Extremely high mortality of newborn juveniles in the nursery colony of Myotis myotis at Žihobce, SW Bohemia, Czech Republic (Chiroptera: Vespertilionidae)

In 2020, an extremely high mortality of juveniles was recorded in a nursery colony of Myotis myotis at Žihobce (Klatovy Dist.), reaching 50.6% of the potential growth. Most juveniles died within two weeks of age (67.4%). The cause of this phenomenon is not known. The results of chemical analyses carried out in five dead juveniles showed neither the presence of heavy metals nor pesticides in the bodies. The dissection of cadavers revealed a good nutrition state. In all dissected individuals, intestinal inflammation was recorded. Most juveniles in the colony were significantly parasitised. In the period of parturition, it was cold and very rainy for several days, the mortality thus may have been caused by chilling of the juveniles. The effort of females to nurse even the dying juveniles which fell down from the colony was observed. The mortality of juveniles was checked in further 28 control colonies in different parts of the Czech Republic in 2020. At eight sites, making up 27.6% of all studied sites, the mortality of 10.1–20.0% was recorded. At the remaining twenty sites (69% of the studied sites), the mortality reached 0–10%. High mortality of juveniles of M. myotis (18.7–39.2%) was also recorded at Vyšší Brod (Český Krumlov Dist.) in the years 1988–1991. In that case, the probable cause was the application of Lastanox (tributyltin oxide), a toxic chemical used for timber treatment in the colony roost.