Building for Wildlife: Living with Bats

by: Matt Sauders

One of my most memorable field experiences occurred in a swamp just after dusk. The night was hot and humid, without even a whisper of wind. I remember wishing for a breeze to scatter the mosquitoes that swarmed about me. The only protection was to tie my hood snugly around my face. My research on bats had brought me here, and as I tuned in my bat detecting equipment - capable of picking up the ultrasonic cries of bats - the speaker came alive with feeding calls. Against the fading light I watched as a lone bat hovered around me- feeding on the insects that tormented me.

Bats make many people uncomfortable. In fact, the inquisitive nature of bats is often misinterpreted as aggression. Although bats appear to fly recklessly, their unpredictable turns and kamikaze dives are essential for catching flying insects. So, even though it may seem like bats are swooping at you, they're only trying to outmanoeuvre their prey. Bats also investigate many sounds. They can be attracted by fluttering your fingers together or by scraping your fingernails on a nylon jacket. Soon their curiosity wears off, and they will resume hunting.

To see bats in action, stand under a streetlight and watch them feed on insects that swarm around the light. Or, visit a pond on a moonlit night and watch for fleeting silhouettes of bats near the water. These glimpses may satisfy the casual observer, but others may find themselves living under the same roof with bats.

Our most common species, the little brown bat, is also the most readily adapted to living with people. In some cases, one or two bats may roost under shingles or clapboard siding. These stragglers- usually males or young bats - often choose a new roost site each day. But some buildings may house hundreds of bats during summer. these colonies usually consist of pregnant females that need a warm place to raise their young. Attics and unoccupied buildings are chosen because the warm temperature and crowded conditions allow baby bats to grow quickly. Rapid development is critical to ensure that the young bats have sufficient time to gain weight before winter and to learn how to feed.

A few bats in your home may go undetected. A colony can be difficult to overlook. Young bats are noisy - calling for their mothers to nurse them - and as bat droppings accumulate, the colony can become smelly. Despite these inconveniences, bats do not chew insulation, wood or wiring. Having the bats can also be helpful. Consider the insect control that they provide. Bats are the most important predator of night flying insects. each bat eats up to half of its weight in insects every night - the equivalent of 1,000 to 3,000 mosquitoes. No man-made insect deterrent can compare.

If bats have chosen to live in your house in the past, they will probably return each year. A number of different methods have been used to discourage this. Non-destructive methods such as placing lights in the attic, or using mothballs and other noxious fumes, are usually ineffective. The bats will roost in shadows cast by lights, or will return if the lights go out. Noxious fumes can be annoying to people and pets - and the bats will be back as soon as the fumes dissipate. Rodent repellers that emit ultrasonic sounds may actually attract bats. Sticky traps will catch a few bats, but most will learn to avoid them.

Poisons have also been used to try to destroy bat colonies. Often they cause more problems than they solve. Dazed and weakened bats can be picked up by children or pets; dead bats may rot in corners and add to the existing odour; and poisons can pose direct health hazards to the occupants of the house. The only sure way to keep bats out of your house is to bat-proof it.

In old houses bat proofing may be difficult because little brown bats can crawl through a hole the size of a quarter. All entrances and exits that bats use should be blocked. One can locate these openings by watching for places where they emerge at dusk, or by looking for droppings on the sides of the house. Bat droppings look like mouse droppings, but can be found stuck to walls where mice cannot reach.

Because bats do not leave their roost every night, place a chute - such as a plastic bread bag with the bottom removed -over the exit for several nights. This device allows bats to leave but prevents re-entry. Once all of the bats are gone, the entrances can be permanently sealed with foam rubber plugs or caulking.

Expect the bats to persist in their attempts to re-enter the house. They will look for an alternate entrance. If they succeed, the eviction process may have to be repeated.

Timing of roost-sealing is critical. The work should be completed before the end of June when the young are born, or after the end of July when the young can fly. If roost entrances are sealed in July, young bats will be locked inside and will die from starvation. Before blocking any entrances, check the attic to see whether young bats are present. Newborn bats are furless and smaller than adults. Furred juveniles look like adult bats but will not fly when disturbed.

Relocated bats will search for new roosts, often staying nearby and taking up residence in a nearby building. Your neighbours may not welcome this, so appease them and the bats by placing a bat house near where the animals once entered your house. Bat houses are simple to construct (see diagram); you can also purchase them through bat conservation organizations. Place the bat house high above the ground on a wall or tree that is protected from wind and direct sunlight. To give the bats a strong grip, construct the house from rough wood or etch the wood with a saw. Paint, stain or wood preservatives will deter the bats, so select wood that weathers well.

The importance of providing an alternate roosting site is obvious when you consider that Nova Scotia's bats have only four or five months to feed, raise young and prepare for winter. Disturbances that disrupt their cycle mean that valuable time is wasted searching for new roosts - time that would normally be spent foraging or nourishing young. Careful placement of a bat house can keep you out of trouble with your neighbours, afford an unusual conversation piece and give you the insect control that you need. most importantly, it will help the bats.

Click here to see a diagram of a small bat house.