LATVIJAS UNIVERSITĀTE
Gunārs Pētersons
LATVIJAS SIKSPĀRŅU (CHIROPTERA)
POPULĀCIJU TERITORIĀLAIS IZVIETOJUMS
UN SEZONĀLĀS MIGRĀCIJAS
DISTRIBUTION PATTERNS AND SEASONAL
MIGRATIONS OF THE BAT (CHIROPTERA)
POPULATIONS IN LATVIA
Promocijas darba kopsavilkums
bioloģijas zinātņu doktora grāda iegūšanai
(specialitāte - zooloģija)
Summary of the thesis for doctoral degree in Biology
(speciality - Zoology)
Rīga, 2004
INTRODUCTION
The bat (order Chiroptera) species occurring in Latvia belong to the
suborder Microchiroptera and to the family Vespertilionidae. They are
insectivores feeding on insects mainly in flight. In temperate climates they
face a lack of food during the winter season. The survival of bats is ensured
by their ability to hibernate. The body temperature of bats decreases
considerably during hibernation thus saving energy (Neuweiler 1993). Bats
require hibernation roosts that meet species-specific ecological conditions.
The hibernation roosts of bats can be divided into two roost types:
underground and above-ground. The underground roosts are better
protected from frost.
Bat species hibernating in underground roosts choose their wintering
sites in the vicinity of their summer roosts. The species preferring the
above-ground roosts can not hibernate in regions with severe winters. It is
considered that the populations of above-ground hibernating species from
northern and north-eastern parts of Europe carry out regular seasonal
migrations to the south-west in autumn and to the north-east in spring
(Strelkov 1969). Hence the availability of appropriate hibernation roosts is
the main factor affecting the winter distribution of bat species.
The summer distribution of bats on a global scale is affected mainly by
temperature. A decrease of temperature northwards is associated negatively
with the number of species. The distribution pattern of species at a regional
scale is affected by different factors such as the abundance of insects (de
Jong & Ahl6n 1991), occurrence of feeding habitats and competition
between species (Baag0e 2001).
The present investigation provides new data on the occurrence and on
the distribution pattern of bat species in Latvia during winter and summer
seasons as well as offers new ideas on the migration strategy of bats using
Nathusius' bat as a model.
IMPORTANCE OF THE STUDY
Bats have a high conservation status both in Latvia (Regulations of the
Cabinet of Ministers Nr. 117 "On specially protected species", annex 2) and
on the European level (EU Directive "On Conservation of Species and
Habitats", 92/43 EEC, annex 2and 4). In 2003 Latvia assumed new
obligations on the conservation and investigation of bats joining the
"Agreement on the Conservation of Bat Populations in Europe".
Complete data on the geographical distribution, territorial and seasonal
pattern of occurrence of bat species are essential prerequisites to ensure
their conservation.
A major study on the status of bat species in Latvia in the period before
1977 was carried out by Ināra Buša (Буша 1980), summarized data both
from her own studies and those of other researchers, mainly based on
checks of winter and summer roosts. This study determined 12 bat species
for the Latvian fauna.
The use of ultrasound detectors for detecting and identifying of flying
bats became widely introduced to study bats in Europe in 1980, It was
shown that the previous methods provided incomplete information on the
distribution of species, as the species assembling in small colonies and tree
species are underrepresented by roost checks compared to species forming
larger colonies in buildings. Moreover, the use of ultrasound detectors
allows to carry out mapping of summer distribution of bats in a relatively
short period of time with minimal resources. The application of ultrasound
detectors in Latvia can be used to test the reliability of knowledge on the
summer occurrence of species provided by previous studies as well as to
clarify the species composition of the Latvian bat fauna. The former
information on bat distributions in Latvia is too scarce to enable the
evaluation of distribution patterns of summer populations of bat species in
Latvia and to identify the factors affecting the population density of species.
Bat survey by means of ultrasound detectors in some neighbour countries e.
g. Sweden, has shown the occurrence of several rare species, which could
potentially occur also in Latvia, as well as demonstrated an uneven
distribution of populations on a local scale (Ahlen 1981).
The geographical distribution and the seasonal occurrence of migrating
bat species in the north-eastern part of range are mainly based on the
discrepancy their summer and winter ranges. Direct proof of long-distance
flights of bats have been limited by few recoveries of common noctules
banded or found in Lithuania and Latvia and by few recoveries of
Nathusius'bats banded in Lithuania.
