Several authors have reported about the high site fidelity of bats to their swarming sites (Parsons & Jones, 2003).
Although it is a known phenomenon already for several decades, swarming is still one of the least investigated aspects for several bat species or geographical regions.
A particular research project of swarming bats at three hibernation sites in Latvia was carried out in 2005-2007.
Its aim was to find out whether swarming can be observed also in Latvia, which species and when do swarm, and whether there are differences in swarming between different age and sex groups of bats.
In 2007 two additional pond bat's winter roosts were checked, one in the north (Daugenu, another natural sandstone cave) and the other, Fortress of Daugavpils City, in the eastern part of Latvia (Fig. Bat catching at the former was performed two times (in August and September) and at the latter, three times (August, September, October).
The dolomite 'Bat' cave in Gauja National Park and Daugavpils' Fortress are the biggest known hibernation sites of pond bat in Latvia.
Swarming is described as a social behaviour of bats, when they circle in high numbers before entrances of hibernation sites or inside the roosts (Fenton, 1969), sometimes staying there in smaller numbers the next day.In all three years swarming of pond bats started in the first half of August and continued until the end of October, when most bats caught had a considerably higher body mass and were ready for hibernation (J. However, the proportion of different sex and age groups varied in different sites.Seasonal changes in the numbers of different sex and age groups of pond bats could be observed both in 20.It is most commonly proposed that the purpose of such an energy-consuming activity of bats can be both mating (Parsons et al., 2003a; Rivers et al., 2005) and introduction of the young bats with potential hibernation sites (Humphrey & Cope, 1976).It is also suspected that during swarming bats may check on the condition of the known hibernation sites before winter, or use them as transitional roosts on their way from summer to winter sites (Fenton, 1969; Whitaker, 1998).RESULTS Altogether 436 pond bats were caught at the entrances of four sites (Table 1), adding up to 10% of all swarming bats caught. Since most pond bats were caught at the entrance of the dolomite 'Bat' cave, data from this site from 20 were used for further analysis. The peak activity of swarming appeared to take place in August-September.Data from the year 2005 were not sufficiently comparable because of the slightly different catching approach during the first times of catching. However, in 20, despite good weather conditions, a drop in the numbers could be observed in the middle of the season (Fig. Most of the captured bats, as shown in Table 1, were adult males.2); net size was adjusted to each roost's entrance size.Nets were erected at about sunset-one hour after sunset, and catching was carried out until dawn.The number of adult males was highest at the beginning of the season and dropped towards its end in both years (Spearman's correlation coefficient [s] = -1.000, P Table 1.Intensity of catching and number of pond bats caught at five sites in Latvia during the study period No. Site, year catching Adult adult age 'Bat' cave (2005-2007) 20 152 95 4 Daugavpils' Fortress (2007) 2.5 5 15 0 Daugenu cave (2007) 2 4 0 0 Kalejala cave (2005) 7 0 0 0 Remdenkalna cave (2005-2007) 23 2 1 0 Total 163 111 4 Females Sub- Indet.