
Nocturnal activity of the endangered Indiana bat (Myotis sodalis)
S. W. Murray* and A. Kurta
Department of Biology, Eastern Michigan University, Ypsilanti, MI 48197, U.S.A.
(Accepted 23 July 2003)
Abstract
Acoustic monitoring and radio-tracking were used to study the nocturnal activity of adult female Indiana bats
Myotis sodalis at a maternity site in Michigan, U.S.A. Pregnant bats did not use the day roost at night, but lactating
females returned 2–4 times/night for 32 ± 7 (SE) min/visit, presumably to feed their young. Both pregnant and
lactating females night roosted as solitary individuals in trees within their foraging areas; night roosting occurred
0–6 times/night for 14 ± 1 min each time. Bats foraged for most of the night, with the total duration of flight
equalling 375 ± 16 min/night. The bats used 13 different foraging areas that were located 0.5–4.2 km from the day
roost. Bats did not fly over open fields but travelled along wooded corridors, even though such behaviour increased
commuting distance by 55 ± 11%. Current models of habitat suitability for this endangered species should be
modified, taking into account the use of wooded commuting corridors and the large home range of these bats.
Key words: foraging, Indiana bat, Myotis sodalis, roost, time budget
INTRODUCTION
Time and shelter are important resources for any animal,
just like food or water, and the temporal and spatial
patterning of rest vs activity helps define the overall niche
of a species (Pianka, 1973; Schoener, 1974; Herbers,
1981). Natural selection should ultimately minimize time
spent performing many activities, such as foraging, because movement is often energetically expensive and
exposes individuals to potential predators and adverse
weather. Similarly, the type and location of resting sites
used by an animal also should be the product of natural
selection because most animals spend most of their time
at rest (Herbers, 1981).
Many bats, for example, display a pattern of prolonged
rest each day and perform bouts of energetically costly
locomotion for varying amounts of time each night. Bats
often spend 12–16 h/day resting in a cave, tree, or manmade structure termed a ‘day roost’ (Kunz, 1982), which
provides protection and a favourable thermal environment.
Individuals must, however, leave the relative safety of the
roost each night, and the time spent away from this shelter
is influenced by various factors including the distance
to foraging areas, temporal and spatial distribution of
prey (Anthony, Stack & Kunz, 1981; Rydell, 1989), the
physiological state of the bat (Racey & Speakman, 1987;
∗ All
correspondence to present address: Department of Biology, Boston
University, Boston, MA 02215, U.S.A.
Speakman & Racey, 1987; Barclay, 1989; Rydell, 1993;
Catto, Racey & Stephenson, 1995), and ambient temperature (Catto et al., 1995).
Flight, however, is energetically expensive, and typically the foraging activity of bats is not continuous, often
being interrupted by bouts of ‘night roosting’. Sites
used for night roosting may or may not resemble those
used for day roosting, but night roosts are often geographically separate from day roosts (Anthony et al.,
1981; Barclay, 1982; Lewis, 1994; Perlmeter, 1996). The
function of a night roost probably varies with species,
but some proposed functions are that night roosts act
as sites for dismemberment of prey, digestion, protection
from predators or adverse weather, transfer of information
concerning successful foraging sites, and/or facilitation of
social interactions (Kunz, 1982).
The Indiana bat Myotis sodalis is an insectivorous
species that occurs throughout much of the eastern
United States (Gardner & Cook, 2002). Most Indiana
bats hibernate in caves and mines, primarily in the states
of Missouri, Kentucky, and Indiana. In spring, females
migrate up to 500 km northward (Kurta & Murray, 2002),
to form maternity colonies consisting of 10–100 adults,
and ultimately each female gives birth to one offspring in
late June or early July (Kurta & Rice, 2002). Maternity
colonies typically roost during the day under the loose bark
of dead trees (Kurta, Williams & Mies, 1996; Callahan,
Drobney & Clawson, 1997; Kurta, Murray & Miller,
2002), but little is known about the foraging or roosting
behaviour of Indiana bats at night.