A mass concentration of bats along the coast of Baltic Sea during the
period of autumn migration, which is comparable with the migration
behaviour known in passerine bird species, was observed at several
ornithological stations in Baltic countries in the 1980's. Systematic
investigations on the phenology of the autumn migration of bats as well as
influence of meteorological conditions on the migration activity based on
direct observation of migration have not been made in Europe.
THE GOAL AND OBJECTIVES OF STUDY
The main goal of the study was to determine the distribution pattern of
bat species in Latvia in the summer and winter periods and to investigate
the migration strategy, using Nathusius'bat as the model species. The
objectives were:
>
to identify the species composition of bats in the summer and in
winter periods and to test the hypothesis that some of the species
reach their north-easternmost range in the territory of Latvia;
>
to test if there are differences in the density of summer populations
of bat species among different parts of Latvia and to analyze the
possible density determining factors;
>
to test the hypothezed sympatric distribution of two recently
discovered sibling species Pipistrellus pipistrellus and P.
pygmaeus in the territory of Latvia;
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to determine if the Latvian populations of P. pygmaeus differ in the
parameters of ultrasound calls from the western and southern
populations of this species;
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to assess the relationship between the activity of bats during
autumn migration and the meteorological conditions;
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to identify the hibernation sites of Nathusius' bats from Latvian
and from north-eastern European populations;
>
to determine the migration distance of Nathusius'bats from the
north-eastern populations and to test the hypothesis that males of
this species demonstrate longer migration flights than females;
>
to estimate the speed of autumn migration in Nathusius' bats and
to test the hypothesis that populations with longer migration
distances migrate with higher speeds than populations with shorter
migration distances;
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to test the hypothesis that the males of Nathusius' bats leave their
summer territories later than females;
>
to test the idea that young males of Nathusius' bats from the north
eastern part of the distribution range do not return to their native
areas and establish territories in areas situated more closely to the
hibernation area.
SCIENTIFIC NOVELTIES
Four new species of bats were proven to occur in the territory of Latvia
- Myotis myotis, Nyctalus leisleri, Eptesicus serotinus and Pipistrellus
pygmaeus. The north-eastern most records in the distribution range of these
species as well as those of Barbastalla barbastellm were recorded in this
study. For the first time, mapping of the distribution of bats covering the
whole territory of Latvia was carried out using ultrasound detectors for the
identification of flying bats. By means of this method, an uneven
distribution pattern was found in following species: Pipistrellus nathusii,
Eptesicus nilssonii, Vespetilio murinus, Pipistrellus pipistrellus, Nyctalus
noctula and Myotis dasycneme. The following factors affecting distribution
of bat species are discussed in this study - competition between species (£.
nilssonii and P. nathusii), climate (V. murinus, N. noctula, P. pipistrellus)
and the availability of feeding habitats (M dasycneme).
The investigation of the autumn migration of bats carried out in Latvia
is unique in the high numbers of captured and banded individuals. The
study site at the Ornithological station Pape is one of the few sites in Europe
where a narrow front migration is observed and systematic and long-term
investigations of the migration behaviour have been carried out. This study
showed that meteorological conditions, especially wind direction and wind
speed, have a significant influence on the activity of bats during the autumn
migration. Marking and recapture data showed that P. nathusii carry out at
least 300 - 400 km longer migration flights than known previously. The
world longest seasonal migration flight for bats (1905 km) is documented.
The mean migration speed for the north-eastern European populations of P.
nathusii was estimated to be 47.8 kilometres per night supporting the
hypothesis that bats from populations with longer migration distances
migrate at higher speed than bats from populations with shorter migration
distances. For the first time, differences between males and females in the
migration distances were recorded for a European bat species. The
emigration behaviour of young male bats was documented — one-way
migration of a banded male to a new summer territory at a distance of at
least 670 kilometres to the south-west from its birth area.
APPROBATION
The results of this study have been presented in 9 scientific
conferences:
5th All-Union Conference of Bats, Penza, Russia, 1989;
10th International Bat Research Conference, Boston, USA, 1995;
27
7th European Bat Research Symposium, Veldhoven, the Netherlands, 1996;
3rd Baltic Theriological Conference, Salaspils, Latvia, 1996; 5th Baltic
Theriological Conference, Birštonas, Lithuania, 2002;
2 nd International Conference "Research and Conservation of biologic al
diversity in Baltic Region", Daugavpils, Latvia 2003;
1st Baltic Bat Research conference, Seili, Finland, 2003;
60 th and 61st Scientific Conferences of the University of Latvia 2002, 2003.
