So, you might have figured out by now that I quite like bats, as I’m always writing about them. Well, in fact a lot of the work I do involves bat surveys, and in particular trying to identify bat roosts. Bat roosts are the places where bats sleep by day, have a quick rest by night while they’re out foraging for food, raise their young in summer, and hibernate in winter, so they are extremely important places in the life cycle of bats. In most countries in Europe bat roosts are protected by law for this reason, but many people seem to be unaware of their presence in the landscape even though they’re quite widespread and fairly common. Therefore in this blog post I’ll walk you through a brief guide to identifying bat roosts, based on my own experience which I’ve gained in the last few years.
The places where bats roost varies a lot depending on each bat species, and the time of year. In Europe, bats’ natural roosting sites consist mostly of trees and caves, and to a lesser extent other types of sites like crevices in rock faces and cliffs. Bats have most likely used these kinds of places for millions of years, but since the arrival of humans they have taken to using man-made structures, including houses, churches, barns and other buildings, bridges, tunnels and mines, among others.
Bats often use different roosts according to the season. In winter, bats will use hibernation roosts, where the temperatures stay a stable, low temperature somewhere between 0 and 10 degrees Celsius. Bats typically hibernate in caves, mines, tunnels, cellars, trees and disused buildings, where they can find these conditions. In summer bats move to different roosts, either maternity roosts where the females rear their offspring, often in large numbers, or to “bachelor pads” where usually small numbers of males build up their body condition in preparation for the autumn mating season. Summer roosts can be found in the whole range of structures that bats occupy, from trees, buildings, bridges, and less often in caves and mines. In spring and autumn, bats use “transitional” roosts between the times when they are in their main breeding and hibernating seasons, and in early autumn many species use specific roosts where they gather together to mate before they enter hibernation. Therefore, bats are always on the move throughout the year, and each individual bat probably uses a whole range of roost sites. However, although this might sound overwhelming, you don’t necessarily need to find the bats themselves to get some idea of whether you’ve found a bat roost site.
Signs of Bats
First, we’ll examine the types of signs to look for that indicate the presence of bats. The best way to begin figuring out whether a site (whether a house, tree, cave, etc.) is a bat roost is to take a good thorough look around it in daytime. If you’re lucky enough, you may find a bat hidden in a crevice, but most of the time they are well concealed (apart from horseshoe bats, which hang out in the open). More often you can find other signs of their presence, especially bat droppings. These are small and look a little like mouse droppings, but are usually quite black and crumbly; if you look closely they appear quite shiny and you can see the little fragments of the insects they’ve been eating. On the other hand, rodent droppings “smear” when you try to crush them if they’re fresh, and when they’ve dried out they’re quite hard. You’ll also recognise bat droppings when you find them in places where a mouse could only leave them by abseiling down a wall! Another tell-tale sign is the little piles of discarded moth and butterfly wings, and other insect prey remains, which are dropped under a bat’s feeding perch. These get left behind underneath a spot where a bat has returned with a large prey item during the night to eat it- the bat tears off the wings and drops them, and sometimes you can even see the little holes left by the bat’s teeth.
Around the entrance hole to a roost, especially where many bats are present, dark grease stains and scratch marks from the bats’ claws may be visible, but if these are high up you might need a pair of binoculars to see them. Similarly, in some places you may be able to see bat urine stains, especially on the interior walls of buildings where bat roosts are present and the bats have access. These are particularly visible on light coloured walls or on metal surfaces (which they cause corrosion to), and often look like an elongated exclamation mark (!), with a long downwards streak and a little round drop at the end. If you are lucky you might find dead bats in some roost sites if you look around carefully enough, especially in water containers, sinks, or other places where bats might fall in and be unable to escape. Finally, it is sometimes possible to hear bats by day, as they can sometimes be heard audibly chirping inside their roosts especially on hot days when their roost becomes particularly warm. Perhaps they are arguing with each other because they’re hot and cranky! So, now that we know the kinds of signs we’re looking for, let’s take a look at the different types of places bats roost in and how to find out if bats are present at a site.