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S. W. MURRAY AND A. KURTA
The Indiana bat was declared endangered in 1967
because of large decreases in the population that were
thought to be caused by disturbance during hibernation
(current IUCN classification is ‘EN A1c endemic’;
Hutson, Mickleburgh & Racey, 2001). Despite protection
of all major hibernating colonies, however, the size of
the overall population continues to decrease, suggesting
that the cause of the decline may actually be operating
during summer (Clawson, 2002). Most previous research
on the Indiana bat in summer has focused on selection
of day roosts and diet (Kurta & Kennedy, 2002), but
development of a comprehensive management strategy
for this endangered species requires that biologists
understand its activities during the 9-h night, as well
as the 15-h day. Consequently, the aim of our present
study was to examine the nocturnal behaviour of the
Indiana bat. This study specifically describes temporal
patterns of nocturnal activity, examines how Indiana bats
use the landscape, and determines whether night roosts
are an important part of their ecology.
MATERIALS AND METHODS
Study area
A maternity colony of Indiana bats, consisting of
about 20 adult females and their offspring, was studied
from May to August in 1997 and 1998. These bats
roosted in dead trees near Norvell, Jackson County,
Michigan, U.S.A. (42◦ 09 N, 84◦ 11 W). Michigan is on
the northern edge of the summer range of this species
(Gardner & Cook, 2002), and the colony studied was
only the second discovered in Michigan (Kurta & Rice,
2002). Glacial features, such as coarse-textured terminal
moraines, kettle lakes, and sandy outwash deposits
(Albert, Denton & Barnes, 1986), dominated the study
area. The River Raisin (c. 14 m wide) was the principal
stream, flowing west to east and bisecting the study
area. Land cover consisted of agricultural and open fields
(55%), wetlands (including lowland hardwood forest;
19%), other forested habitats (17%), urbanized zones
(6%), and lakes/ponds/rivers (3%; Kurta, Murray et al.,
2002). Although most studies of tree-roosting bats occur in
large state-owned forests (e.g. Barclay & Brigham, 1996),
our study area was divided into many privately owned
parcels.
Capturing and radio-tracking
Adult female Indiana bats were caught in mist-nets set
across flyways near roosts, along commuting corridors,
or at foraging sites; netting typically occurred only
once every 7–14 days. After capture, the reproductive
condition of each female was assessed by palpation of
the abdomen and the condition of the nipples (Racey,
1988). For radio-tracking, miniature transmitters (Holohil
Systems, Ltd., Carp, Ontario, Canada) were attached
to the dorsal surface, using a surgical adhesive (Skin-
Bond Cement, Smith & Nephew United, Inc., Largo, FL,
U.S.A.). Transmitters weighed 0.5–0.7 g and represented
c. 8% of the body mass of the bat (Kurta & Murray,
2002). Radio-tracking did not start on the initial night
that a transmitter was attached, but on subsequent nights;
a receiver (TRX-2000S, Wildlife Materials, Carbondale,
IL, U.S.A) and a directional antenna were used to locate
the bats.
Nocturnal activity at a day roost
Use of a day roost at night was examined in 2 ways.
One gave an overall pattern of activity for the colony
and the other revealed individual patterns of behaviour.
Overall activity was quantified with a tunable, ultrasonic
detector (Mini-2, Summit, Birmingham, U.K.) that was
pointed toward a focal roost tree (sensu O’Donnell, 2000)
and placed on a stand, 1 m above the ground and 5 m
from the roost tree. From sunset to sunrise in 1997
and 1998, the observer remained near the detector and
counted ‘bat-passes,’ which were defined as sequences of
vocalizations that included 3 or more pulses (Walsh &
Harris, 1996). To exclude calls of other species of bat,
the detector was tuned to 50 kHz. This frequency was
within the range of frequencies used by the Indiana bat
(O’Farrell, 1999) but above those used by the big brown
bat Eptesicus fuscus, which was the most common species
in the area. Other species that consistently include 50 kHz
in their calls, such as the little brown bat Myotis lucifugus
or northern long-eared bat Myotis septentrionalis, are
rarely encountered in this part of Michigan (Kurta, 1980,
1995).
Although the overall activity of the colony was determined by counting bat-passes, the nocturnal activity of
individuals at the day roost was determined by monitoring bats fitted with radio-transmitters (‘transmitter
bats’). These bats were being radio-tracked as part of a
companion study that examined day-roosting behaviour
(Kurta, Murray et al., 2002). In our study, a radio
receiver was stationed c. 5 m from the day roost, and
the presence or absence of the transmitter bat at the
day roost was determined throughout the night by an
observer sitting nearby. Presence or absence was recorded
every 10 min, except for pregnant bats in 1997 that were
monitored at 30-min intervals. How often bats returned
to the day roost each night and the amount of time
spent inside the day roost at night were determined
by monitoring the presence/absence of transmitter
bats.
