Bat workers make a count in one of the tunnel networks
A Scottish bat conservation worker has returned from Poland where he helped count bats hibernating in derelict war-time underground bunkers.
John Haddow, from Dunblane, encountered greater mouse-eared bat and bunkerowcy, a group of people who spend weekends in the tunnels.
The bunkers and tunnels provide shelter to tens of thousands of bats, the latest count found a drop in numbers.
Here Mr Haddow throws some light on life inside the Polish bat caves.
Hibernating bats are not easy to find in Scotland, in spite of having some of the largest summer colonies in the UK in Scotland's central belt.
Even the largest bat hibernation sites in the UK rarely reach three figures.
In complete contrast, I visited western Poland in mid-January as part of an international team counting a total of 33,473 bats of 10 different species.
All the more remarkable since 21,581 of these bats belong to one of the largest European species, the greater mouse-eared bat. In the UK there is only a single male of this species known, since he turns up most winters in a disused mine in Sussex.
The hibernation site in Poland is remarkable in many ways. Now known generally as Nietoperek after a village in the area where there is an underground complex of tunnels.
The tunnels connect a series of fortifications at ground level, built by the Germans before World War II as part of the defensive east wall extending from the Baltic Sea to the now Czech Republic along Germany's eastern border.
In 1945 Poland's borders were re-drawn by the allies and effectively the whole country shifted westwards.
Tourism poses a threat to the bat habitat
Nietoperek is now 70km inside Poland, and the villages which had German names have been renamed in Polish.
Sometimes the new name was a translation, so the village that Hitler visited in 1936 to inspect the fortifications, Hochwalde (high wood) became Wysoka (high in Polish).
Other names were just "Polish-ised" including Niepter which became Nietoperek, a remarkable coincidence since it means "little bat" in Polish.
We can be fairly certain that neither Hitler, nor the new occupants of the land, foresaw that these defences would become famous for their bats.
The underground system includes 30km of tunnels extending 10km from north to south, linking a number of ground level defensive bunkers.
Many of the ground level structures have been destroyed, but some are intact and allow entry to the lower levels.
Bats must have begun to find their way into the system from the 1950s, but it was not until the 1980s that serious investigation of these bats began.
Although the bats present in winter have been counted since then, annual counts of the whole underground system started in the last decade.
In order to keep disturbance of these bats to a minimum, this happens on one day in mid-January, under a special licence from the Polish government.
This year, a team of 50 led by Dr Tomasz Kokurewicz, a Polish biologist with a long association with Nietoperek, stayed in an old farmhouse in order to carry out the census.
We were divided into 10 teams made up of a mix of Polish, German, Czech, Dutch, Belgian and British.
The first groups left to drive over snow-covered roads at five in the morning and the last returned at eight at night, some people walking up to 15km underground and seeing no daylight that day.
That evening, following a warming meal and a few bottles of Polish beer, the results were compiled, presented and discussed by the assembled team in the farmhouse cellar.
The outcome this year was a surprise, since the total was "only" 33,500, compared with 37,500 in 2008.
It is thought that bats moved into the tunnels in the 1950s
From the year 2000 the count had increased steadily to the highest ever count last year. We have to wait and see if this indicates a real downward trend. January 2009 was very cold, with temperatures locally down to -24C.
The last three winters were relatively mild, so perhaps that had some influence on the increases. However, since the temperatures in the underground system remains at a fairly stable 8C the "missing" 4,000 bats are a bit of a mystery.
Nietoperek is recognised as a site of European importance for bats - those wintering there come from as far as 250km away in Germany and Poland.
Because of its history and character, many visitors are attracted for reasons other than bats. This is not a problem in the summer months, but in the winter it is legally closed to visitors.
There is however pressure to officially open up sections for tourism in winter, and then there are the unofficial "bunkerowcy" - bunkermen and women - who like to spend their weekends in the underground.
Since one of the steel grilles barring an entrance had been destroyed in December, many of these bunkermen have visited the tunnels.
I met two of these illegal groups during the census day, about 17 people in all. One group shook hands with us but complained that they had been woken up too early by the "bat people".
However, it is the official winter tourism which is likely to be more of a threat to the hibernating bats, since sections of the system will be visited regularly, and it is known that bats cannot survive the winter if exposed to frequent sounds, lighting and warmth from human bodies.
Currently there are negotiations to find a solution which protects the bats and permits limited and controlled visiting.
Meanwhile, the bunkermen continue to hack and blast their way through any obstacle to their entry to the underground system, and resources have to be found to repair the damage.