We analysed the distribution and relative abundance of Myotis daubentonii in the lowlands and uplands around the Lahn river near the city of Giessen (Hessen, Central Germany). We assumed a positive correlation between distribution and relative abundance of the trawling M. daubentonii with the amount of water surface in our study area. We further expected an unequal distribution of male and female M. daubentonii especially during the energy demanding pregnancy and lactation period of females. Daubenton's bats were found at 75% of the 64 ponds and lakes we surveyed by standardized nightly spotlight counts. The number of Daubenton's bats correlated positively with the area of the water surface and negatively with the distance to the nursery colonies. Nursery colonies were located with radio-tracking and existed predominantly in the lowlands close to the Lahn river. Mean flight distance between nursery colonies and foraging areas was 2.3 km (mean ± 1.4 km, range: 0.6–6.3 km). Sex ratio was determined at three sites studied in detail from 1992–2003 by mist-netting along regular used flight paths (n = 1,847 caught individuals). The number of female per male M. daubentonii was unequal and differed significantly between the three sites. In the lowland we found one site clearly dominated by females (median = 4.3 females per male, percentages of females 79.3%, n = 169 individuals) and a second site with an almost balanced sex ratio (median = 1.1 females per male, percentages of females 50.3%, n = 939 individuals). At the third site in the uplands males outnumbered females (median = 0.2 females per male, percentages of females 13.5%, n = 739 individuals). The percentages of females remained largely constant from spring to mid summer and changed at the beginning of September. The study revealed that a detailed knowledge of the spatial distribution of gender specific roost sites and key foraging habitats is necessary for the establishment of meaningful monitoring and conservation measures for bats.
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