Continuous radio-tracking
To detect nocturnal roosting away from the day roost and to
identify foraging areas, transmitter bats were followed in
a car and/or on foot throughout the night in 1998. Changes
in strength and direction of the signal were used to
determine whether a bat was flying or roosting (Entwistle,
Racey & Speakman, 1996; Kalko et al., 1999; O’Donnell,
Foraging and night roosting
199
roosting, and night roosting; number of maternityand night-roosting bouts; duration of maternity- and
night-roosting bouts; number of different night roosts;
number of foraging bouts; minimum commuting distance
travelled.
2002), and this behaviour was recorded every 10 min.
The amount of time spent flying or roosting was then
calculated by multiplying the number of intervals spent
in each activity by 10. The amount of time spent in
the day roost before evening emergence but after sunset,
time spent in the day roost during the night but before
the morning return (‘maternity roosting’), time spent
in a night roost different from the day roost (‘night
roosting’), and time spent in the day roost after the
morning return but before sunrise were quantified. In
addition, external ambient temperature was recorded
every 15 min with a data logger (Optic Stowaway, Onset
Computer Corporation, Pocasset, MA, U.S.A.) that was
placed within 100 m of a day roost in a white, perforated,
wooden box.
Flight paths used by the bats also were recorded
whenever possible, and activity centres were identified
using the homing-in method of White & Garrott (1990).
An activity centre was conservatively identified as one
in which any bat was flying for 30 min or more
(Entwistle et al., 1996). Because of the amount of time
the bats spent flying in localized activity centres, it
was assumed that they were foraging in these areas
(Entwistle et al., 1996; Güttinger et al., 1998; O’Donnell
et al., 1999). No attempt was made to estimate the
absolute size of activity centres or to quantify habitat
types within them, because it was not possible to determine the precise boundaries of these areas. Typically
only 1 observer was in the field at any time, which
prevented triangulation, and the large number of privately owned parcels of land restricted our movements
mostly to public roads and lanes, which often prevented
a close approach to the bat and an exact ‘fix’ on its
position.
Nocturnal activity was monitored at a focal dayroosting tree with the bat detector on 13 nights during
pregnancy (25 May–13 June). The average number
of bats seen leaving the roost was 12 ± 1 (SE), and
mean number of bat-passes/night was 133 ± 36. Activity
(bat-passes) differed significantly among segments of
the night (χ 2 = 17.10, d.f. = 2, P < 0.001), with fewer
passes during the middle of the night (11 ± 6 passes)
compared with early (14 ± 2 passes; Z = –2.20, P = 0.03)
or late segments (108 ± 32 passes; Z = –3.11, P =
0.002).
Activity at the day roost was also acoustically monitored
on 16 nights during lactation (26 June–23 July). The
average number of bats leaving a roost was 16 ± 1, but
the total number of bat-passes/night was 764 ± 60, a
33% increase in population size but a 570% increase in
activity during lactation compared with pregnancy. Unlike
pregnancy, there was no significant difference in activity
among the three nightly segments for lactation (χ 2 = 3.57,
d.f. = 2, P = 0.17), with an average of 243 ± 25, 279 ± 28,
and 250 ± 27 passes, during the early, middle, and late
segments, respectively.
Statistical analyses
Nocturnal activity of transmitter bats at the day roost
To examine overall activity of the colony at the day roost,
the night was divided into 3 equal segments (early, middle,
and late), and differences in activity (bat-passes) looked
for using the Friedman test (Zar, 1999). If the Friedman
test indicated a significant difference in activity across
the night, a priori comparisons were made between the
middle segment and the early and late segments using
the Wilcoxon signed-ranks test. Comparisons were made
only between middle vs early and middle vs late segments,
because use of separate night roosts during the middle of
the night was predicted from the behaviour of the closely
related little brown bat (Anthony et al., 1981; Kunz,
1982).
Although ultrasonic detection gave a view of the overall
activity of the colony, the activity of individuals was
determined by radio-tracking. Not all bats were radiotracked for the same number of nights, however, and
to avoid non-independence, only mean values for each
bat were used in statistical comparisons. To compare
activity of pregnant and lactating individuals, 1-way
analyses of variance were conducted on means of each of
10 categories: total time per night spent flying, maternity
Attempts were made to monitor the nocturnal activity
of 16 bats with radio-transmitters at a focal tree used
as a day roost, on 36 transmitter-nights (one transmitternight was equivalent to an individual bat being monitored
throughout one night). Indiana bats, however, often
changed their day roost, particularly during pregnancy
(Kurta, Murray et al., 2002), and on some nights,
bats did not return to the focal tree that was being
monitored. Consequently, only transmitter bats that
were in the observational roost tree or a nearby tree
(within c. 50 m) at sunrise were considered. The final
analysis, therefore, was based on 12 bats yielding 23
transmitter-nights: eight pregnant bats for 11 transmitternights and four lactating bats for 12 transmitternights.
