Why and How Bats Use Buildings

Why bats roost in buildings

With the loss of natural roost sites such as veteran trees or undisturbed cave systems, bats have had to find alternative roosts in buildings, ideally with suitable foraging habitat nearby, such as parkland, gardens, farms, lakes and woodlands. Bats may also use buildings because they are more thermally stable and safer environments, and there is less competition from birds and other mammals.

Building features that serve as bat roosts

As long-lived animals, bats get to know a large number of suitable roosting structures over the areas they need for their exacting life cycle requirements. Different species of bat at different times of their annual cycle will make use of a variety of building ‘habitats’. In summer they need stable, warm, dry roosting environments to give birth and raise their young. In the winter they need stable, cold and humid roosts to hibernate.

Roosting places are sought where no rain, frost, direct sunlight or wind can penetrate. This is particularly important because although bats are able to regulate their body temperature, direct exposure to the weather can disrupt this, rapidly leaving them vulnerable.

Depending on the number of microclimates and size of internal spaces, a building may accommodate just one or a number of different species and types of roost. Bats may visit these roosting sites occasionally or frequently, however some buildings have records of continuous use by bats for decades.

The illustration below shows the range of building features that may serve as bat roosts.

Bats prefer surfaces on which they can get a good grip with their toes and thumb claws to hang on to or crawl along, such as clay tiles or brickwork. Their claws are so sharp they can even grip onto roughly skimmed concrete.

The aspect, situation and construction of a building will create different temperatures and roosting and hibernation opportunities that can make a site attractive for bats. For example:

• Roofs or walls facing south will provide hotter roof voids or wall cavities for at least part of the day
• High roofs not shaded by trees are likely to be warmer
• The roofing material will also affect roof void thermal stability. For instance, a dark slate roof void will have a different thermal property to a corrugated iron roof. Roof underlays and membranes will also affect conditions inside, as will insulation at floor level in the roof void
• An underground room will be more buffered from outside conditions than one above the surface

The area around a building and its wider landscape setting are used by bats for feeding and other roosting sites. Unlit walls, buildings, hedges and trees are used by bats to navigate between feeding areas and roost sites. The loss of vegetation, the demolition of a wall or a new opening in a hedge close to a roost may sever commuting routes and cause roost abandonment.

Bats’ entry points into roosts

The size and height of the bats’ entry point depend on the species however all UK bats are small creatures.

Crevice-dwelling bats crawl into their roosts via gaps from as small as the width of a thumb.

Roof-void dwelling bats require similar-sized gaps to access their roosts. The height of entry is usually 2–7 metres. They typically need timber joists or beams on which to roost on or behind. Horseshoe bats need a larger entry point so they can fly (instead of crawl) into the roost. The roosting area should not be trussed or cluttered.

The bats' entry point may be close to their roosting place. In some cases, the roosting place may be a little removed from the access point, such as a mortar gap in a wall which may lead into cavities between the inner and outer walls.

There may also be a first access point some distance away, such as where the bats enter over the top of a poorly fitting door or through a broken window before finding their way to their roost in the roof.

Tunnels, mines, bridges and cellars

Bats use underground sites and bridges in a variety of ways depending on the species, roost type and time of year. Some bats breed in underground sites but this is uncommon as they are normally too cool.

Cold sites, such as disused railway and canal tunnels, ice houses, stone bridges and unheated cellars, are in most use during the bats' hibernation period. Favoured sites typically have stable and low temperatures and high relative humidity.

In these sites, most bats typically use crevices between brick or stonework, and other cracks and crevices. They are unlikely to be seen or detected. However, where horseshoe bats can fly into underground sites you may find these bats are more visible as they roost hanging out in the open.

Stone bridges are used extensively in summer by species such as Daubenton's bats. Cellars that receive waste heat for example from a boiler are sometimes used by horseshoe bats as breeding sites.

Bats also use underground sites in the autumn, when some aggregate at the entrances to certain stone mines or tunnels to 'swarm' at night.

The section Managing properties for bats and people provides advice about features like lighting and grilles.

Signs of bats

Finding bats requires skills and expertise and should be undertaken by a bat specialist.

As well as tending to choose dark, inaccessible places, bats are small and usually quiet and inactive when roosting. A single bat squeezed and motionless in a mortar gap is likely to remain unnoticed. This is a particular problem when looking for hibernating bats, but even in summer, a cluster of sleeping bats in the corner of a dark roof void can be easily overlooked.

Bat species regularly move between roosting places between and within buildings, so even a known roosting place can have no bats using it at that point of time.

In summer, females gather to give birth and raise their young (called pups) in maternity roosts, and although they may occasionally be heard chittering during the heat of the day or just before emergence, they may not be seen if roosting in an inaccessible location such as within a cavity wall.

Signs of bats include:

• Droppings
  Bat droppings are often found below the places where bats roost. Bat droppings can be distinguished from rodent droppings as they are usually found in piles. They can be found on higher surfaces such as walls or windows as well as the floor. Bat droppings crumble to a fine powder when crushed, whereas rodent droppings usually harden with age. Fragments of insects will be visible in the bat droppings with a hand lens. It is sometimes possible to identify the bat species from the appearance of the droppings but good practice is to confirm species identification with DNA analysis.
• Insect remains
  Some species of bat capture moths, butterflies, cockchafers or other large insects on the wing and return to a feeding perch to consume them. The wings or wing-covers are discarded and may accumulate below the perch.
• Smell
  Well-used roosts may have a distinct musky smell. Occasionally this can be very strong, particularly in unusually large roosts where there is inadequate ventilation or dampness.
• Urine
  Shiny surfaces, such as polished wood, plastic or stone, may show urine spotting or staining.
• Grease marks and staining
  Bat fur contains oils to keep it in good condition. A characteristic dark stain might be seen on wood or stone near a roost entrance or favoured roosting spot.
• Sound
  If bats are present in a roost, their chittering can often be heard, most commonly just before the bats emerge in the evening or when the weather is very warm.

Bat detectors are not a reliable way of telling if there are bat roosts as bats are unlikely to echolocate within a roost.

The absence of signs does not necessarily mean that no bats use the site, especially during the winter months when hibernating bats remain motionless for long periods of time or where the weather may remove signs from external facades.