The results of this study are incorporated into the National Reports on
the implementation of the "Agreement on the conservation of bat
populations in Europe" (2003, 2004).
PUBLICATIONS
The results of the study are presented in five published scientific
papers and three published short communications as well as in the abstracts
of six scientific conferences.
VOLUME AND STRUCTURE OF THE THESES
The total volume of the theses is 137 pages containing 24 figures and
12 tables, 157 references are used. The main part of the theses consists of
three chapters corresponding to three studies. Each chapter has subdivisions
typical for scientific papers: Introduction, Material and methods, Results,
Discussion and Conclusions. The chapters uniting the whole paper are as
follows: summary in Latvian and English, general introduction, references
and acknowledgements. Distribution maps of bat species and a table with
list of long-distance flights of P. nathusii are annexed as appendices.
CONTENTS
Material and methods
Seasonal occurrence and distribution patterns of species
The distribution of the bat populations in Latvia were studied between
1 January 1992 and 31 December 1998. The data on bat records were
divided on the basis of whether they were made in winter season (all
observations between 1 December and 31 March, during hibernation torpor
between 1 October and 30 April) or in summer (all observations between 10
May and 10 August). Each record was referred to one of 728 10x10
kilometres size UTM squares. Distribution maps were produced for each
species — both winter distribution and summer distribution maps for
sedentary species and only summer distribution maps for migrating species.
The whole territory of Latvia was divided into four parts to analyse the
distribution patterns: north-west (comprising 164 UTM squares), south-west
(153), north-east (193) and south-east (218). The number of squares with
records of species was used as an index of population density. Four methods
were used for detection and identification of species: 1) identification of
flying bats by means of ultrasound detectors, 2) capturing of flying bats
with mist nets, 3) inspection of summer roosts, 4) inspection of winter
roosts. Most of the summer records were done by means off ultrasound
detectors. This method was applied for the identification of species
belonging to genus Pipistrellus, Nyctalus, Eptesicus and Vespertilio and of
the Myotis species - M. dasycneme. Heterodyning detectors Pettersson
Electronics D90, D95 and D100 were used to identify species in the field. In
problematic cases the ultrasound calls of bats were stored for further
analysis on a tape recorder using the time expansion ultrasound detector
Pettersson Electronics D980 and tape recorder Sony TCD-D3. The calls
were analysed with the sound analysis program Pettersson Electronics LP
900 version 3.
Occurrence of Pipistrellus pygmaeus in Latvia
The ultrasonic orientation calls recorded at 25 sites in Latvia and at one
site in the northern part of Byelorussia were analysed to determine the
occurrence of Pipistrellus pygmaeus. The recordings were made in feeding
habitats of bats between 1993 and 2001 using an ultrasound detector
Pettersson Electronics D980 and a digital tape recorder Sony Walkman
TCD-D100. The calls recorded as time expansion signals were analysed
using the computer programme Pettersson Electronic AB BatSound, version
3.3. A total of 95 calls with pulse length > 5 ms were selected for further
analysis. The frequency containing most of the energy (fmaxe) was used as
the main feature to distinguish the species. Calls with fmaxe 44-49 were
referred to P. pipistrellus, and calls with fmaxe>52 kHz were assigned as P.
pygmaeus (Jones & van Parijs 1993).
Migration strategy of the Nathusius'bat Pipistrellus nathusii
Study of the autumn migration of bats was carried out at the
ornithological station of the Institute of Biology of the Latvian University in
Pape (56°11'N 21°03'E) and in bird boxes located at four sites in pine
forests, 5 to 20 km NE of Riga in central Latvia. Survey at the
Ornithological Station Pape was made in 1986-1992, covering the peak
period of the autumn migration - the second part of August and the
beginning of August. A modified Helgoland bird trap was used to capture
bats. In addition, two frame traps were installed inside the Helgoland trap
and hand-held nets were used to capture bats. The capturing was started
shortly after sunset and followed up until the end of bat activity before
sunrise. The following meteorological conditions were recorded three times
per night: air temperature, wind speed, wind direction and precipitation. The
meteorological conditions were classified as follows: air temperature (°C
rounded to the closest 1°C); combination of wind direction and wind speed
(0 - no wind; CL - light following wind; CM - moderate following wind;
IL light inland wind; IM - moderate inland wind; PL - light opposing wind;
PM - moderate opposing wind; JL - light sea wind; JM - moderate sea
wind; S - strong wind, all directions), precipitation (0 - no precipitation;
fog, light rain; heavy rain).
