Why are bats found roosting in buildings?

Bats and people have been sharing dwellings for thousands of years. As natural roosting sites have become scarce due to development and land use change, so the number of artificial roost sites has increased in the form of houses, bridges, mines, barns, icehouses, etc.

Man-made roosts can provide stable micro-climates for the bats. Loss of natural roosts has increased the importance of man-made structures for bats to the point that artificial roosts are becoming essential in the survival of many bat species. However even these man-made roosts are now under threat; demolition of old buildings, renovations, changes in use, artificial lighting and the move towards air-tight buildings, all have implications for bat populations using buildings.

What do bats look for in a home?

Like us, bats are looking for shelter that is clean (free of disease and parasites), comfortable (protected from the weather) and secure (safe from predators and disturbance). However, bats also have unique requirements and may live in several different roosts throughout the year depending on their seasonal needs.

A number of factors are very important when considering roosts for bats including:

Temperature

Temperature is very important to bats. Bats are different to us in that they look for warm roosts in the summer and cool roosts in the winter.

Summer: Bats usually only have one young a year. Mother bats gather together and find somewhere warm to rear their young called a maternity roost. Males will form smaller roosts elsewhere that are often cooler than the needs of the females. Generally maternity roosts are usually between 30 and 40°C, however species can have their own unique preferences and studies have shown that even a 1.5°C difference may influence bats to choose one building over another. It is always best to provide a number of different options for bats, so that they can choose the right roost with a temperature based on their needs.

Winter: British bats feed on insects which are scarce in the winter. Bats have adapted to this seasonal scarcity by going into hibernation where they lower their body temperature to the ambient temperature or just above. Hibernation roosts are often a cool space with high humidity. For most species this is generally between 0 and 6°C, however horseshoe bats prefer slightly warnmer conditions of 6 - 10°C.

Aspect and orientation

Summer maternity roosts in the northern hemisphere often have a southerly or westerly aspect for maximum solar heating. Male roosts and hibernation sites typically have a northerly aspect.

Which bats roost in buildings?

All UK bat species have been known to use buildings, but some species are more building reliant than others. Different bat species select different types of roosts in buildings since they look for sites comparable to those chosen in natural habitats.

Bats that use buildings can generally be divided into four categories, although there is regional variation and some species can occupy more than one category:

  • Crevice-dwelling bats (which tend to be hidden from view) include the common pipistrelle, soprano pipistrelle, Nathusius’ pipistrelle, Brandt's bat and whiskered bat.
  • Roof-void dwelling bats (that may be visible on roof timbers) are serotine, Leisler's bat, Daubenton's bat and barbastelle.
  • Bats that need flight space in certain types of roost are Natterer's bat, and brown and grey long-eared bat.
  • Bats that need flight space and flying access into the roost are greater horseshoe and lesser horseshoe bat.
Bats in Buildings

John Hooper