Evaluating the Effectiveness of the Standard Mist-netting
Protocol for the Endangered Indiana Bat (Myotis sodalis)
Lynn W. Robbins, Kevin L. Murray, and Paul M. McKenzie
Northeastern Naturalist, Volume 15, Issue 2 (2008): 275–282
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2008 NORTHEASTERN NATURALIST 15(2):275–282
Evaluating the Effectiveness of the Standard Mist-netting
Protocol for the Endangered Indiana Bat (Myotis sodalis)
Lynn W. Robbins1,*, Kevin L. Murray1,2, and Paul M. McKenzie3
Abstract - Standardized mist-netting protocols set guidelines regarding the best
way to sample a bat species or community. We evaluated the current mist-netting
guidelines designed to determine presence or absence of endangered, Myotis sodalis
(Indiana bat). This test was conducted in Deer Ridge Conservation Area
(Lewis County, northeastern Missouri), an area known to have an abundance of
Indiana bats, including several primary maternity colonies. We mist-netted according
to recommended guidelines for Indiana bats for two consecutive nights
at three different times during the reproductive season. Anabat II detectors were
used in conjunction with mist nets to sample bat activity at the same locations.
Captures and detections of Indiana bats and other species of bats varied substantially
among the sampling periods. In addition, there was a significant decrease in
number of Indiana bats captured with mist nets from night one to night two, although
activity levels remained the same. Finally, our data show that augmenting
mist nets with ultrasonic detectors can enhance the probability of determining the
presence or absence of Indiana bats.
Introduction
A standard mist-netting protocol should be designed to provide an accurate
sample of the chiropteran biodiversity in an area. However, the
emphasis shifts to maximizing the accuracy of determining presence or
absence when monitoring for an endangered species. Myotis sodalis (Miller
and G.M. Allen) (Indiana bat) was listed as endangered by the US Fish and
Wildlife Service in May 1967. Since then, populations in the southern states,
including Missouri, have continued to decline or have become stable at substantially
lower numbers, although overall the population is increasing (US
Fish and Wildlife Service 2007). Observations suggest (Gardner and Cook
2002) that most females of this species migrate north in the spring, where
they form summer maternity colonies under the exfoliating bark of dead
trees. Because Indiana bats inhabit forests, any disturbance to this habitat
within the summer range of the species could disrupt their reproductive success
in an area. As a result, the US Fish and Wildlife Service (1999, 2007),
in collaboration with the Indiana Bat Recovery Team, provided standard
mist-netting guidelines to determine presence or absence of Indiana bats in
a particular area.
1Department of Biology, Missouri State University, Springfield, MO 65897. 2Current
address - Department of Biology, University of Miami, Coral Gables, fl33124. 3US
Fish and Wildlife Service, 101 Park Deville Drive, Suite A, Columbia, MO 65203. *Corresponding
author - lynnrobbins@missouristate.edu.
276 Northeastern Naturalist Vol. 15, No. 2
Two recent studies have evaluated the effectiveness of these mist-netting
guidelines. MacCarthy et al. (2006) tested a subset of the mist-netting guidelines
by video-recording bat activity at net sites. They showed that 23% of
bats were able to avoid the mist net and concluded that the overall bat capture
rate would be increased by decreasing or eliminating the time between
net checks. This decreased time period (10 min.) has been included in the
2007 revision. Carroll et al. (2002) suggested that, in addition to potential
flight corridors, nets should be placed in forest interiors to more effectively
document the presence of Indiana bats. Both of these studies demonstrate
how mist-netting protocols can be improved with empirical data.
The purpose of this project was to test the effectiveness of the accepted
mist-net protocol in determining presence or absence of Indiana bats. Specifically, we investigated three questions regarding the Indiana bat mist-net
protocol: 1) Does the probability of capturing an Indiana bat vary throughout
the maternity period (15 May to 15 August)? 2) Does the probability
of capturing an Indiana bat at a net site vary from night 1 to night 2? and
3) Are mist nets alone able to reliably detect Indiana bats when present? We
conducted our study at Deer Ridge Conservation Area in northeastern Missouri.
Because it had abundant Indiana bats, this site was an ideal location to
evaluate the mist-netting protocol. We followed the standard guidelines used
to survey Indiana bats except that sampling sites were always within 1.6 km
of known maternity colonies.
Methods
The study was conducted May–August 2002 in conjunction with a larger
study on the breeding and foraging habitat of Indiana bats and other vespertilionid
bats in managed and unmanaged forests (Miller 2003, Timpone
2004, Timpone et al. 2006). The study area was Deer Ridge Conservation
Area (DRCA) and adjacent private lands (Lewis County, northeastern Missouri).
