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The effects of illuminating the roost entrance on the emergence behavior of Pipistrellus pygmaeus


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In an attempt to increase the accuracy of roost emergence counts for a monitoring programme, the exits of two Pipistrellus pygmaeus roosts were illuminated with light of different colours and intensities. Light intensity affected bat emergence more than light colour. At one roost there was no significant difference in the bat emergence pattern between when the roost exit received no illumination and when it was illuminated with red light. The use of the latter is proposed to increase the accuracy of bat roost emergence counts.
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The effects of illuminating the roost entrance on the emergence
behaviour of Pipistrellus pygmaeus
N.C. Downs
*, V. Beaton
, J. Guest
, J. Polanski
, S.L. Robinson
, P.A. Racey
University of Aberdeen, Department of Zoology, Aberdeen AB24 2TZ, UK
University of Aberdeen, Central Workshop Services, Aberdeen AB24 3UE, UK
Received 12 December 2001; received in revised form 15 May 2002; accepted 25 September 2002
In an attempt to increase the accuracy of roost emergence counts for a monitoring programme, the exits of two Pipistrellus pygmaeus
roosts were illuminated with light of different colours and intensities. Light intensity affected bat emergence more than light colour. At
one roost there was no significant difference in the bat emergence pattern between when the roost exit received no illumination and when
it was illuminated with red light. The use of the latter is proposed to increase the accuracy of bat roost emergence counts.
#2003 Elsevier Science Ltd. All rights reserved.
Keywords: Pipistrelle, Pipistrellus pygmaeus; Light intensity; Light colour; Roost emergence; Bats
1. Introduction
Monitoring of bat populations is a requirement of the
European Bats Agreement 1991. The agreement came into
force in 1994 and currently includes 23 European coun-
tries. Following the establishment of a monitoring pro-
gramme for lesser horseshoe bats by the Countryside
Council for Wales (Warren et al., 2002), the Department
of the Environment contracted The Bat Conservation
Trust in 1996 to undertake a National Bat Monitoring
Programme (NBMP). This comprised the development
of methods to monitor selected bat species and the
application of these methods to establish population
trends (Walsh et al., 2001). One NBMP project involves
counting the number of pipistrelle bats emerging from
selected roosts. Illuminating the exit of some roosts may
improve the accuracy of counts. However, this may also
affect the behaviour of the bats, particularly since
ambient light levels affect emergence (Church, 1957;
ˆte, 1974; Swift, 1980; Usman et al, 1980; Erkert,
1982; Rydell et al., 1996). The aim of this study was to
determine the extent to which light of different colours
and intensities affects bat emergence, and whether roost
counts can be improved by illuminating exits.
2. Materials and methods
2.1. Experimental design
The influence of light colour and intensity on bat
emergence was investigated over 20 nights between 28/
07/00 and 20/08/00 at two Pipistrellus pygmaeus roosts
in Aberdeenshire (57N, 02W). Each experimental
night was proceeded by a control night, during which
the number of bats emerging per 30-s interval was
counted, but no light treatments were used. Recording
began when the first bat emerged, and ended after an
interval of 10 min had elapsed since the emergence of a
bat. On experimental nights, the first treatment always
consisted of a period of no light. This was followed by
treatments of white, red, and blue light which were used
in random order and changed every 30 s. A hand held
halogen light source and commercially available light
filters (Omega Night Vision Systems, Godalming) were
used to illuminate a roost exit. The angle of the light
beam varied between 45and 65with distance from the
roost. The number of bats emerging from the roost
during each 30-s interval was noted. Data from a con-
trol and experimental night pair were collected from
each roost on an alternate basis. Two investigators
collected data on each night. One changed the light
treatment, and the other recorded data and time. Both
0006-3207/03/$ - see front matter #2003 Elsevier Science Ltd. All rights reserved.
PII: S0006-3207(02)00298-7
Biological Conservation 111 (2003) 247–252
* Corresponding author. Fax: +44-1224-272396.
E-mail address: (N.C. Downs).
investigators counted the number of emerging bats.
Data was not collected during heavy rain and strong
wind, conditions which may have affected bat emer-
gence patterns. The influence of light intensity was
investigated by placing the light source 3 m away from
the roost at site 1 and 5 m away at site 2.
The influence of red light intensity on bat emergence
was investigated over four nights between 24/07/00 and
27/07/00. The methodology used was the same as pre-
viously described, with the exception of using differing
numbers of red light filters rather than filters of different
colours. The intensity of light illuminating the roost was
decreased by increasing the number of filters used from
one to three. Data from one control and one experi-
mental night were collected from each roost.
A radiometer/photometer (Macam Photometrics Ltd.,
Livingston) was used to determine radiometric and pho-
tometric readings from each light filter. A radiometer
measures electromagnetic power (in watts) linearly over
its entire spectral range, while the response of a photo-
meter is calibrated (in lux) to reflect the nonlinear
response of the human eye over the visible spectrum only.
2.2. Roosts
Roost 1 was situated in a small single storey cottage
surrounded by mature deciduous woodland and large
lakes. The distance between the light source and the
roost was approximately 3 m. Between 51 and 508 bats
were counted emerging from this roost on study nights.