For each individual of P. nathusii (n= 13061), the hour after sunset (0 10) when it was captured was recorded as well as sex and age. The bats
were aged as adults or juveniles. Adults were defined bats born at least one
year previously; juveniles as bats born during the last summer. The captured
bats were banded with aluminium wing rings and released. During the study
period, five individuals banded in nurseries elsewhere were captured at the
Pape Station. Recapture data on 61 individuals banded in Pape were
received until 2003.
The investigations in mating roosts of P. nathusii were conducted
mainly in bird boxes located near Jaunciems, 5 kilometres NE of Riga in
1983-1989. The bird boxes (99 to 135 boxes per visit) were checked for bats
26 times in May-September. The captured bats were sexed and aged
(subadults or adults). A total of 306 individuals were banded with wing
rings. One individual was recaptured outside of the banding site. Data on
two individuals found in another mating ground near Garupe and banded
outside this area were used as well. The long-distance flights (n=7) of P.
nathusii banded (n=714) or found in the nurseries in Latvia in 1980-1991
were included in the data analysis.
A total of 73 long-distance flights ranging from 88 to 1905 kilometres
were recorded and analysed.
Main results
1. Seasonal occurrence and distribution patterns of species
Fifteen bat species have been recorded in Latvia during the period
1986-1998 (Table 1).
The north-eastern most records in the distribution range are found for
four species. Three species, N. leileri, E. serotinus and B. barbastellus occur
in Latvia at the north-eastern edges of their distribution range. The only
record of M. myotis is considered as occasional. Five bat species, N. leisleri,
N. noctula, P. nathusii, P. pipistrellus and V. murinus, were found
exclusively in the summer period or during the autumn migration and are
classified as migratory species hibernating in areas situated south-west of
Latvia.
Bats of eight species, M dasycneme, M. daubentonii, M. brandtii, M.
mystacinus, M. nattereri, E. nilssonii and B. barbastellus are found
hibernating during the investigation period. Although the occurrence of two
hibernating species, M. mystacinus and B. barbastellus, in summer was not
proven in this study, their repeated capturing during the period of autumn
migration at the south-western sea coast of Latvia allows to regard them and
the other hibernating species as sedentary or non-migrating species.
A higher population density was found in the southern Latvia in
comparison to the northern part for three species, TV. noctula (P<0.01), V.
murinus (P<0.05) and P. pipistrellus (P0.01). This can be explained by
differences in climate in summer and by larger migration distances to be
covered in the more northern populations. Two common bat species in
Latvia, P. nathusii and E. nilssonii, differ in population density in the southeastern Latvia where the first species is more common (PO.001) and the
second species is less common (P<0.001). The population densities of these
two species, which have similar feeding habitats and hunting techniques, are
probably regulated by interspecific competition. P. nathusii, a species with
origin in areas of broadleaf forests, is more competitive with boreal species
E. nilssonii in areas with a continental climate (higher summer
temperature). M. dasycneme was relatively rare in north-western part of
Latvia (P<0.5), which can be explained by a lack of large water bodies, a
typical feeding habitat of this species. M. daubentonii is comparatively rare
species in south-eastern Latvia probably due to dominance of the
ecologically similar species M. dasycneme in this area.
2. Occurrence of Pipistrellus ppygmaeus in Latvia
The analysis of ultrasound calls, previously recorded as Pipistrellus
pipistrellus calls, showed a bimodal distribution of peak frequencies of
calls, indicating the sympatric occurrence of two sibling species, P.
pipistrellus and P. pygmaeus, in the territory of Latvia in summer (Figure
1).
P. pygmaeus was recorded at five sites in southern or western Latvia
and at one site in north-eastern Byelorussia. The study showed that the
Latvian population of P. pygmaeus lies at the north-eastern edge of its
distribution range.
P. pygmaeus individuals from Latvian and Byelorussian populations
had calls with lower frequencies than bats from Western Europe and
Sweden. It can be explained by a lower disruptive selective pressure on call
frequencies is lower in the periphery of the range of species, in Latvia,
where competition between the sibling species is reduced compared to
regions where both species are common.
3. Migration strategy of the Nathusius' bat Pipistrellus nathusii
A concentration of Nathusius' bats during the autumn migration was
observed at the Baltic Sea coast in south-western Latvia, near Pape village.
