Introduction to Bats

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Bats are the world's only flying mammal and they comprise about one quarter of all the worlds mammal species.

All British bats are insectivorous and they emit their own sonar called echolocation which they use to find their prey and navigate in the dark with incredible accuracy. These echolocation calls are at a very high frequency (usually between 20 - 110 kHz) so humans cannot hear them very well.

In the summer, females gather together to form maternity roosts where they give birth and raise their young. Females usually only have one baby a year. Maternity roosts are often in trees, buildings or barns. Babies are usually born in June or July. After about four weeks the young are almost full size and can fly around with their mother.

In winter, when insects are scarce, bats hibernate, usually in caves, cold parts of buildings or trees. However, we still only know where a very small proportion of the bat population in Britain go to roost and hibernate.

Bats usually mate in the autumn but sometimes it also occurs in the winter, but the females store the sperm throughout the entire winter, and wait until the spring to ovulate and fertilize the egg. This enables the females to wait until they percieve the conditions to be right to become pregnant. This reproductive strategy is unique to bats.

Serotine summer roost (left) and hibernating Natterers bat (right). Phil Richardson.

Bats in decline

All British bats feed on insects. One of the most serious problems facing bats is the loss of habitat which supports insects. The other major problem is the loss of roost sites.

A huge reduction in woodland area across Britain has deprived bats of roosting and foraging sites.

The intensification of agriculture has further reduced insect abundance in the countryside through the loss of hedgerows and uncultivated field margins and by increased use of herbicides on crops and also and pesticides on livestock, which reduces numbers of chafers and dung beetles.

The removal of hedgerows and construction of roads and towns has fragmented the countryside and isolated roosts. Bats use linear features, such as hedgerows, for navigation and for sheltered commuting routes. Bats will often fly many miles in a night to forage, and they also frequently move roosts, so they need to be able to safely navigate around the landscape. Unfortunately some human activites are impairing this. Hedgerows that become severed by a road can lead to abandonment of roosts and foraging sites. This can also cause direct mortality of bats from traffic as they attempt to cross the road too low and are hit by cars.

Building work has often disturbed and excluded bats from roosts and poisonous chemicals used to treat timber killed many bats.

We still have so much to learn about the behaviour of bats, which is complex and often unpredictable. We also need to better understand their distribution and habitat preferences if we are to reverse the decline of these intriguing mammals and we encourage you to report any bats you see.