A family walk in the lovely Cassiobury Park, Watford, was much enhanced by seeing bats flying in broad daylight. It was the first hot day of the year, hitting 20c, and I wonder if that had anything to do with it, though I visit the park only rarely so for all I know they're out and about most days.
The first was hunting in a clearing in the trees by the river, about 20-30 feet up. The second, unless it was in fact the same individual, was skimming low over the canal, patrolling a stretch of a couple of hundred feet. I managed to get a video of the second, though a fast-flitting bat filmed at 12X zoom on a compact camera by an idiot like me does not add up to BBC HD quality results.
What sort of bat is it? I guess either a Daubentons or a Pippistrelle, the former being particularly noted for its water-skimming, though others sometimes do the same.
What drives bats to come out during the day? This academic paper gives a really detailed insight into daytime flying, but suffice to say it's not that unusual and it's probably to make up for poor feeding at night, or to take advantage of good hunting. Or one of many other possible reasons.