Both pregnant and lactating bats with transmitters
typically emerged 20–30 min after sunset and began day
roosting 10–40 min before sunrise. Although pregnant
animals never returned to the day roost before dawn,
each lactating individual returned 2–4 times/night, with
a mean of 2.3 visits/night to the day roost. The average
duration of each visit was 36 ± 8 min for a total of
RESULTS
Acoustic monitoring of nocturnal activity
at the day roost
200
S. W. MURRAY AND A. KURTA
Table 1. Nocturnal behaviour of Indiana bats Myotis sodalis in Jackson County, MI, U.S.A., based on all-night radio-tracking of seven
individuals (four pregnant and three lactating) on 17 transmitter-nights (nine during pregnancy and eight during lactation)
Parameter
Pregnancy
(meana ± SE)
Lactation
(meana ± SE)
Time in contact (% of night)
No. of maternity-roosting bouts/night
Duration of maternity-roosting bout (min)
Total time spent maternity roosting/night (min)
No. of night roosts
No. of night-roosting bouts/night
Duration of night-roosting bout (min)
Total time spent night roosting/night (min)
Time spent in flight (min)
No. of foraging areas/night
No. of foraging bouts/night
Minimal commuting distance/night (km)
91 ± 6
0.2 ± 0.2
15b
2.5 ± 2
1.5 ± 0.5
3.1 ± 1.0
12 ± 0.3
41 ± 14
392 ± 12
1.6 ± 0.3
2.1 ± 0.5
8.3 ± 1.2
91 ± 2
2.0 ± 0.2
35 ± 12
67 ± 21
0.9 ± 0.2
1.4 ± 0.5
16 ± 2
20 ± 6
352 ± 34
2.0 ± 0.4
3.1 ± 0.5
13.3 ± 1.9
F
P
0.09
51.91
0.78
0.0008
12.87
0.55
4.81
1.96
4.12
0.25
0.54
1.97
5.79
0.02
0.40
0.25
0.22
0.25
0.25
0.50
0.22
0.06
Combined
(meana ± SE)
91 ± 2
1.3 ± 0.3
2.4 ± 1
14 ± 1
32 ± 9
375 ± 16
1.8 ± 0.2
2.5 ± 0.4
10.4 ± 1.4
To preserve independence, average values for each bat were used; hence, n = 4 for pregnancy, n = 3 for lactation, and n = 7 for combined
data.
b
Only two bouts of maternity roosting, for 30 min total, were recorded during pregnancy, and both were by the same bat on the same night
during periods of rain.
a
79 ± 13 min/night. Adult females were outside the day
roost (flying or roosting elsewhere) for 8.0 ± 0.05 h/night
during pregnancy compared with 6.8 ± 0.2 h/night when
lactating. The average length of the night (sunset to
sunrise) was 8.9 ± 0.04 h and 9.1 ± 0.05 h for observational nights during pregnancy and lactation, respectively.
Radio-tracking throughout the night
Estimated time budgets and use of roosts
During 1998, nine different bats were followed throughout
the night for 34 transmitter-nights. Bats occasionally flew
out of range of the receiver, however, and to develop
nocturnal time budgets, only nights during which contact
was made with the transmitter bat for > 80% of the
sampling periods were used. Final analyses, therefore,
were based on seven bats or 33% of the approximate
number of adult females in the colony. Four pregnant and
three lactating bats were tracked for 17 transmitter-nights
(nine nights during pregnancy and eight nights during
lactation), and during these nights, contact was made with
the transmitter bat for 91 ± 2% of the sampling periods
(Table 1).
The results of continuous radio-tracking were similar to
those obtained by stationary monitoring of echolocation
calls or transmitter bats at the day roost in that pregnant
females typically did not return to the day roost before
dawn. The only exception was a single pregnant individual
on one night that returned twice to the day roost (once for
10 min and once for 20 min), but both visits coincided
with periods of heavy rain. In contrast, lactating animals
made 2.0 ± 0.2 visits/night to the day roost. The duration
of each visit was 35 ± 12 min, and each bat spent a
total of 67 ± 21 min inside the day roost each night.
