Lesser horseshoe bat summer roost counts (2015)

Yep, that’s right, there are soooo many lesser horseshoe roosts in North Wales that they need bat workers from Wales, Merseyside, Cheshire and Lancashire to get together to have a snowball’s chance of counting all those bats!

Every year, Natural Resources Wales pick two dates in June to do simultaneous, co-ordinated emergence counts at as many roosts as possible in the area around Llanrwst in an attempt to monitor the population of lesser horseshoes.  Whilst it’s not as common for lesser horseshoe bats to switch maternity roosts as, say, common pipistrelles, they do move around regionally and are aware of and use various roost sites across the year.  This means that bats which used one roost last summer might be somewhere else this summer, according to the weather or other roost conditions, so we need to count all the roosts, or as many as possible, to have any idea of what is going on with the total population at any one time.

The counts are in June when females congregate in maternity roosts but have not yet given birth to their pups.  This year I went to the same roost on both count nights, a charming olde worlde building just outside Llanrwst which is blessed with lesser horseshoe bats both in the main building and in the cellar.  We think that the primary roost site is in the main building, but that the bats also roost in the cellar.  Clwyd Bat Group speculate that the cellar is cooler and that the bats might be using it to regulate their pregnancies in some way, presumably to better integrate with weather conditions and availability of prey.

What this means in reality is that on any one night, some bats are in the main roost, some are in the cellar, and when night falls and the bats begin to emerge, all hell breaks loose…… because while bats from the cellar are going in and out of the cellar entrance “light sampling” and checking conditions outside before they decide to emerge for the night, bats from the main roost are coming out, whizzing over the wall and ALSO bombing in and out of the cellar, in what looks like the ultimate bat party !   Trying to count bats emerging from the cellar was like trying to count a cloud of dancing midges – tricky to say the least.  It’s chaos to human eyes, particularly as the light levels fall.

By the second count we had got wise to this behaviour and decided to post two observers at the cellar, each with a clicker, one person to count bats in and one to count bats out.  The difference between the two is the number of bats which actually emerged from the cellar.  We worked out that about 50 bats came over the wall from the main roost to visit the cellar, and that about 70 bats emerged from the cellar.  Not bad !!


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