Roost 2 was situated in the cavity wall of a swimming
pool surrounded by a playing field and mature deciduous
woodland. The distance between the light source and the
roost was approximately 5 m. Between 170 and 536 bats
were counted emerging from this roost on study nights.
2.3. Analysis
Due to the non-parametric nature of the data, Krus-
kal–Wallis ANOVA and Chi-square goodness-of-fit
were used to test for differences between light treat-
ments. Wilcoxon’s matched pairs tests were used to test
for differences between control and experimental nights.
3. Results
At both roosts most bats emerged during no light
treatments, and fewest during white light treatments,
with an intermediate number of bats emerging dur-
ing red and blue light (Fig. 1). There is a statisti-
cally significant difference in the number of bats
emerging between different light treatments at both
roosts (Kruskal–Wallis ANOVA, P<0.01). There was
Fig. 1. The median nightly number of emerging bats with different light treatments for roosts 1 and 2 (IQ range).
248 N.C. Downs et al. / Biological Conservation 111 (2003) 247–252
no statistically significant differences in the emergence
pattern between red and blue light at both roosts,
and between red and no light at roost two (Table 1,
Mann–Whitney U-Test, P<0.05). More bats emerged
during control than experimental nights (Fig. 2). This
difference was statistically significant at roost 1 (Wil-
coxon’s Matched Pairs, Z=0.043, P<0.05) but not
roost 2 (Wilcoxon’s Matched Pairs, Z=0.416,
P<0.05). At both roosts an increasing number of
bats emerged as the number of red filters was
increased (Fig. 3a and 3b). These differences in the
number of bats emerging with an increasing number
of red filters are statistically significant (Chi-square
goodness-of-fit, P<0.05). Radiometric and photo-
metric light responses decrease with increasing dis-
tance from the light source to the roost (Fig. 4a and
4b). In both cases the red light response is stronger
than the blue. An increase in the number of red light
filters used also results in a decrease to both the
photometric and radiometric light response (Fig. 4c).
4. Discussion
The effects of light on bat emergence may be due to a
difference in light colours or intensities. More bats
emerged during red light than blue (Fig. 1). This is
contrary to the predictions of the radiometric and pho-
tometric responses, since those of red light are higher
than blue at the short distances between the light source
and roost exit relevant to the present investigation
(Fig. 4a and 4b). Therefore the eyes of the bats are likely
to be more sensitive to blue light than to red. However,
the difference in the number of bats emerging between
increasing numbers of red light filters (Fig. 3) is greater
than the difference in the number of bats emerging
between red and blue light (Fig. 1). This suggests that
light intensity is more important than colour in deter-
mining the onset of bat emergence. Additionally, there
is less difference in the numbers of emerging bats
between control and experimental nights at roost 2 than
at roost 1 (Fig. 2). The same pattern is shown between
the three red light filters and the no light treatments
(Fig. 3). A likely reason concerns the shorter distance
between the torch and roost at roost 1 (3 m rather than
5 m), resulting in increased radiometric and photometric
readings. The response curves for these readings drop
sharply between 3 and 5 m (Fig. 4). This indicates a
light threshold, beyond which illumination does not
affect emergence. Since there is no significant difference
in the pattern of bat emergence between no light and red
light when the light source is 5 m away from the roost
exit (Table 1), this level is at least 150 lux/1800 mw per
square cm (Fig. 4a and 4b).
Table 1
Nightly Mann–Whitney U-test factorial analysis between light treatments
Roost/treatment Date None/red None/blue None/white Red/blue Red/white Blue/white
1 (light source
3 m away)
02/08/00 2408.0
11/08/00 1972.0
15/08/00 1419.5
17/08/00 1372.0
19/08/00 924.5
2 (light source
5 m away)
04/08/00 1982.5
12/08/00 1705.0
16/08/00 1020.0
18/08/00 1045.0
20/08/00 831.0
N.C. Downs et al. / Biological Conservation 111 (2003) 247–252 249
Fig. 2. The median nightly number of emerging bats during both control and experimental nights at roost 1 and 2 ( IQ range).
Fig. 3. The total number of bat emergences within different intensities of red light at roosts 1 (a) and 2 (b).
250 N.C. Downs et al. / Biological Conservation 111 (2003) 247–252
Therefore, low intensity red light may be used to
improve pipistrelle roost counts. Torches should have
strong red filters and be used at least 5 m away from the
roost exit. If these actions reduce illumination to such
an extent that it is no longer effective, the torch may be
placed closer to the roost. However, it is advisable to err
on the side of caution and not illuminate the roost any
more than is strictly necessary for effective counting of
emerging bats.
This study was supported by a NERC postgraduate
studentship. We thank Dr. Sue Swift for commenting
on earlier drafts of the manuscript, Isobel Davidson and
Iain Mackie for providing field assistance, and Victor
Kapoor for allowing us roost access.
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Fig. 4. The radiometric and photometric responses to the red and blue filters used in the main investigation (a and b), and to the red filters used in
the light intensity investigation (c).