3.1 Phenology
The autumn migration peak along the sea coast occurred during the
second half of August and beginning of September. The latest migrants
were recorded in the middle of October (the latest date 20/21 October). The
mean sex ratio (0.85) of bats captured in 1986 - 1992 was significantly
female biased (P<0.001) (Table 2). The uneven sexual ratio can be
explained by a prevalence of females among adults in the summer
populations in NE Europe. The recapture data of a male documented in our
study indicates that a some young males do not return after their first
hibernation to their birth areas in NE Europe, and they establish their
mating territories in Central European areas situated close to the hibernation
areas.
Females migrate slightly earlier than males. Comparing the observed
sex ratio of bats sampled during each 10-day period to the expected ratio
0.85 calculated for the entire migration period, more females than expected
were captured during the second decade of August (PO.01) and more males
than expected were captured during the first decade of September (PO.01)
(Figure 2). The later migration in males can be explained by their mating
behaviour. Observations from bird and bat boxes indicate that adult males
occupy mating roosts during the period of autumn migration and mate with
females migrating through their territories. The males leave the territories as
the latest when the migration of females is over.
3.2 Nightly activity
A monomodal pattern of migration activity of bats was found in this
study (Figure 3). A low migration activity, observed during the first two
hours after the sunset, can be explained by the necessity to feed and to
accumulate the energy needed for nightly migration flight.
A significant influence of weather conditions (wind direction, wind
speed and the air temperature) on the migration activity of P. nathusii was
observed (Figure 4). More than expected bats were captured in nights with
light or moderate inland wind and light or moderate opposing wind. No bat
migration was observed during strong winds (>10 m/s). A similar behaviour
is known for passerine birds, when they carry out narrow-front migrating
along the sea coast. The air temperature of the migration was correlated
positively with the number of individuals captured (P<0.01).
3.3 Hibernation sites, direction, distance and speed of migration
The proved or potential hibernation sites of banded P. nathusii
migrating through the area of Pape station cover an area of about 800,000
km2 ranging from 700 to 1905 km SW, to the SW border of Latvia (Figure
5). The SW direction of the autumn migration coincides with the direction
of autumn migration found in other studied populations of this species.
The migration distances covered by males ranged from 785 to 1905 km
(mean 1365.5 km, SD 324.2 km, n=22) and the distances covered by
females ranged from 700 to 1620 km (mean 1216.5 km, SD 251.3 km,
n=23). The mean migration distance of males significantly (P<0.1)
exceeded that of females by about 150 km. Of the 11 individuals that
covered distances of over 1500 km, eight were males and only three were
females (Figure 6). A shorter migration distance found by females can be
explained by selection pressure forcing females to return earlier than males
in spring to their breeding areas and to give birth as early as possible. The
males start the reproduction season only at the end of summer.
The average migration speed, estimated in 12 individuals that were
recaptured during the day, ranged from 31.7 to 76.9 km/night (mean 47.8
km/night, S.D.= 12.4 km/night). This speed is approximately two -times
higher than that estimated in several studies in Germany, where bats were
captured and banded in boxes during the autumn migration period. It is
possible that bats reduce their migration speed when they come closer to the
hibernation area.
CONLUS1ONS
Study 1. Seasonal occurrence and distribution patterns of species
>
The occurrence of 16 bat species in the territory of Latvia was
documented during the investigation period. The species are
classified according to their seasonal occurrence in the following
groups: stationary species (eight species), migratory species (six
species), and vagrants (one species). One species is regarded as a
potentially stationary species, although its hibernation in the
territory of Latvia is not proven yet.
>
Four bat species reach in Latvia the north-eastern edge of their
distribution range: Barbastella barbastellus, Eptesicus serotinus,
Nyctalus leisleri and Pipistrellus pygmaeus.
>
Eight species demonstrate an uneven distribution pattern in
different parts of Latvia. The uneven distribution can be explained
by differences in climate between southern and northern Latvia
(Nyctalus noctula, Pipistrellus pipistrellus un Vespertilio murinus),
an uneven distribution of specific feeding habitats (Myotis
dasycneme), and interspecific competition (Pipistrellus nathusii un
Eptesicus nilssonii). A decreased population density in northern
Latvia was found in three species - Nyctalus noctula, Pipistrellus
pipistrellus and Vespertilio murinus.
Study 2. Occurrence of Pipistrellus ppvgmaeus in Latvia
>
A sympatric distribution of two sibling species Pipistrellus
pipistrellus and P. pygmaeus in Latvia was confirmed.
>
P. pygmaeus from populations in Latvia and northern Belarus
demonstrate orientation calls with lower frequencies than bats from
populations in western and southern Europe. Probably, the
disruptive selective pressure of leading to the separation of call
frequencies in both species is weaker in the north-eastern part of
range, where both species have a low population density.
