This video from Latvia says about itself:
On the 14th of February 2009 we searched for bats in winter-sleep inside old bunkers in Mangaļsala, Rīga, Latvija.
The task was to assess the population, classify individuals by species and disturb them as little as possible.
In total 5 people in 2 teams we found 78 bats from 5 species – considered not bad for that place.
Translated from Rogier Lange, Vleermuiswerkgroep AWD in the Netherlands:
The Vleermuiswerkgroep AWD [Bat Working Group] has been counting bats for 25 years in the [World War II German] bunkers of the nature reserve Amsterdamse Waterleidingduinen. And successfully so, because every year they find more bats. With some minor adjustments the bunkers prove to be ideal hibernation places. …
The numbers in the course of the years increased from 76 to over 260, an average increase of over 5% per year!
The most numerous species is the Daubenton’s bat, with a maximum of 221 individuals. In the early years more than 90% of the bats counted belonged to this species, in recent years this is below 80% due to the emergence of other species. Second is the brown long-eared bat (up to 35 individuals, 8 to 15% of the total number), followed by the Natterer’s bat (up to 14 individuals, last year 5% of the total).
The pond bat is emerging. In the early years it was a rarity, last winter 18 specimens of this species exceeded the 5% limit for the first time. Finally, in the early years occasionally a single whiskered bat was observed.