By involving citizen scientists, biologists can gain a detailed overview of the animals sheltering near human settlements. Without their help, specialists simply could not physically inspect thousands of home cellars. The resulting dataset now allows researchers to better understand how the winged mammals survive in changing conditions.

Estonian Theriological Society chairman Oliver Kalda explained that animals struggling through subzero winter temperatures often leave their natural habitats. This winter, even large sandstone caves failed to keep out the cold. "Monitoring in the Piusa Caves this year showed that in some places it had become very cold and quite a lot of bats had died," the zoologist said.

Kalda nevertheless described the monitoring campaign itself as a complete success. Around 100 people who responded to the zoologists' appeal photographed hibernating bats and sent their locations to researchers. Based on the photographs, biologists identified the species and recorded around 280 individuals. "The number of bats was the highest compared with previous years, so the monitoring program has really taken off. People are happy to let us know who is visiting their cellars," the scientist said.

More than one-off campaigns, zoologists are interested in long-term trends. Data series stretching back decades vividly show how populations cope with a changing climate over time. Some species appear to be adapting relatively well to the new conditions. "What is interesting is that if you look at a 40-year perspective, the number of bats in wintering sites has increased enormously for species such as the northern bat or whiskered bat — we do not distinguish between them in winter," Kalda said.

Alongside native bats, biologists are also closely monitoring migratory species. A significant share of Estonia's summer bat population flies hundreds of kilometers south each fall. The animals spend the winter in Central Europe and return here only in the spring. Fresh data from Finnish zoologists, however, points to a new trend. Species that traditionally migrate are now also trying to survive the winter locally.

As habitats shift northward, researchers are seeing more and more unexpected visitors. "Last fall, Lauri Lutsar recorded a western barbastelle. It was only the second sighting of the species in Estonia. We still do not know whether it is becoming established here or whether it was simply an individual that had strayed off course," Kalda said.

By now, bats have begun waking up with April's warmer temperatures and the appearance of the first insects. Daubenton's bats are already flying near the bridges in Tartu. Above the Emajõgi River, a common noctule can be seen hunting for food.

Because the animals are now flying more actively, they are also increasingly encountering homeowners. Since it takes bats time to raise their body temperature, startling a bat half-way through can cause it to waste its last energy reserves. If necessary, people should therefore use a thick work glove simply to move a half-dormant bat to a safe place.

Kalda urged people above all to remain calm when dealing with a bat. Many homeowners panic when they see one fly into the house, even though it is a completely ordinary wild animal. Kalda added: "The golden rule is to think about what you would do with a fox. If you had a fox in your house and wanted to catch it, the same applies to a bat."

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