Some whale-watchers and bird-watchers out on a pelagic trip looking for seabirds and dolphins in the Irish Sea on 21st September were surprised to see a bat flying past in the middle of the afternoon. John Stewart-Smith managed to get a photo of it. The bat was flying eastwards, about twenty miles off the Pembrokeshire coast towards the Smalls lighthouse.
After some consultation with others, including Bob Stebbings, we have established that the bat is either a Leisler or Noctule, but without a size reference, or more details, it's not possible to be sure which.
Leislers are found in Ireland, and in the rest of Britain except the far north. Only occasionally have they been recorded in Pembrokeshire. Noctules, on the other hand, are relatively common in Britain, including Pembrokeshire, but are apparently absent from Ireland.
(the following information is from Bob Stebbings)
Now the real question is how do bats navigate. Clearly bats do a lot of water crossing eg between Norway and Scotland/England at least 350 miles and that migration route has been assumed for over 130years. Migration all across northern Europe east to the Urals is all NNE to SSW in autumn and back again in spring. The direction is similar to many birds. It has long been known bats often fly with flocks of birds as they are seen at the south coast bird observatories in particular. That is by day, but John Parslow, who used to work at Monks Wood, studied migrating birds at night crossing the Channel using radar. He could identify the different species of bird from their flight ‘signature’ but he saw these other flying animals which were not birds – ie bats. These were flying up to 10,000 feet high in moderate numbers.
So a bat flying across the sea by day is not unusual, but seeing one flying low over the water is just luck. It's not as if it was out looking for food with the birds in the Celtic Deep!