The duration and number of visits by three lactating
females determined by continuous radio-tracking were
similar to those determined by stationary monitoring of
four lactating transmitter bats at the day roost, as reported
earlier (2.3 visits/night for 36 min each). Combining
results from the two techniques indicated that there were
2.2 ± 0.2 visits/night, with each visit lasting 32 ± 7 min,
for a total of 68 ± 11 min/night in the day roost; these
combined results are based on 20 nights of data for six
different lactating bats (to preserve independence, a mean
value was used for one lactating bat that provided data
for both stationary monitoring and for continuous radiotracking).
No other statistical differences in nocturnal behavior
of pregnant and lactating Indiana bats were identified
(Table 1), so data from the two groups were combined
for further analyses. The seven Indiana bats were in flight
for 375 ± 16 min/night. They used night roosts separate
from the day roost 0–6 times/night (2.4 ± 1 roosting
bouts/night), for a total of 32 ± 9 min/night. Each nightroosting bout lasted 14 ± 1 min, but the duration of 71%
of the bouts was only 10 min (i.e. one sampling period).
Use of multiple night roosts was indicated when
roosting occurred in separate activity centres or when
stationary signals came from distinctly different directions
within a particular centre. Some transmitter bats occupied
as many as three different night roosts within the same
night, although the average was 1.3 ± 0.3 roosts/night. All
bats night roosted in areas in which they foraged, i.e. bats
did not fly from a foraging area to a different site just to
night roost.
It was not possible to determine the exact location of
any night roosts because resting periods were so brief.
Nevertheless, one bat was approached to within c. 15 m of
its night roost before it resumed flight. This bat was radiotracked in a riparian woodlot dominated by green ash and
silver maple and within which there were no buildings or
other man-made structures, which suggested that the bat
was roosting in a tree.
Foraging and night roosting
201
River Raisin
Day roost
N
Lakes / ponds
Forested areas
Roads
Streams and rivers
Foraging areas
km
Fig. 1. Study area, showing approximate location of 13 foraging areas (circles) used by Indiana bats Myotis sodalis in 1998. Shaded areas,
forested sites or bodies of water, as indicated; c. 90% of the unshaded area consisted of agricultural fields and pastures.
In addition, as part of our study of day-roosting
behaviour (Kurta, Murray et al., 2002), a transmitter was
detected in a dead black ash Fraxinus nigra after dawn on
22 June 1997. Subsequent observations at dusk indicated
that the transmitter remained in the tree that night and for
the next several days; no bat, however, was seen leaving
the tree, and none was heard with an ultrasonic detector.
These observations, coupled with recapture of the bat,
which had been banded, 11 days later, indicated that the
transmitter had detached while the bat was using the ash
as a night roost.
Foraging activity
Thirteen activity centres or foraging areas were discovered
by radio-tracking in 1998 (Fig. 1). Five of these were
used only by pregnant individuals, four were used only by
lactating bats, and four were used both during pregnancy
and lactation. Females visited 1–4 (1.8 ± 0.2) foraging
areas/night. The smallest circle that included all foraging
areas had a diameter of 7.5 km.
Members of the colony rarely seemed to forage together.
When two transmitter bats were monitored on the same
night, they spent most, if not all, of the night hunting in
different areas. The presence of two radio-tagged bats in
the same area occurred on only four occasions, and the
longest that two bats foraged near each other was 30 min.
During two nights, three bats were monitored at the same
time, and they generally were not present in the same
foraging areas, indicating concurrent use of at least three
foraging sites by members of this small colony. Different
transmitter bats were detected simultaneously in foraging
areas located up to 5.5 km apart.
Minimum straight-line distances from the day roost to
the estimated nearest edge of each activity centre ranged
from 0.5 to 4.2 km, with a mean of 2.4 km (Fig. 1).
However, these bats were never detected crossing open
areas on any of the 34 transmitter-nights, which indicated
that the bats seldom took the shortest route to a foraging
site. Day roosts used by this colony in 1998, for example,
were on the southern edge of an open wetland (Fig. 1;
also see Kurta, Murray et al., 2002: fig. 2, photo), with
five foraging areas located north of the day roost and
the others, mostly to the south. Upon emergence, all bats
immediately flew south of the roost tree into a wooded
area, regardless of where they eventually foraged, and
did not fly over the open wetland. In addition, northern
and southern foraging areas were separated by extensive
agricultural fields bordering the River Raisin, and the only
route between northern and southern sites used by the bats
was along a fence, separating two fields and lined with a
single row of mature trees; this tree-lined corridor was
used consistently and predictably by commuting Indiana
bats in at least 5 different years (Kurta & Murray, 2002).