Roosts in Buildings
Bats will roost in a wide range of buildings, including houses, sheds, barns, churches and almost any other building type you can think of. These can also range from almost complete ruins to practically brand new houses, although most bat species do tend to prefer older buildings (say 100+ years old), and those which are built from more natural materials such as timber and stone, as opposed to concrete. Building roost preferences vary by species, as pipistrelles seem to be the most tolerant of modern buildings followed by brown long-eared bat and Leisler’s bat (in my experience at least- these also happen to be the commonest bat species in Ireland), while the Myotis bats (e.g. Daubenton’s) and lesser horseshoes prefer older structures. Rural buildings will probably be more likely to contain bat roosts as they are more likely to be near suitable bat feeding habitat, but buildings in areas of towns and cities where there is some suitable habitat nearby (e.g. large gardens, parks, rivers) also have some potential. However, in a small number of places I have even found bats roosting in the most urbanised part of a city centre, so it seems that most buildings probably have at least some potential for bat roosts.
Bats will use several typical places in buildings to roost in, often in the roof space, either in the attic (if there is one) or otherwise between slates and felt, or simply under a loose slate. Warm roof spaces provide ideal maternity roost sites for many bat species. Bats may also roost in cracks or spaces in other parts of buildings, within walls or around windows, between floorboards, and some species often use cool cellars as winter hibernation sites.
As I mentioned above, droppings are probably the easiest signs of bats to find. The first place to look is on the outside of the building; do a circuit around and look along all the walls, windows and windowsills, doors, and on the ground just around the building. If you’re lucky, you’ll spot bat droppings somewhere, often stuck to walls or windows just beneath where the bats emerge from their roost. Also take a look with a torch into any cracks you find in the walls, as you might just find a bat tucked away in there. Depending on the type of building and whether it’s lived in, have a look inside the building and look for bat signs such as droppings, butterfly wings, dead bats and urine stains. Be systematic, and look in as many parts of the building you have access to. As with the outside of the building, look into any cracks or crevices that you can find. Cobwebs often catch falling bat droppings, and keep an eye out for surfaces where droppings will stand out, such as windowsills, uncluttered floors, tables, etc. On some hot days it’s possible to hear bats chattering inside their roost, if there’s a good number of them present and the site is quiet.
Bridges are another common potential roost type, which quickly becomes obvious if you look at a map of your local area and see how many of them are actually out there. In Ireland and Great Britain the majority of our bridges are old stone bridges, with a smaller number of more modern concrete bridges. However, many older bridges have been rebuilt, widened or reinforced over the years with concrete, which affects their suitability as bat roosts. In general, in the British Isles it seems that bats prefer to roost in the older stone bridges instead of concrete ones- certainly I’ve never found signs of bats roosting in concrete bridges here, but I wouldn’t completely rule the possibility out. In contrast, in North America it seems that concrete bridges are used quite often by bats there, and one modern bridge in the city of Austin in Texas is home to one of the biggest summer colonies of Mexican free-tailed bats in the world (at 1.5 million of them, there are more bats than people living in Austin!).
Bridges are fairly straightforward to survey for bats- you just need to search in the cracks present in the stonework under the arches of the bridge. As old stone bridges deteriorate over time they accumulate often quite deep cracks, which provide ideal bat roosting opportunities. Usually you’re looking for the bats themselves, as other signs like droppings just fall out of the crack the bat might be roosting in and get washed away by the river. However, if you’re lucky and you look carefully enough you might find some droppings stuck to the stonework in a crack, indicating where a bat has been. Cracks in the underside of bridges can often be quite deep, so it isn’t always as easy as it sounds to be fully sure that there isn’t a bat hidden away in some recess that’s out of sight- professionals use an endoscope to be fully sure that they haven’t missed anything.
Bat species which commonly roost in bridges include Daubenton’s and Natterer’s bats, and I’ve heard of pipistrelles being found on some occasions. As the cracks that bats use in bridges can eventually become a structural problem, they’re prone to being repointed with concrete- bridges that have recently been repaired in this way often have little potential left for bat roosting sites, but it’s still probably worth a look just in case.
