I have been checking bat boxes at work and outside of work most years since 1992. I have found many bats in that time, but many other species inhabiting the boxes have come to my attention, including other protected species.
The best find…
By far the most interesting bat box I have ever opened contained a dormouse nest at the bottom, a bird nest on top of the dormouse nest, and on top of the bird nest were Daubenton’s bat droppings. Which goes to show that sometimes one bat box can benefit not just one, but three different protected species throughout the course of a year!
Other interesting finds...
While I have never encountered a dormouse in a woodcrete box, in some instances I have found nearly as many dormice as bats inhabiting boxes in several woods in Devon in the wooden Chapman Wedge boxes that are popular in that region. Usually I have just found old nests, but sometimes a little yellow face has poked out from the underside of the box, right before shooting up the tree. They may have a reputation for being lazy, but most of the ones I’ve seen are quick as lightning.
Blue tits and great tits commonly nest in bat boxes in the spring. Care is taken if active nests are discovered, and the nests are left alone – as active nests are protected, but if discovered later in the year when the birds have finished nesting they are usually cleaned out to leave an empty box.
Again, I’ve yet to find a nest in a woodcrete box, but on several occasions I have found old bumblebee nests in bat boxes in late autumn.
Spiders are present in most boxes and I have found a variety of species, including house spiders (Tegenaria spp.), walnut orb-weavers (Nuctenea umbratica), Drassodes spp., several “money” spiders (Linyphiidae) and perhaps the most interesting being a false widows spider (Steatoda grossa) and the Local and unusual-looking Cyclosa conica. Some species will make their webs inside the box, whereas others make their webs outside of the box and hide in wait inside the box.
Moths are regularly encountered inside bat boxes. By far the most frequently encountered in my experience is the Large Yellow Underwing (Noctua pronuba), a favourite food of the brown long-eared bat.
I don’t come across too many beetle species in bat boxes, but one particularly interesting species that turned up in several boxes at a site in Gloucestershire was a species of chequered beetle, Opilio mollis (Nationally Notable B), a predator on the larvae of bark and longhorn beetles, and is thought to be an asset to forestry.
I’ve yet to find a hornet nest in a bat box, but several times now I have encountered hibernating queens in early spring. Although large (and a little un-nerving if you are at the top of a ladder), they are usually docile, and do not usually bother you if you gently close the lid. That said, I was once chased down a ladder by a particularly active one…
Slugs are usually uncommon in woodcrete boxes, but I have on occasion found wooden boxes full of them. Fortunately they are relatively easy to remove (which is necessary if there are a lot of them).
On one particular site the boxes are usually full of earwigs. Otherwise, they are often found in smaller numbers, and often end up as spider prey.
In autumn I sometimes find green lacewings hibernating in boxes. As well as being food for bats, lacewings are valuable as pest control, consuming many aphids in their lifetime.