Thursday, 15 May 2008

Dawn Swarming

Someone once said that bats are hard to study "...because you can't see them and you can't hear them." It's certainly true that they don't give away their secrets easily, though bat detectors have allowed us to start the long hard job of understanding them.

A behaviour exhibited by most British bats which tips the balance a little on our side is dawn swarming, in whcih bats returning to a roost fly around the roost entrance for a period of time at dawn, often making false landings at the roost entrance. In doing so they give away the roost location. This is in stark contrast to evening emergence, when they dive out of the roost, flying hell for leather to avoid any possible predators, making it hard to spot where they have emerged from!

Of course the downside is that dawn is a horribly antisocial time of day to be about, something I was reminded of when my alarm clock went off at 2.45 am this morning. With dawn in Edinburgh at 5 am I wanted to be alongside a roost site at a suburban house well beforehand, to watch swarming develop. It wasn't just for fun: I had a suite of things I needed to know about these bats. All I knew was where the roost entrance was: at the end of a flat roof bargeboard, giving access to a hollow wall; and that they were Pipistrelles. I needed to find out which species they were, whether they were roosting in any other parts of the building and approximately how may bats roosted there.

For the first half hour there were steady comings and goings, with Soprano Pipistrelles (Pipistrellus pygmaeus) entering and leaving the roost in small groups and occasional bats flying close to the entrance in twos and threes. At 4.25 am swarming started, with numbers gradually building up to a peak five minutes before dawn, with around 60-70 bats zooming round the adjacent garden. You can see a brief snippet of the action here:

Soon after this bats started to enter the roost. Up until this point, everything was as I expected: no other part of the house roof was involved, I knew the species of bat and could see an approximate number. All I had to do was count them as they entered the roost, giving an accurate count and the job was done...or was it?

Only 30 bats actually entered the roost. The remainder gradually gained height until they were around 5-10m higher then the house, then one by one they flew off to the north-east. I have never heard of this behaviour before and was quite astonished to see it. Stuart Smith, of Lothians Bat group was later able to tell me of another roost site about half a kilometre away in that direction, so perhaps that is where they were heading. It is quite common for a colony to move around between several roost sites, but I have never before heard of them swarming at one site and then splitting up, to roost in mor than one location.

Whatever the cause, it was worth being up early to see the spectacle: the video doesn't really do it justice!

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