Three primary maternity roosts (Callahan et al. 1997), each with >100
individuals, were located within DRCA. All mist netting was conducted
according to the Indiana Bat Recovery Plan standardized mist-netting guidelines.
The guidelines are as follows: 1) netting must be conducted between
15 May and 15 August; 2) for every 1 square km (100 ha), two net sites are
required; 3) each net site must consist of two nets no less than 30 m apart,
each of which must be monitored for a minimum of two nights; 4) although
not required, nets are usually placed in potential flight corridors (e.g.,
streams, road ruts, logging roads), perpendicular to the corridor, covering as
much of the corridor as possible; 5) the sampling period must begin at sunset
and last a minimum of 5 hours; and 6) surveys must not be conducted during
precipitation, if temperatures are <10 °C (50 °F), during strong winds, or in
bright moonlight.
We surveyed three net sites (2 net sets each) on service roads and a stream
within 1.6 km of primary roosts. Site 1 consisted of an open-canopy service
road (12–15 m wide), site 2 consisted of an open-canopy service road and
2008 L.W. Robbins, K.L. Murray, and P. McKenzie 277
stream (10–12 m wide), and site 3 consisted of a closed-canopy service road
(7–10 m wide). For each net set, we used standard 50-dernier/2-ply nylon
mist nets with 38-mm mesh size (Avinet, Inc.; www.avinet.com) and stacked
three 2.6-m high nets. Mist-net width varied from 6 m to 12 m depending
on the site. We monitored each net site during three different reproductive
periods of Indiana bats: pregnancy (10–20 June), lactation (15–21 July), and
post-lactation (6–12 August). Within each reproductive period, we monitored
each net site for two consecutive nights whenever possible (always
within two nights). One bat detector set-up (Anabat II detector, ZCAIM, and
laptop) was placed at each net site to record echolocation calls throughout
the sampling period (see Britzke et al. 2002, Murray et al. 1999). Detectors
were placed >25 m from all mist nets. We identified all bats captured in mist
nets to species. Echolocation calls were identified to species according to the
methods of Britzke et al. (2002).
We compared the difference in capture success and species richness
from night one to night two for both nets and bat detectors using a Wilcoxon
signed ranks test. We conducted these tests for all species combined and for
Indiana bats. To determine if bats are sampled more effectively with nets or
detectors, we compared differences in species richness between techniques
using a Wilcoxon signed ranks test. Species richness was defined as number
of species. All statistical tests were two-tailed with α = 0.05.
Finally, we calculated capture/detection success for each technique and
both techniques combined for four of the most common species encountered
in our survey. Mist-net success is often expressed as capture rate (number of
bats per unit effort) and reported as a percent. We were interested in estimating
the probability that a particular technique was successful in determining
presence or absence of the target species. Therefore, we defined the capture/
detection success as number of net sets (detectors) that captured (detected)
the target species divided by the total number of nets sets (detectors) used.
A net set was defined as a set of 2 nets used on a single night.
Results
We captured 139 individuals representing seven species. Bat captures
for each reproductive period and night are shown in Table 1. Data from
three species, Lasiurus cinereus Palisot de Beauvois (Hoary bat, n = 1),
Myotis lucifugus LeConte (little brown bat, n = 3), and Perimyotis subflavus
F. Cuvier (eastern pipistrelle, n = 3), were not included due to small
sample size. Substantial variation in capture success occurred among reproductive
periods (Table 1). Patterns were not consistent among species.
Small sample size precluded statistical analysis of variation among reproductive
periods.
With the exception of Nycticeius humeralis Rafinesque (evening bat),
captures were consistently lower on the second night of netting than on the
first night (Table 1). Both Indiana bats (meannight1 = 1.33 ± 0.55; meannight2 =
0.11 ± 0.11; Z = -2.07; P = 0.038) and total captures (meannight1 = 10.00 ± 1.68;
278 Northeastern Naturalist Vol. 15, No. 2
meannight2 = 5.56 ± 0.11; Z = -2.16; P = 0.031) were significantly lower on the
second night of netting. This difference did not reflect a change in bat activity
at net sites because neither the number of Indiana bats (meannight1 = 20.67 ±
10.12; meannight2 = 12.89 ± 4.65; Z = -0.840; P = 0.401) nor total bats (meannight1
= 124.56 ± 54.15; meannight2 = 98.11 ± 31.49; Z = -0.652; P = 0.514) recorded
with bat detectors were significantly different from night 1 to night 2. Finally,
we found no significant difference in species richness from night 1 to night 2
for either nets (meannight1 = 3.78 ± 0.49; meannight2 = 2.78 ± 0.49; Z = -1.558;
P = 0.119) or detectors (meannight1 = 3.56 ± 0.44; meannight2 = 4.11 ± 0.35; Z =
-1.121; P = 0.262).
