Bats still on the wing prior to hibernation
BATS are still on the wing hunting for insects on mild nights. However, as autumn progresses flying insects will become scarcer. Bats cope with this seasonal lack of food by hibernating.
Hibernation is not just sleep; not even deep sleep. It is a state of dormancy that is totally alien to us humans and involves major changes in how the body functions.
We have at least ten species of bat in Ireland. The number has to be vague as rare migrants and/or vagrants occasionally turn up. Ten species have been confirmed to be resident and the most common of these and the one that is likely to be seen is the Common Pipistrelle, our smallest bat.
Bats are extremely difficult to identify in flight. Generally, they need to be caught and identified in the hand by experienced bat workers. Otherwise, their calls identify them. However, since they are our most common species it is reasonable to assume that small bats flitting about in the fading light at this time of year are probably Common Pipistrelles.
All of our bats are nocturnal. They emerge at dusk to hunt for midges, small moths and other flying insects. As we advance deeper into autumn and as the weather turns cooler, the number of insects out and about will decline and bats will be forced to go into hibernation.
The timing of hibernation depends on the weather. If the weather is mild, insects will be active and bats will continue to feed. In general, Common Pipistrelles usually hibernate between November and April although they may come out to feed on warm days anytime during the winter.
Little is known about where Common Pipistrelles bats hibernate in winter. Since they are no longer flying their activity cannot be monitored. Since they effectively stop defecating they leave no clues in the form of droppings to indicate their whereabouts.
There is no evidence that Common Pipistrelles hibernate in large numbers so it appears that they survive alone or in small numbers in a variety of hideaways scattered throughout the countryside.
Since bats are so small, in a worst case scenario, they have to survive the winter on just a few gramme of stored fat. To conserve energy, their body temperature drops, their metabolism slows and their heart beat decreases dramatically.
If the winter is a tough one, they may emerge from hibernation the following spring having lost roughly half of their body weight.