N.C. Downs et al. / Biological Conservation 111 (2003) 247–252 251
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Terrestrial organisms live in an environment that, by its geophysical nature, is subject to profound rhythmic alteration. The rotation of the earth brings about a 24-hr periodicity in light intensity, temperature, and humidity. As the earth revolves around the sun there are annual changes in day length, temperature, and the duration of twilight, which become more pronounced at increasing latitudes; that is, over the period of a year the form, level, and amplitude of diurnal variables themselves vary to different degrees. Other effects result from the revolution of the moon about the earth. The rhythmic gravitational fluctuations so produced are of little relevance to terrestrial animals, but there are also variations in night-time brightness with lunar periodicity. Organisms have adapted to this temporal structure of their environment in different ways. Very early in evolution they developed endogenous diurnal rhythms with periods of approximately 24 hr. These circadian rhythms were coupled with the external diurnal cycle by special synchronizing mechanisms so that a certain relatively stable phase relation, characteristic of each species, was maintained. In many cases circalunar and circannual endogenous rhythms evolved in addition to the circadian rhythm; these, too, are synchronized with the corresponding environmental variables by special mechanisms.
Volunteers counted lesser horseshoe bats (Rhinolophus hipposideros) emerging from 79 roosts throughout Wales prior to parturition over a 5-year period. Analysis of the count data and environmental variables revealed that there was no statistically significant change in the number of bats counted during the monitoring period. A maximum count of 5118 lesser horseshoe bats for one of the years of the study, suggests a Welsh population of >10,000. Within years, date had a strong influence on roost count with higher counts occurring later in the year. Analyses of roosts for which data was available in most years failed to find any evidence of changes in number of emerging bats over time either between roosts or across geographical regions. Over the time period studied, the population of R. hipposideros was stable. Power analysis suggested that the present monitoring programme should be able to detect a 5% increase or decrease over a 5-year period and could detect smaller trends over longer periods of time even when some data were missing. Roost surveys can therefore be a valuable tool to assess the status of a bat species, alert conservationists to population declines and provide feedback regarding the success of conservation management for bats.
Pipistrellus pipistrellus emerge from their nursery roosts in north-east Scotland about 35 minutes after sunset, at light intensities of between 15 and 35 lux. Cloud cover, windspeed, ambient temperature, rain, light mist and moonlight have no apparent effect on the time or pattern of emergence. Throughout pregnancy and lactation, emergence lasts for about an hour. After weaning, when the adult females have left the roost, their young take about 40 minutes to emerge. The average rate of initial emergence is proportional to colony size, and the maximum rate of emergence occurs half way through the exodus. During pregnancy in May and June most bats leave the roost once each night soon after dusk and return between midnight and dawn. After parturition in late June the activity pattern becomes bimodal and the numbers of bats outside the roost show peaks after dusk and immediately before dawn. There is intermittent activity in the vicinity of the roost all night and bats make two or three flights each night. After weaning in August the activity pattern gradually ceases to be bimodal, and the number of flights per bat falls to between one and two. The average time spent outside the roost varies between 2–5 and 5 hours during the summer. The recorded activity patterns of night-flying insects are all bimodal, with peaks after dusk and before dawn, corresponding with the maximum number of bats outside the roost during lactation.
1. The activity cycle in a colony of about 150 pond bats is studied throughout the summer season in Berlikum (Netherlands). The natural diurnal roosts of the bats are sited between rafters of a church loft where a nocturnal darkness reigns day and night. The onset of activity generally takes place in two phases: I—descending into and waiting in a narrow exit chamber from where daylight can be seen, II—flying out to the feeding grounds. The investigation includes electronic recording of passages of bats, and of light intensity during morning and evening twilight.
The hunting activity of tropical bats was observed during a lunar eclipse at night. During the eclipse, the activity was significantly higher than before and after when the bright full moon was visible. The decrease of hunting activity in bright light is interpreted as a direct adaptation to the light conditions, whereas endogenous factors seem not to be involved. The possible role of predators feeding on bats is discussed.
The times of emergence of the pipistrelle. Proceedings of the Zoological Society of London
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Church, H.F., 1957. The times of emergence of the pipistrelle. Proceedings of the Zoological Society of London 128, 600-602.
Fig. 4. The radiometric and photometric responses to the red and blue filters used in the main investigation
  • K Usman
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  • R Subbaraj
  • G Gopalkrishnaswamy
Usman, K., Habersetzer, J., Subbaraj, R., Gopalkrishnaswamy, G., Fig. 4. The radiometric and photometric responses to the red and blue filters used in the main investigation (a and b), and to the red filters used in the light intensity investigation (c).
The UK's National Bat Monitoring Programme-Final Report
  • A Walsh
  • C Catto
  • T Hutson
  • P Racey
  • P Richardson
  • S Langton
Walsh, A., Catto, C., Hutson, T., Racey, P., Richardson, P. & Langton, S., 2001. The UK's National Bat Monitoring Programme-Final Report 2001. DEFRA.
The times of emergence of the pipistrelle
  • Church