Planning a wind farm requires a thorough surroundings analysis, including bat hibernacula, which determine bats’ survivability during rough period of winter. Not including them during planning process might lead to severe, often irreversible, ecological loss.
A bat hibernaculum is a location where bats spend the winter in a state of hibernation, allowing them to survive the months when insect prey is scarce. During hibernation, bats lower their body temperature, heart rate, and metabolism, conserving their limited fat reserves (which constitute up to 30% of body mass before winter!). In temperate regions like Poland, this strategy is essential for survival, as food shortages can last for several months. Because of their importance, many hibernacula are protected under the EU Habitats Directive, with some designated as Natura 2000 sites specifically for their bat populations.
Daubenton’s bat wintering in a brick cellar, Ambiens 2025
Unused brick cellar, potential bat hibernaculum, Ambiens 2025
The ideal hibernaculum offers a stable, cool environment, typically just above freezing, often between 0 – 10 °C. These conditions must remain constant, as sudden temperature changes can prematurely wake the bats, forcing them to use up needed energy. High humidity, usually above 85 percent, is equally important, helping to prevent dehydration. In Poland, hibernacula can be found in a variety of settings, both natural and man-made. Limestone caves, abandoned mines, old fortifications and underground tunnels are among the most common, providing the necessary stable microclimate. Old cellars, root stores, and wartime bunkers can also serve as winter refuges, while some animals may make use of rock crevices or deeper tree cavities – different site types support different species depending on their size, humidity, and temperature.
Controlling hibernacula in the context of pre-construction surveys is particularly challenging. Many sites are small, hidden, or located deep underground, making them difficult to access. Some hibernacula may only be used by a few individuals and can be easily overlooked, especially outside the winter period when bats are absent or difficult to detect. Some may be located in private buildings, to which access is restricted. Seasonal restrictions further complicate surveys, as inspections must be timed to avoid disturbing hibernating bats while still allowing accurate assessment (in January or February). In addition, incomplete records and the mobility of bat populations mean that previously unknown hibernacula can be present around a planned construction area, making thorough field checks and interviews with local communities essential for a thorough assessment.
That is why accurate identification of hibernacula is essential for wind farms to be built in an environmentally safe way and in line with principles of sustainability.