Wide variety of bats overwinter in Finnish rock habitats

A recent study shows that bats require diverse rock habitats for overwintering. New insights into inter-species differences in overwintering, and the importance of overwintering sites at various stages of hibernation, support bat conservation.

All bat species found in Finland are insectivorous and retreat into hibernation in autumn as food becomes scarce. However, their hibernation sites have remained largely unknown, hindering efforts to ensure a favourable conservation status for these protected species.

Researchers from the University of Helsinki and the Turku University of Applied Sciences recently conducted an extensive study in southwestern Finland and the Åland Islands to explore the overwintering behaviour of bats.

“We spent a long time wondering where our bats go in winter, as only a fraction of those seen in summer were found in the few known hibernation sites, such as cellars and bunkers. Our study shows that bats can actually overwinter almost anywhere in our surroundings, as long as rocky features are present,” says Postdoctoral Researcher Anna Blomberg of the University of Helsinki’s Finnish Museum of Natural History (Luomus). 

The new study sheds light on not only inter-species differences in using various rock habitats, but also the habitats’ significance to bats at various stages of hibernation. 

“While vesper bats begin hibernating relatively early in the autumn and remain in place until spring, the northern bat and the endangered Nathusius’ pipistrelle tend to arrive later and often fly outside their hibernation sites even during the winter months,” explains Thomas Lilley, Senior Curator for Vertebrates at Luomus. 

The researchers were surprised by the wide range of bat species overwintering in rock habitats. All of the most common species use them, including the northern bat, Daubenton’s bat, whiskered bat, Brandt’s bat and brown long-eared bat, as well as rarer, endangered Finnish species, such as the above Nathusius’ pipistrelle and the Natterer’s bat. The data were gathered using recorders left at hibernation sites, which captured species-specific echolocation calls. 

“Until now, the Natterer’s bat has been the least well-known overwintering species in Finland, with permanent hibernation previously recorded in only a single cave in the Turku region. The hibernation sites we discovered demonstrate the benefits of modern technology for bat research. Simultaneous and wide-scale data collection using high-quality equipment enables acquiring information even on rare species. This supports both research and conservation,” notes Specialist Ville Vasko of the Turku University of Applied Sciences. 

So far, most international bat hibernation research has taken place in large caves and abandoned mines in the karst regions of Central Europe and on North America’s east coast. Although Finland lacks such cave systems, glacial activity during the ice ages has left us with a diverse range of rocky environments, including shingle beaches, glacial erratics, boulder fields and rock outcrops.

Environmental impact assessments for zoning currently consider bats primarily through summer surveys, which aim to identify breeding grounds, roosts, foraging habitats and key routes.

“Environmental impact assessments cover only a quarter of the bats’ annual cycle, overlooking hibernation sites that are vital to their survival and highly vulnerable to land-use change. To complement summer monitoring, surveys must determine whether the areas in question contain these key sites,” says Lilley.