Warmer winters may lead to changes in the hibernation behaviour of bats, such as the barbastelle Barbastella barbastellus, which prefers to hibernate at low temperatures. The species is also known for its large annual fluctuations in the number of wintering individuals, so inference about population trends should be based on long-term data. Prior to 2005, analyses indicated stable or even increasing barbastelle population in Poland. We analysed the results of 13 winter bat counts (2005-2017) of the species from 15 of the largest hibernacula, and additional site of 47 small bunkers, in Poland. The total number of wintering individuals remained stable during the study period, because the barbastelle is not a long-distance migrant, this likely reflects the national population trend. On the basis of mean winter air temperatures we divided the country into four thermal regions. Analyses of barbastelle abundance in hibernacula in the four regions revealed a 4.8% annual mean increase in numbers in the coldest region, where mean winter temperatures were below-2˚C, annual mean declines of 3.3% and 3.1% in two warmer regions of western Poland, but no trend in the region of intermediate mean winter temperatures of between-1˚C and-2˚C. Overall, there was a significant, but weak, negative correlation between the abundance of hibernating individuals and the mean winter temperature. On the other hand, the number of individuals hibernating in small bunkers increased, even though the site was located in one of the warm regions. The results indicate a warming climate will likely reduce the use of large, well-insulated winter roosts by species that prefer colder conditions-and that this is already

Temperate zone bats are associated with forests and affected by forest management practices. However, practices vary among regions and countries, and the relationship between bats and managed forest stands is not well understood. We compared the activity of bats in three forest management areas across four stand ages of managed Scots pine (Pinus sylvestris) in western Poland. Stand ages included clear-cut stands, young (2-5 year) stands, middle-aged (41-60 year) stands, and mature (> 80 years) stands. We sampled bat activity by walking transects with a broadband ultrasound Pettersson D-1000X detector. Across our study area, highest bat activity was in clear-cut and young stands and lowest in mature stands. Bat species adapted to foraging in open habitats had high activity in clear and young stands, while those adapted to closed habitats had high activity in middle-aged and mature stands. Our results suggest that the presence of mature pine forests is important for closed-habitat foragers, including rare and threatened bat species, and active management to increase mature forest areas is important. At the same time, a mosaic of different growth stages of stands can support high activity of open-and edgehabitat foragers.

The aim of this study was to compare the foraging activity of bats in coniferous, deciduous, and mixed forests and to test whether this activity was subject to seasonal variation. Sample points were selected in stands of similar spatial structure in coniferous (Pinus sylvestris L.), in mixed (Pinus sylvestris and Quercus petraea (Matt.) Liebl.), and in deciduous (Quercus petraea) managed forests in western Poland. Bat calls were recorded using automated ultrasound recording devices (Batcorder 3.0, ecoObs, Nürnberg, Germany) during five consecutive nights from May to September in each of the six stands. A total of 4,250 bat passes were recorded. Overall, 63.1% of bat passes were identified to species, 31.6% were identified to genus or sonotype group, and 5.3% remained unidentified. In total, eight species of bats and seven sonotype groups were recorded. The dominant species in all types of forests were Pipistrellus pygmaeus (44.5% of recorded bat passes), followed by Nyctalus noctula (10.3%) and Pipistrellus nathusii (5.7%). There were no significant differences in the total activity of bats between the three types of forests; however, high seasonal fluctuations in bat foraging activity were found. This study demonstrates that when coniferous, deciduous, and mixed stands with similar spatial structure are compared, forest type does not affect the foraging activity of bats.

Monitoring studies in Strzaliny, one of the greatest hibernacula in Poland, comprised 31 annual bat censuses (1989–2019). The abundance peaked in 2002 for Myotis myotis, 2009 for Myotis nattereri and 2008 for the whole assemblage. Comparison of the maximum abundance in the monitoring period with that from 1980 to 1982 showed an almost fourfold increase for the whole assemblage, tenfold increase for M. nattereri and fourfold increase for M. myotis. In 1989–2019, the numbers of M. myotis, M. nattereri, Myotis daubentonii and Plecotus auritus were fluctuating, but most of the recorded changes could not be explained by methodological problems or a direct human impact. Therefore, the cumulative results largely reflected the real trends in the species abundance. A long-term upward trend in the whole bat assemblage was recognisable, but with a stable or slightly decreasing phase in the last decade. An upward trend in M. nattereri was even stronger and has only slightly flattened recently. In M. myotis, the trend was clearly upwards up to the early 2000s, but weakly downwards in the following years. In M. daubentonii and P. auritus, no significant trend was determined. In strongly fluctuating M. daubentonii, the numbers were mostly moderate or high, and even increasing, up to 2008 and only moderate or low in the following years. In P. auritus, an increase occurred in the 1980s and early 1990s, and then, after the stochastic human-induced drop in 1994, its abundance remained relatively stable. The population trends in Strzaliny largely reflected the general trends assessed for a large part of Europe. This suggests that the general population trends may be recognisable even in one large winter assemblage if it is reliably and consistently monitored through a long period. In this context, the hibernaculum in Strzaliny appeared to be a model object for such studies.