Counting bats in the winter roosting sites

Every year in February Carsten Harrje sets out to count bats which roost during the Winter in former air raid shelters in Kiel. Most are closed but for small openings to allow the circulation of air. These ventilation shafts are used by bats in Fall to enter the tunnel systems. They leave the tunnels again in Spring.

The old air raid shelters are mostly used by Myotis daubentonii and rarely by Plecotus auritus or P. austriacus. In 2000, the number of counted overwintering bats decreased slightly to almost 1.000. This is, however, a substantial part of the population considering many bats are probably overlooked when they climb to extremely sheltered spots.

The air raid shelters are very attractive to bats because of the way they are constructed, made of ribbed concrete which has mainly not been covered by other material. Most bats prefer to roost in the gaps in the ceiling, but some can also be seen in the walls. The highest density is close to the ventilation shafts, but some are always found as far as possible from the entrance - therefore a complete counting envolves checking every concrete rib in the air raid shelter.

Some tunnels are less popular due to the fact that the concrete ribs have been covered, but some bats will always find a suitable spot.

Parts with high moisture are especially popular with bats, even if water drips from the ceiling. This Myotis is covered in dew - a good way to protect these sensitive wings from drying out and becoming brittle. Another important factor is a constant temperature which may not fall below freezing. These tunnels offer a year-round temperature of 9 degrees Celsius.