Species richness was not significantly different between nets and detectors
(meannets = 3.28 ± 0.36; meandetectors = 3.83 ± 0.28; Z = -0.991; P = 0.322).
Capture/detection success varied for nets, detectors, and combined (Table 2).
Detectors usually had a higher capture/detection success than mist nets.
Combining both techniques generally increased capture/detection success of
a particular species.
Table 1. Total bat captures by reproductive period and night (see methods). MYSO = Myotis
sodalis; MYSE = Myotis septentrionalis; NYHU = Nycticeius humeralis; EPFU = Eptesicus
fuscus Palisot de Beauvois (big brown bat); LABO = Lasiurus borealis. We sampled for 12
net nights during each reproductive period, 18 net nights each on 1st and 2nd nights, and 36 net
nights total. Detector nights are always one half of net nights.
MYSO MYSE NYHU EPFU LABO TOTAL
Reproductive period
Pregnant 11 7 0 4 12 34
Lactating 1 9 14 21 15 60
Post-lactating 1 6 5 4 22 38
Night
1st 12 17 9 16 33 87
2nd 1 5 10 13 16 45
Table 2. Capture/detection success measured as percent of nets (N%), detectors (D%), or combined
(C%) capturing or recording at least one of the target species. Number of net nights is the
same as Table 1. Total capture/detection success is per net site (2 nets x 2 nights) rather than
per net set (2 nets x 1 night).
MYSO MYSE EPFU LABO
N%, D%, C% N%, D%, C% N%, D%, C% N%, D%, C%
Reproductive period
Pregnant 67, 100, 100 33, 83, 83 50, 67, 83 83, 100, 100
Lactating 17, 83, 83 83, 83, 100 83, 100, 100 83, 100, 100
Post-lactating 0, 50, 50 83, 33, 100 50, 67, 67 100, 100, 100
Night
1st 44, 56, 56 89, 78, 100 44, 67, 78 100, 100, 100
2nd 11, 100, 100 44, 56, 89 78, 89, 89 78, 100, 100
Total 44, 100, 100 89, 78, 100 78, 89, 89 100, 100, 100
2008 L.W. Robbins, K.L. Murray, and P. McKenzie 279
Discussion
Two previous studies (Carroll et al. 2002, MacCarthy et al. 2006)
have suggested that the mist-netting guidelines of the US Fish and Wildlife
Service (1999) Indiana Bat Protocol should be modified to more
accurately assess the presence or absence of Indiana bats. Carroll et al.
(2002) showed that nets set in non-traditional (forest-interior) locations
accounted for Indiana bats at 50% of their netting locations, and MacCarthy
et al. (2006) recommended that nets be monitored either at intervals
of less than 10 minutes or continuously. The present study followed the published
guidelines as closely as possible and addressed three questions: 1) Does
the probability of capturing an Indiana bat vary throughout the maternity
period? 2) Does the probability of capturing an Indiana bat vary from
night 1 to night 2? and 3) Are mist nets alone able to reliably detect Indiana
bats when present?
We found that capture rates varied seasonally for all species. However,
our sample size (n = 3) was small and we were unable to explain the
observed patterns of variation. The variation may not necessarily be associated
with reproductive season, but may reflect factors such as ambient
temperature or the availability of food resources. Our data highlight how
much we do not know regarding the activity and foraging patterns of Indiana
bats and other vespertilionids. It is difficult to explain why we did
not capture more net-naïve, newly volant, juvenile Indiana bats during the
post-lactation netting sample. Both Miller (2003) and the present study
showed that activity in forest corridors remained high during all three
reproductive periods. However, Miller (2003) also showed that activity
increased over open-water sources during the post-lactation period. Perhaps
juvenile Indiana bats are preferentially using these more open areas
as they perfect their foraging strategies and therefore were not present at
our net sites.
Our most striking result was the decrease in bat captures, but not activity,
from night 1 to night 2. Indiana bats and all species combined were captured
significantly less on night 2. Kunz and Brock (1975) were the first to document
a similar reduction in captures from one night to the next. In our study,
bat detectors did not have a detection bias between nights, as bat activity
levels were not significantly different between night 1 and night 2. This result
indicates that prior knowledge of the mist-net location on the part of the
bats is the most likely explanation for decreased capture success. We assume
that bats use spatial memory to remember the location of nets, and that experience
with a net makes them more inclined to avoid it in the future. Kunz
and Brock (1975) drew a similar conclusion. Regardless of the mechanism
of avoidance, it seems that netting two nights in the exact same location is
of limited value, particularly for Indiana bats.
