Bats in a haunted house

Last night I was out until the early hours surveying bats. It was an emergence survey at a deserted and crumbling old mansion house which is allegedly haunted. Most of our time was spent outside the house identifying and counting bats as they emerged, but we also went into the house before and after the survey to look for roosting bats or bats flying around inside the house. It was a really spooky experience, especially down in the cellar where there was water dripping and toads and frogs hopping around.

We didn't find any roosts but we did see bats flying around inside, and their presence was betrayed further by bat droppings and numerous wings of various lepidoptera all over the house. This was particularly interesting to me because most of the wings were of butterflies, especially small tortoiseshell and peacock. I would say that these accounted for around 90% of all of the wings we saw. There were a few moth wings, mainly large yellow underwing, but also dark arches.

Small tortoiseshell wings.

The dominance of butterfly wings and the fact that they are inside the building may seem surprising, but actually they give a clue to the identification of the bats in the house. Bats are well known to use echo location to catch flying prey, but some species, especially brown long-eared "glean" prey from the foliage of trees. This is a technique of hovering around leaves and picking off insects, which means that day flying insects such as butterflies are as vulnerable, perhaps even more so, than nocturnal insects such as moths. Finally the bats then often carry their prey to a night time roost  which can be a cave or tree, or in this case an old building, where they remove the wings and eat the more edible parts, leaving the wings scattered all over the floor below.