Conversely the selection pressure is stronger in regions where both
species are more common.
Study 3. Migration strategy of the Nathusius' bat Pipistrellus nathusii
> A mass concentration of P. nathusii at the coast of Baltic Sea in
south-western Latvia during the autumn migration occurs in nights
with specific weather conditions - light or moderate opposing or
inland wind.
>
The peak of P. nathusii autumn migration in the coastal area of the
Baltic Sea is during the second and third decade of August and first
decade of September. Late migrating individuals are recorded up to
end of second decade of October. The males of this species migrate
later than females.
>
The hibernation sites of the populations of P. nathusii from Latvia
and the other parts of north-eastern Europe are situated in a
territory of 800,000 km2 at distances ranging from 700 to 1905 km
from the southern border of Latvia. The winter distribution covers
the territories of the Netherlands, Belgium, France, Germany,
Switzerland, Czech Republic, Croatia and the northern part of
Italy. The hibernation areas of P. nathusii from north-eastern
populations and those from western populations overlap. Thus, the
animals from north-eastern populations carry out considerably
longer seasonal migrations than the animals from western
populations.
>
The mean migration distance covered by males of P. nathusii
exceeds the mean distance covered by females by about 150 km.
The shorter migration distance in females can be explained by the
selection pressure forcing them to return to their breeding areas
earlier than males.
> The mean speed of the autumn migration off. nathusii from northeastern populations is about two times higher than that of P.
nathusii from western reproduction populations, showing shorter
migration distances. There is evidence that bats reduce their
migration speed nearing the hibernation sites.
>
Some young males born in the north-eastern part of the distribution
range do not return to their native area in summer after their first
hibernation and establish their mating territories in areas situated
closer to the hibernation area of this species.
ACKNOWLEDGMENTS
I specially thank my first teacher in bat research Ms. Ināra Rūce, who
introduced me to field methods and taught me to distinguish bat species. I
am very grateful to Prof. Ingemar Ahlen and Dr. Johnny de Jong, who
taught me how to identify bat species with bat detectors.
My greatest thanks go to Viesturs Vintulis and Ainis Platais for the many
hours we spent together in the field. They both were key persons in the
survey on bat distribution. Many people contributed to the collecting of
information on bats or helped me at the field work. I would like to
particularly thank Ēriks Dreibants, Didzis and Jānis Grunduļi, Zigrīda
Jansone, Māris Jēkabsons, Oskars Keišs, Ineta Kruste, Edvards Kušners,
Viesturs Lārmanis, Varis Liepa, Ilona Liniņa, Vita Līcīte, Martiņš
Pētersons, Andris Piterāns, Edmunds Račinskis, Ināra Rūce, Pāvil s
Silenieks, Juris Smaļinskis, Andris Stīpnieks, Roberts Šiliņš, Irisa Šmite,
Ilze Štrausa and Ilze Vilks.
Special thanks are given to the Head of the Ornithological laboratory,
Institute of Biology, University of Latvia, Dr. Jānis Vīksne, for allowing us
to conduct fieldwork at the Pape Bird Station. Many thanks are extended to
supervisors of the bird station, Dr. Jānis Baumanis and Mr. Agris Celminš,
who started to capture and ring bats in 1985 and helped me to develop
capture and observation methods. Appreciation is extended to Dr. Matti
Masing, who introduced me with the experience in capturing migrating bats
at the Bird Station Kabli, Estonia, in 1985. I am very grateful to all field
assistants who helped me capture bats and produce bat rings, in particular:
Inara Rūce, Ēriks Dreibants, Māris Smiltnieks, Antra Balode, Vita Līcīte,
Dace Valbe, Haralds Barviks, Marija Pētersone and Viesturs Vintulis. Many
thanks go to A. Celmiņš and M. Smiltnieks, who continued trapping of bats
in late autumn and kindly provided me with the capture records.
I would like to thank my supervisor Dr. Janis Priednieks, especially for his
valuable comments and suggestions on drafts of this paper. I thank Oskars
Keišs and Dr. Ieva Vilks for comments on the draft of summery of the
thesis and Dr. Guntis Brumelis for checking the English.
Parts of this study were supported by the World Wide Fund for Nature
Sweden, the Swedish National Environmental Protection Board, the Latvian
Fund for Nature, the Department of Wildlife Ecology of the University of
Agriculture Uppsala and Naturschutzbund Deutschland e.V.