It is notoriously difficult to capture an Indiana bat with a mist net. In our
study, the capture success for Indiana bats was often very low using nets
alone (Table 2), but higher when using ultrasonic detectors. However, both
280 Northeastern Naturalist Vol. 15, No. 2
nets and detectors have their own limitations and biases when used alone.
The best results, for both successfully documenting presence/absence and
estimating biodiversity (i.e., species richness) were found using both mist
nets and ultrasonic detectors. As other studies have shown, the most effective
way to survey bat communities is to incorporate both of these techniques
whenever possible (Flaquer et al. 2007, Kalko and Handley 2001, Murray et
al. 1999, Sampaio et al. 2003, ).
Other studies (Carroll et al. 2002, MacCarthy et al. 2006) showed that
there was room for improvement in the original Indiana Bat Recovery
Plan mist-netting protocol. The revised protocol was modified based upon
these studies. We feel that the current guidelines should also be modified
to enhance the probability of capturing or detecting Indiana bats at a site.
Currently, there are simply too few data available to determine how variation
in seasonal abundance and activity affect capture success. Future
studies should attempt to address this issue. Our results provide good evidence
that bats are not fooled twice by the same net. We see limited utility
to netting multiple nights in the exact same location and advocate moving
nets to a new location on the second night to enhance the probability of
capturing Indiana bats. Finally, we recommend that ultrasonic detectors
be used to augment survey work involving Indiana bats. Numerous
studies in the past decade have made acoustic identification of bat echolocation
calls much more commonplace (Biscardi et al. 2004, O’Farrell et
al. 1999, Parsons and Jones 2000, Preatoni et al. 2005, Russo and Jones
2002, Rydell et al. 2002, Vaughan et al. 1997), and a substantial amount
of work has been done to characterize and identify the echolocation calls
of Indiana bats (Britzke et al. 2002; Ford et al. 2005; Murray et al. 1999,
2001; Yates and Muzika 2006). Overall, we recommend that comparable
studies in other portions of the range of Indiana bats be conducted and
that further work be done to characterize the echolocation calls of Indiana
bats, particularly in different clutter conditions and with different types
of ultrasonic detectors. Acoustic identification is only as good as the call
library it relies upon (Broders et al. 2004, Tibbels 2000). In conclusion,
we suggest that the protocol as written may be improved by moving mist
nets between night one and night two and by using ultrasonic detectors to
supplement mist-netting effort.
Acknowledgments
Funding for field work was provided by the US Fish and Wildlife Service,
Missouri Department of Conservation, Dickerson Park Zoo, and Missouri State
University. We thank the Missouri Department of Conservation for permission to
work on Deer Ridge Conservation Area and in particular, Richard Clawson and
Brian Root for their helpful interactions. For their considerable help in the field,
we thank Matt Miller, John Timpone, Justin Boyles, and Scott Kelly. We thank T.
Carter and one anonymous reviewer for providing comments that improved the
quality of this manuscript.
2008 L.W. Robbins, K.L. Murray, and P. McKenzie 281
Literature Cited
Biscardi, S., J. Orprecio, M.B. Fenton, A. Tsoar, and J.M. Ratcliffe. 2004. Data,
sample sizes, and statistics affect the recognition of species of bats by their echolocation
calls. Acta Chiropterologica 6:347–363.
Britzke, E.R., K.L. Murray, J.E. Heywood, and L.W. Robbins. 2002. Acoustic
Identification. Pp. 221–225, In A. Kurta and J. Kennedy (Eds.). The Indiana
Bat: Biology and Management of an Endangered Species. Bat Conservation
International, Austin, TX.
Broders, H.G, C.S. Findlay, and L. Zheng. 2004. Effects of clutter on echolocation
call structure of Myotis septentrionalis and M. lucifugus. Journal of Mammalogy
85:273–281.
Callahan, E.V., R.D. Drobney, and R.L.Clawson. 1997. Selection of summer roosting
sites by Indiana bats (Myotis sodalis) in Missouri. Journal of Mammalogy
78:818–825.
Carroll, S.K., T.C. Carter, and G.A. Feldhamer. 2002. Placement of nets for bats:
Effects on perceived fauna. Southeastern Naturalist 1:193–198.
Flaquer, C., I. Torre, and A. Arrizabalga. 2007. Comparison of sampling methods for
inventory of bat communities. Journal of Mammalogy 88:526–533.