Roosts in Caves, Mines, Tunnels and Other Underground Sites
Caves, mines, tunnels, and other underground sites are good potential bat roosts, especially in winter when they may be used by hibernating bats. However, finding hibernating bats can be tricky, for the obvious reason that the bats aren’t active and aren’t leaving behind as many signs as in other times of the year. Looking for bats in underground sites, whether manmade or natural, is broadly quite similar. As with bridges, you’re looking into every nook and cranny you possibly can to see if you can find any bats wedged into them, which in this case is often easier said than done. Caves can be rather problematic, especially ones in limestone bedrock which can be quite extravagantly complex with deep, inaccessible cracks, fissures and passages in the floor, walls and roof. You have to do your best to look into as many of these as possible, and if you’re lucky you may be able to find a hibernating bat, although they tend to hide themselves as far away into crevices as possible. On the other hand, if you look carefully you may well find bat droppings on the caves floor or walls, but it’s difficult to tell from these whether the bats are hibernating or just passing through at other times of the year.
Trees are one of the main categories of natural bat roosting sites, and they’re undoubtedly still an important roost type for many bat species, although some bat species rely more heavily on tree roosts than others (e.g. Bechstein’s and barbastelle bats). Despite them being probably one of the most important roost types, the systematic study of tree roosts (in Europe at least) really only seems to have taken off in recent years. I’m not really sure why this is, but there could be several factors. They seem to be fairly inconspicuous a lot of the time, as even a bit of peeling bark or a small crevice can be enough to shelter a bat. Bats also seemingly move between different tree roosts quite often, apparently once every 3-4 days in summer. Tree roosts can also be inaccessible, either because they’re often located well out of reach of anyone except an expert tree climber, or because they’re hidden well inside a deep crevice or hollow within a tree.
I have to admit I know very little about tree roosts and I’ve never one, but it’s a field I’m very interested in learning more about. As with other roost types, two of the most common signs of roosting are either the bats themselves or bat droppings. If you want to find out more about tree roosts, have a look at Henry Andrews’ fantastic dichotomous key on the subject here.
Safety and Legislation
Before you decide to rush out the door and start hunting for a bat roost, there some things you should be aware of, on top of the signs of the presence of bats.
First and foremost, you need to think of your own safety. Some of the places where bats roost can be potentially dangerous, such as derelict buildings, bridges and caves. In particular, disused mines are especially dangerous due to the risk of collapse or rock falls and should probably be avoided altogether. Before investigating a spot you think might be inhabited by bats, think about the potential risks- rotten floorboards, fast flowing water, steep slopes, etc. – and consider whether it’s safe to visit the site, and if you do, take measures to reduce or eliminate the risks. Ideally you should take someone with you, don’t go to risky places on your own.
Secondly, consider the safety of bats that you might disturb. Bats are very sensitive to disturbance at their roosts, especially during hibernation in the winter (especially December to February) or during the breeding season (especially June to mid-August); outside these periods is less sensitive, but care is still needed. This isn’t really a problem if you’re looking for signs outside a building for example, but if you have direct access to where you think bats are roosting (e.g. an attic space) and you will be at close quarters with them, you should consider whether you really need to enter. If you have a strong suspicion that bats are present there are other ways of confirming this without disturbing them, such as by watching them exit the roost at dusk.
Closely tied to this last point is the need to be aware of wildlife legislation. All bat species and their roosts are protected by law in Ireland, the UK and the EU (if you’re based outside Ireland or the UK you should check your local wildlife laws, but they’re likely to be very similar). Without wanting to scare you off, it’s important to be aware of these laws, but they boil down to two fairly simple rules: don’t harm bats, and don’t disturb or damage their roosts. To do any of these needs a government-granted licence, so if you know that a site contains a bat roost you should avoid entering or otherwise disturbing the roost (as an example, it’s ok to enter the building, but avoid entering the attic if that’s where the bats are roosting). If a site is not known to contain a bat roost you’re safe to look around as you please, but if you do find a roost it is automatically protected and the safest course of action is withdraw from where bats are roosting to avoid disturbing them.
How to Find Out More
If you’re interested in learning more about bat surveying, two good starting points are Bat Conservation Ireland and the Bat Conservation Trust, who both run training workshops and courses. Another good place to try might be your local bat group- there are many county bat groups in the UK and there are also several in Ireland, and these may also organise surveys where you can pick up a bit of experience of identifying bat signs, as well as having experienced surveyors who can give you pointers.