Delineating the complete path that each bat took from its
roost to a foraging area was difficult, because of the large
distances covered in a short time, but this was achieved for
six bats (eight trips). The actual distance flown from the
day roost to a foraging area averaged 55 ± 11% greater
202
S. W. MURRAY AND A. KURTA
than the straight-line distance; the extra distance flown
varied from 0.2 to 3.4 km.
Foraging habitat
Activity centres were not determined by triangulation;
hence it was not possible to quantify reliably the size
of these areas or the proportions of various habitats
within them. Nevertheless, some qualitative statements
can be made concerning the 13 foraging sites. One
activity centre was over a small lake (Watkins Lake),
and one consisted of c. 50% wooded areas and 50%
open fields; the other 11 were dominated by woodland,
especially forested wetlands. In addition to Watkins Lake,
lakes/ponds occurred in five other areas. Only two sites
were in riparian forest, and only one of these actually
included the River Raisin. Ten sites were >1 km from the
River Raisin, and no other streams were associated with
any foraging centre.
Effects of temperature
During all-night radio-tracking, minimum ambient
temperature varied from 10.5 to 20.8 ◦ C. After correcting
for differences owing to reproductive condition, there was
no significant correlation between minimum temperature
and number of night-roosting bouts, time spent in night
roosts, or time spent flying (partial correlation analysis;
all P > 0.05).
DISCUSSION
Nocturnal activity at the day roost
Acoustic monitoring during pregnancy indicated that
activity was lower during the middle of the night compared
with equivalent periods after sunset and especially before
dawn. Pregnant individuals typically did not use the day
roost at night and spent 7–8 h/night outside the day roost.
In contrast, intermittent activity occurred throughout the
night at the day roost during lactation, with transmitter bats
returning 2–4 times/night. The pattern of activity at the day
roost during pregnancy and lactation was the same whether
it was determined through acoustic means or through
radio-tracking, indicating that carrying a transmitter did
not grossly alter behaviour.
Most previous studies of the nocturnal activity of bats
at a day roost have involved species that roosted in
buildings (e.g. Swift, 1980; Maier, 1992; Catto et al.,
1995; Entwistle et al., 1996; Henry et al., 2002). Although
the exact pattern of nocturnal behaviour varied among
species, most were similar to our study in showing
increased activity at the day roost during lactation compared with pregnancy. Some of the added activity that
was observed (133 bat-passes/night during pregnancy vs
764 bat-passes/night in lactation) was owing to a slightly
larger population size (16 adults during lactation vs 12 bats
during pregnancy), and much of the increase presumably
was caused by females repeatedly returning to nurse their
offspring. In addition, a large proportion of the increased
activity indicated by acoustic monitoring was attributed
to ‘checking behaviour’ (Gardner, Garner & Hofmann,
1991: 43), which involves individuals leaving the roost,
repeatedly circling it, often approaching and/or landing on
the tree, and occasionally re-entering the roost. Checking
behaviour, particularly at dusk, is not common during
pregnancy.
Night-roosting behaviour
Indiana bats used night roosts that were distinct from
the day roost, both during pregnancy and lactation. They
typically night roosted for multiple, brief periods each
night at sites distant from the day roost and within activity
centres (foraging areas). In a single night, individual bats
used up to three different night roosts, night roosted up to
six times, and spent 10–30 min roosting during any one
bout. The number of night roosts used and the number
of night-roosting bouts were potentially underestimated
because our protocol could have missed roosting sessions
of <10 min (Audet, 1990; Entwistle et al., 1996).
Although Indiana bats were recently discovered night
roosting under bridges in southern Indiana (Kiser et al.,
2002), Indiana bats in Michigan probably used trees
as night roosts. Most activity centres did not contain
buildings or bridges that could be used as night roosts,
and rocky cliffs, such as those occupied by pallid bats
Antrozous pallidus (Lewis, 1994), are non-existent in this
part of Michigan (Dorr & Eschman, 1970). Moreover, one
transmitter became detached from an Indiana bat while it
was apparently night roosting in a tree.