Ford, W.M., M.A. Menzel, J.L. Rodrigue, J.M. Menzel, and J.B. Johnson. 2005. Relating
bat species presence to simple habitat measures in a central Appalachian
forest. Biological Conservation 126:528–539.
Gardner, J.E., and E.A. Cook. 2002. Seasonal and geographic distribution and quantification of potential summer habitat. Pp. 9–20, In A. Kurta and J. Kennedy
(Eds.). The Indiana Bat: Biology and Management of an Endangered Species. Bat
Conservation International, Austin, TX.
Kalko, E.K.V., and C.O. Handley. 2001. Neotropical bats in the canopy: Diversity,
community structure, and implications for conservation. Plant Ecology
153:319–333.
Kunz, T.H., and C.E. Brock. 1975. A comparison of mist nets and ultrasonic detectors
for monitoring flight activity of bats. Journal of Mammalogy 56:907–911.
MacCarthy, K.A., T.C. Carter, B.J. Steffen, and G.A. Feldhamer. 2006. Efficacy of
the mist-net protocol for Indiana bats: A video analysis. Northeastern Naturalist
3:25–28.
Miller, M.N. 2003. Activities within a myotine bat community with emphasis on the
endangered Indiana bat, Myotis sodalis. M.Sc. Thesis. Missouri State University,
Springfield, MO. 62 pp.
Murray, K.L., E.R. Britzke, B.M. Hadley, and L.W. Robbins. 1999. Surveying bat
communities: A comparison between mist nets and the Anabat II bat detector
system. Acta Chiropterologica 1:105–112.
Murray, K.L., E.R. Britzke, and L.W. Robbins. 2001. Variation in search-phase calls
of bats. Journal of Mammalogy 82:728–737.
O’Farrell, M.J., B.W. Miller, and W.L. Gannon. 1999. Qualitative identification of
free-flying bats using the Anabat detector. Journal of Mammalogy 80:11–23.
Parsons, S., and G. Jones. 2000. Acoustic identification of twelve species of echolocating
bat by discriminant function analysis and artificial neural networks.
Journal of Experimental Biology 203:2641–2656.
Preatoni, D.G., M. Nodari, R. Chirichella, G. Tosi, L.A. Wauters, and A. Martinoli.
2005. Identifying bats from time-expanded recordings of search calls: Comparing
classification methods. Journal of Wildlife Management 69:1601–1614.
282 Northeastern Naturalist Vol. 15, No. 2
Russo, D., and G. Jones. 2002. Identification of twenty-two bats species (Mammalia:
Chiroptera) from Italy by analysis of time-expanded recordings of echolocation
calls. Journal of Zoology 258:91–103.
Rydell, J., H.T. Arita, M. Santos, and J. Granados. 2002. Acoustic identification of
insectivorous bats (order Chiroptera) of Yucatan, Mexico. Journal of Zoology,
257:27–36.
Sampaio, E.M., E.K.V. Kalko, E. Bernard, B. Rodriguez-Herrera, and C.O. Handley.
2003. A biodiversity assessment of bats (Chiroptera) in a tropical lowland rainforest
of Central Amazonia, including methodological and conservation considerations.
Studies on Neotropical Fauna and Environment 38:17–31.
Tibbels, A. 2000. Do call libraries reflect reality? Bat Research News 40:153–155.
Timpone, J.C. 2004. Roost-site selection of bats in northeast Missouri with emphasis
on the endangered Indiana Bat (Myotis sodalis). M.Sc. Thesis. Missouri State
University, Springfield, MO. 63 pp.
Timpone, J.C., J.G. Boyles, and L.W. Robbins. 2006. Potential for niche-overlap in
roosting sites between evening bats (Nycticeius humeralis) and big brown bats
(Eptesicus fuscus). Northeastern Naturalist 13:597–602.
Vaughan, N., G. Jones, and S. Harris. 1997. Identification of British bat species by
multivariate analysis of echolocation call parameters. Bioacoustics 1997:189–
207.
US Fish and Wildlife Service. 1999. Agency draft Indiana Bat (Myotis sodalis) revised
recovery plan. US Fish and Wildlife Service, Fort Snelling, MN. 60 pp.
US Fish and Wildlife Service. 2007. Indiana Bat (Myotis sodalis) draft recovery plan:
First revision. US Fish and Wildlife Service, Fort Snelling, MN. 258 pp.
Yates, M.D., and R.M. Muzika. 2006. Effect of forest structure and fragmentation
on site occupancy of bat species in Missouri Ozark forests. Journal of Wildlife
Management 70:1238–1248.