Distribution patterns and seasonal

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Gunārs Pētersons







Promocijas darba kopsavilkums

bioloģijas zinātņu doktora grāda iegūšanai

(specialitāte - zooloģija)

Summary of the thesis for doctoral degree in Biology (speciality - Zoology)

Rīga, 2004


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.


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 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;

  • 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;

  • to assess the relationship between the activity of bats during
    autumn migration and the meteorological conditions;

  • 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;

  • 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.


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.


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;


7th European Bat Research Symposium, Veldhoven, the Netherlands, 1996; 3rd Baltic Theriological Conference, Salaspils, Latvia, 1996; 5th Baltic Theriological Conference, Birštonas, Lithuania, 2002;

2nd International Conference "Research and Conservation of biological 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).


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.


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.


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.


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
    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
    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

  • 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.


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, Pāvils 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.

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