Unlike the closely related little brown bat that typically
night roosts in large groups (Anthony et al., 1981),
Indiana bats apparently night roosted and foraged as
solitary animals. This conclusion is based on several lines
of evidence. First, night roosting occurred within the
foraging areas, but whenever more than one transmitter
bat was monitored, different individuals typically foraged
in different areas. Secondly, Indiana bats night roosted for
short periods of time, which presumably would not allow
a group to form before some bats began foraging again,
especially considering the small size of the day-roosting
colony (≤ 21 adults in 1998). Thirdly, night roosting often
occurred in multiple locations during a single night, and
it was unlikely that bats dispersed over many kilometres
could communicate the location of the next night-roosting
site, as well as pinpoint the time to be there. Finally,
bats that use communal night roosts typically are highly
faithful to a specific roosting site (Barclay, 1982; Lewis,
1994). Therefore, if Indiana bats were roosting in groups,
individuals would be expected to night roost consistently
in the same area, but there was no evidence for this.
Many functions have been proposed for night roosting.
However, the solitary behaviour of Indiana bats, the brief
duration of night roosting, and the lack of correlation
between duration and temperature exclude information
transfer, energetic savings from clustering, and protection
from predators and/or weather as the primary function
of night roosting in this species (Kunz, 1982). Indiana
Foraging and night roosting
bats may have night roosted simply to rest or perhaps
because they were satiated and needed to digest their latest
meal before feeding again (Barclay, 1982). In addition, day
roosts of Indiana bats are typically located within foraging
areas (Murray, 1999; Kurta, Murray et al., 2002), and
short bouts of night roosting may involve assessment of
potential day roosts.
Temporal patterns of foraging behaviour
Indiana bats foraged or commuted for much of the
night (375 min), despite the high energetic costs of flight
(Thomas & Suthers, 1972; but see Winter & Helversen,
1998). Although insectivorous bats in tropical areas forage
for as little as 60 min nightly (Fenton & Rautenbach,
1986), those from temperate areas typically spend a
much greater time out of the day roost, presumably
foraging (Entwistle et al., 1996; Hickey & Fenton, 1996).
For example, in Germany, radio-tagged notch-eared
bats Myotis emarginatus were active for 362 min/night,
with short periods of night roosting lasting 5–67 min
each (Krull et al., 1991), and mouse-eared bats Myotis
myotis foraged up to 363 min/night, night roosting 1–
12 times per night, for 1–37 min each time (Audet, 1990).
Similarly, long-tailed bats Chalinolobus tuberculatus in
New Zealand foraged for an average of 354 min/night
(O’Donnell, 2002).
Based on field measurements of daily metabolic rate
in little brown bats, Kurta, Bell et al. (1989) predicted
that pregnant and lactating insectivorous bats would
forage for similar amounts of time, even though average
daily energy requirements during lactation were much
greater than during pregnancy. Insects are typically more
abundant during lactation than pregnancy (Anthony &
Kunz, 1977), which would reduce the cost of searching
for prey, and lactating females weigh less than pregnant
individuals, which would reduce energy expended per
gram of prey captured. Although sample size was low,
our data supported this prediction because pregnant and
lactating bats did not statistically differ in the amount
of time they spent flying. Similarity in duration of flight
between pregnant and lactating individuals, based on
radio-tracking, has also been reported for other Myotis,
including little brown bats (Henry et al., 2002) and mouseeared bats (Audet, 1990).
203
1983). Based on radio-tracking, Gardner et al. (1991)
estimated that pregnant and lactating Indiana bats in
Illinois typically flew only about 1 km from their
roost to the centre of their foraging range. Both
the largest (this study) and some of the smallest
distances (Gardner et al., 1991) to foraging areas were
determined by radio-tracking, indicating that the observed disparity in the size of the home range is not the
result of methodological differences. We speculate that the
variation in home-range size reflects differences in quality
of habitat.
Our observations indicated that Indiana bats did not fly
over open areas but seemed to follow tree-lined paths.
Studies on other vespertilionid species also documented
bats following edges, such as tree lines, hedgerows,
forest edges, and banks of rivers (Limpens & Kapteyn,
1991; Entwistle et al., 1996; Verboom & Huitema, 1997;
Verboom & Spoelstra, 1999). Tree-lined sites may offer
protection from predators or from wind and may include
landscape features that aid orientation (Entwistle et al.,
1996; Verboom & Huitema, 1997; Verboom & Spoelstra,
1999). Although wooded corridors may provide higher
densities of insects compared with open areas (Verboom &
Spoelstra, 1999), the flight of Indiana bats along the fenceline connecting northern and southern foraging areas was
rapid and direct, indicating that the bats were not feeding.
