Characteristics of winter bat aggregations in Poland: a review

Abstract

The distribution of natural wintering roosts of bats in Poland is not homogenous, and covers almost exclusively the south of Poland: the Kraków–Wieluń Upland, the Sudetes, the Carpathian and the Świętokrzyskie mountains. Man-made underground shelters, on the other hand, are found across the country, including areas outside the range of natural caves. Based on literature data and own observations, the size and species composition of winter bat aggregations occupying natural and artificial roosts were compared. The data concern records both published and collected between 1992 and 2019. The start date of the range was assumed due to the well-established formula of nationwide bat monitoring that started in 1988 (important factor: correctness of species identification), while the end date was based on the availability of publications. We arbitrarily adopted 50 individuals as the minimum number of bats in a certain wintering site, found at a minimum of one during the study period. In total, we collected data on 128 wintering sites: 42 natural and 86 man-made. The overall size of the analysed aggregations reaches at a minimum of 88 800 individuals of which man-made gathered at minimum 77 600 individuals, while natural gathered almost 12 200 individuals. The most abundant bat aggregations were recorded in man-made underground sites: up to 38 594 individuals, while in natural sites only up to 2900 individuals, but the average aggregation size did not differ between the two types of shelters (169 vs. 156, F = 3.368, df = 1,128; p = 0.069). In contrast, the number of species wintering in natural shelters is higher than in man-made ones (8 vs. 6 species, F = 9.785, df = 1.128, p = 0.002179). P. pipistrellus was found only in anthropogenic shelters. In addition, this type of shelters is characterised by a higher proportion of M. daubentonii (16.4% vs. 3.7%), M. myotis (42.0% vs. 29.1%), M. nattereri (26.4% vs. 16.5%), and B. barbastellus (11.3% vs. 7%), as well as species recorded incidentally: P. austriacus (0.09% vs. 0.04%) and E. serotinus (0.17% vs. 0.07%). In contrast, R. hipposideros (16.7%) and M. emarginatus (5.7%) were recorded almost exclusively in caves; moreover, M. brandtii/mystacinus (15.6% vs. 0.3%) and P. auritus (4.2% vs. 1.9%) were clearly predominant in these shelters. Some of these differences could be explained by the effect of geographical location: some species are found only in the south of the country (R. hipposideros, M. emarginatus), on the other hand, artificial shelters are usually characterised by more dynamic microclimate, preferred, in turn, by species rarely found in natural shelters.