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Bats on the Reserve

Description

The only bat so far recorded at Holtspur Bank is the common pipistrelle (Pipistrellus pipistrellus), which is the smallest and commonest bat in the British Isles. There are two other types of pipistrelle, the soprano pipistrelle and Nathusius’s pipistrelle, the latter being very rare.

It has a tiny body, short legs, broad flat head, short broad ears, fairly narrow wings and a short tail. Adults vary in colour from place to place, some colonies are mainly orange-brown and others mainly pale grey-brown. Ears and muzzle are dark. This bat has a jerky, erratic flight, flicking its wings rapidly as it pursues its prey. Its head and body length is 35 mm – 45 mm, its wingspan can be anywhere between 190 mm – 250 mm (6 inches), and it weighs between 3 grams – 8 grams (about the weight of a 2p coin!)

Habitat

Pipistrelles can be found in a wide range of habitats, including mature woodlands, wetlands, grasslands, farms, parks and gardens. They prefer to hunt in open grassy areas surrounded by trees or bushes and are often seen flying low over water. In spring and summer pipistrelle bats congregate in large colonies: roosting in trees; under bark; or in hollows; and buildings, such as churches; also any rock crevices. Their favourite summer roosts are small warm spaces behind tiles or weather-boarding on a south-facing wall. These bats are so tiny that they can squeeze through gaps of only 15 mm. They sleep lightly during the day and usually begin streaming out from their roost 15 - 30 minutes before sunset to hunt for insects. In midsummer they sometimes appear during the daytime. Each bat spends 2 - 5 hours per day away from its colony, using temporary roosts in between short feeding flights. In winter the bats hibernate from about mid-October onwards. The colony gradually stops feeding and finds a suitable hibernation site, such as a church roof or bell tower, a quiet place in a large house, a hollow tree or rock crevice. Each bat hangs or wedges itself head down, gripping the surface with its feet. During warmer spells pipistrelles will wake up and fly out to look for food.

Feeding and Flying

They tend to hunt over a regular beat, flying at between 2 - 13 metres, usually up to 5 metres, above the ground. These bats eat mostly gnats, tiny moths and small caddis-flies, hunting for them in open spaces around a building or tree, or above water. If a large insect is captured, it is taken to a perch to be eaten. A single bat can eat up to 3000 insects every day.

Echo-location

When the pipistrelle is flying fast in the dark, it can avoid bumping into obstacles and track down prey by using a system of echo-location, similar to the radar-scanning equipment used in ships and aircraft. A hunting bat frequently emits very high-pitched ultrasonic squeaks, which bounce back from any solid object into its ears. In each ear there is a fleshy spike, known as a tragus, which receives this sound reflection. The bat is able to interpret the time taken for the echo to return and virtually see its surroundings. A bat probably carries a sound picture of familiar territory, comparable to our own visual memory. Their calls range in frequency from 39 – 60 kHz: the common pipistrelle having a peak of intensity at about 45 kHz; the soprano about 55 kHz; and the rare Nathusius’s are down at 39 kHz. However, pipistrelles do have a social call frequency which is somewhat lower at 20 – 30 kHz and which can sometimes be heard as a ‘chonk’ by some children and young adults.

 Often the best way to detect and observe bats is using a ‘Bat Detector’ while there is still some light in the evening sky. This is something done regularly on the very popular ‘Bat and Glow-worm Evenings’ in early July. The bat detector simply picks up the ultra-sound from the bat and lowers it into the range of human hearing. By identifying the frequency of the sound and studying its nature it is often possible to identify the type of bat and also gain further knowledge of its feeding habits. Almost all of the bats detected in this way on the reserve have been the common pipistrelle at a frequency of 45 kHz; only on one occasion has the bat detector detected pulses at 55 kHz indicating a soprano pipistrelle.

Breeding

Mating takes place in the autumn, just before hibernation, but the sperm does not fertilise the female's egg cells until the spring - this is called delayed implantation. Once the embryo starts to grow, its development depends on the weather and the food supply. It will stop developing if conditions are poor. Female pipistrelles form their own nursery colonies in early summer, often in a roof space behind tiles. At this time the males roost in separate small colonies, only joining the females during the autumn and winter months. Gestation is normally 44 - 50 days and usually only one baby is born in June. The baby bats are tiny, hairless and blind for about a week. Warmth is very important and the warmer they are the faster they grow. Being mammals, the young are fed on mothers’ milk and are weaned and ready to fly independently at three weeks old. The females reach sexual maturity in one year and the males in two years.

Protecting the pipistrelle

Although the pipistrelle is the commonest bat in Britain and the smallest, most widespread and abundant bat in Europe, it has been declining over the years along with all other bats. The 14 species of British bat are all protected under the Wildlife and Countryside Act. If any of them have made a home in your roof it is against the law to interfere with them. They do no harm to you or your property - so there is no need to worry if you have resident bats!

The widespread use of organochlorine pesticides, such as DDT, in the 1960s, drastically reduced insect populations thus depriving bats of food - they may also have eaten insects poisoned with chemicals. Over the years the habitats e.g. hedges, ponds and old grassland, where bats like to hunt have declined in number and they have also lost many of their traditional roosting places, such as hollow trees.