How bats rule the roost

VAMPIRES are evil spirits who force corpses to rise at night and seek out sleepers, whose blood they suck.

The victims become vampires when they die and only rest if a stake is driven through the heart. When blood-drinking bats were discovered in South America, their relatives all over the world were tarnished with the vampire brush. People may fear them but Irish bats don’t suck blood, nor become entangled in ladies’ hair.

These harmless insect-eaters need our support just now. Their babies were born a few weeks ago and bat maternity roosts are a hive of activity. The nurseries can be noisy, which alerts householders to the presence of uninvited guests. Bats are protected by law. It’s an offence to harm them but this doesn’t always deter householders. Eviction notices may be issued and the lodgers expelled.

Non-breeding females and a few immature males frequent the roosts but most of the inmates are nursing mothers. Normally, they have one youngster each. Twins are rare. An infant’s eyes open three to four days after birth. Mothers can carry their babies in flight, but they do so only when moving between roosts.

The youngsters remain at home when the adults go hunting. Breastfeeding continues for about six weeks, by which time the juveniles are able to fly and forage for themselves. Interference with bats when babies are present can be catastrophic. If a colony is threatened, an entire community, and not just a few individuals, suffers. Householders should be patient, the roosts will be abandoned in the autumn. In any case, bat droppings are just digested insect parts, fairly inoffensive. Bats hardly ever carry diseases which infect humans. We have nine bat species in Ireland. All will roost in buildings at least occasionally but bats are choosy when it comes to roosts. Michael Viney in his book Ireland – A Smithsonian Natural History, gives a colourful account of their preferences.

The long-eared bat, he says, is “the bat of the clergy, as old rural churches are its most distinctive” choice. A survey around Dublin found long-eared bats in over half of old churches. The lesser-horseshoe, which likes large airy attics, is “the bat of the aristocracy”. Stately homes are its preferred location. Daubenton’s, the “water bat”, likes to roost under bridges.

The ones which squat in the homes of ordinary mortals are usually pipistrelles. These, the smallest European species, may be found even in town houses, provided there is woodland nearby where they can hunt. It’s not that the bats like modern architecture; they need confined snug places where their babies will be warm and safe. Cavity walls and spaces behind fascia boards, on the south facing side of houses, are ideal. There can be up to 600 bats in a colony and individuals remain loyal to their tribe for life. A colony will have several roost sites, so the bats in your attic won’t necessarily return next year.

It was thought that there was only one pipistrelle species. Then it was noticed that three different frequency bands are used for echo-location pulses. DNA examination shows that we have three pipistrelle species in Ireland, all so alike that it takes an expert to tell them apart. Two of the pipstrelles roost, almost exclusively, in houses.

From September onwards, males will establish territories. Courtship display flights will entice females into harems. Mating takes place in the autumn but fertilisation is delayed until spring. As day-length shortens and temperatures fall, bats enter winter roosts to hibernate. Quiet locations with a fairly constant temperature are ideal. Damp conditions are a bonus as dehydration can be a problem during the long sleep. Caves, large barns and old stone buildings provide just the right conditions.

Bat numbers are declining throughout Europe. The Irish Leisler’s population is particularly important; we have more of them than any country in Europe. Chemicals used to treat timbers in attics may be a factor in their decline. The lesser horse-shoe bat has also suffered and Ireland has a large population of this species. British pipistrelle numbers have been halved in the last 15 years and a similar decline may be occurring here.