Foraging habitat
Over 60% of the study area was agricultural fields or
urbanized areas (Kurta, Murray et al., 2002), yet 12 of
13 foraging sites were dominated by forest. Our results
were consistent with those of other studies that showed
Indiana bats foraging preferentially in wooded areas
(Humphrey et al., 1977; Brack & LaVal, 1985; Garner &
Gardner, 1992). The diet of this colony included large
amounts of aquatic-based insects, especially dipterans and
trichopterans (Murray & Kurta, 2002), which correlated
with the presence of forested wetlands in most foraging
areas and existence of open water (lakes, ponds, or the
River Raisin) in or near most sites. Although the River
Raisin often was used as a commuting corridor, only one
foraging area actually encompassed a portion of the river.
Humphrey et al. (1977), in contrast, indicated that Indiana
bats in the state of Indiana restricted foraging to riparian
areas, and Garner & Gardner (1992) reported that Indiana
bats in Illinois preferred to hunt in floodplain forest.
Spatial patterns of foraging behaviour
Members of this colony of Indiana bats apparently used
13 distinct foraging areas that were located up to 4.2 km
from the day roost (Fig. 1), and consequently, members
of the colony commuted long distances each night
(≤ 20 km/night). In contrast, members of a maternity
colony in the state of Indiana only foraged along a
0.82-km strip of riparian woods near the day roost
(Humphrey, Richter & Cope, 1977), and in Missouri,
light-tagged individuals did not fly farther than 2 km
from the release point (LaVal et al., 1977; Brack,
Management implications
The mobility of bats allows them to disperse daily
over large expanses of land compared with non-volant
mammals of similar size, making the management of bats
more difficult because of the increased area that must be
protected or regulated (Kurta, 2001). In a single maternity
season, members of this colony of Indiana bats used at
least seven day roosts (Kurta, Murray et al., 2002) and
13 distinct foraging areas that were up to 4.2 km from the
204
S. W. MURRAY AND A. KURTA
day roost (Fig. 1). Management of the Indiana bat also
is complicated by its existence in a highly fragmented
agricultural landscape (Gardner & Cook, 2002). Although
Indiana bats in Michigan used multiple feeding areas,
these seemingly isolated patches of foraging habitat were
interconnected by wooded corridors as small as a single
line of trees. It seems probable that both the size of
habitat patches (de Jong, 1995; Carter et al., 2002)
and their wooded connectivity are important factors
in determining whether an area is suitable for this
species.
Thus, wildlife stewards must manage habitat for the
Indiana bat on a landscape scale, taking into consideration
the long distances that these bats fly each night and
the corridors necessary to move safely from day roosts
to foraging areas, as well as essential day-roosting and
foraging habitat. Current models of habitat suitability
for the Indiana bat (Rommé, Tyrell & Brack, 1995;
Farmer, Cade & Staufer, 2002) are insufficient, because
they presume small home ranges (circles of 1-km radius)
and do not consider the need for wooded commuting
corridors. The inclusion of night-roosting habitat into
these models, however, is probably not necessary because
Indiana bats apparently use trees within their foraging
areas, and presumably good foraging habitat will generally
contain suitable night-roosting sites. Although this bat
once was considered a riparian specialist (Humphrey
et al., 1977), only one of the 13 foraging areas identified
in this study included a stream; consequently, future mistnetting or acoustic surveys for this species should be
carried out in a variety of forested areas and not only over
streams. Finally, further studies are needed to quantify
foraging/night-roosting habitat for this species, and to
determine whether our results are broadly applicable to
the Indiana bat in other parts of its range and to other
tree-roosting species.
Acknowledgements
We thank S. Carson, R. Trachet, and J. Trolz for access to
their land, and J. Caryl, J. Kappler, C. King, and J. Werner
for aiding in fieldwork. J. Knapp provided statistical
assistance, and C. Shillington created the map of our study
area. B. Betts, T. Kunz, and C. O’Donnell commented on
the manuscript. Funding was provided by grants to SWM
from Bat Conservation International and to AK from the
Michigan Chapter of the Nature Conservancy and the
Nongame Wildlife Fund of the Michigan Department of
Natural Resources.
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