A common misconception is that bats build a nest in a similar way to some bird and rodent species, by dragging in bedding material and prey to feed on. Instead they seek out and use existing structures to roost in. A roost can be defined as any place or structure that a wild bat can use for shelter and protection.
Bats can remain loyal to particular roosting sites, but can also move between many sites during the year depending on their needs. The suitability and choice of a roost location will depend on the season, the availability of food, and the age, sex and species of the bat. Most UK species form social groups for at least part of the year and this is when the presence of a roost may become more obvious.
In summer, pregnant adult female bats will gather together in a warm, safe place to form a maternity colony, to give birth and rear their young. These colonies are particularly sensitive to disruption during this period and any major disturbance may cause the mothers to abandon their pup before they are ready to fly. These colonies will begin to disperse once the young become independent. In late summer male bats will setup a mating roost to attract females and defend their territory from other males.
In winter when their insect food is scarce, bats will seek out a cool sheltered location with a stable temperature in order to hibernate. This is an extended period of deep sleep called torpor where they lower their metabolic rate and temperature to conserve energy, and survive on their stored fat built up in autumn. If disturbed during hibernation, energy reserves are used up in waking from this deep sleep and may prevent the bat from surviving the rest of the winter.
Most UK bat species have evolved to roost in caves and trees, but some species have now adapted to use man-made structures all year round, while others only use occasionally.
The three main roost types are:
Buildings barns, bridges, churches, factories, farms, houses and schools. Crevice dwelling species will prefer gaps in features such as barge boards, cavity walls, fascias, hanging tiles, lead flashing, roofing membrane, slates and tiles. Other species may roost in the open in the roof void.
Trees cracked loose or split sections of bark, dense ivy cover, or hollows in the branches or trunk. As well as providing shelter, trees attract many insect species for bats to feed on.
Underground caves, cellars, mines and tunnels. Particularly important as hibernation sites due to their stable cool conditions.
This article was brought to you by Total Ecology, ecological consultants for bat surveys and other wildlife in the UK.