After building up fat reserves during Autumn, bats will find a roost in which to hibernate, often in small groups. Most British bat species will choose to roost in tree cavities or woodpecker holes but also many will roost in manmade structures like barns, churches and even houses. They will enter a state known as torpor in which their breathing, heart rate, metabolism and body temperature are greatly reduced to conserve energy. They will remain like this until the weather begins to warm in early March when they will slowly begin to venture out. By April, on most nights, bats will be out taking advantage of the warmer evenings and feeding, so it is a great time to see them, writes our Conservation Officer, Will Scott-Mends.
International Dark Sky Week is also in April from 22nd to 30th. It is a celebration of the night sky organised by the International Dark-Sky Association (IDA) during which people are encouraged to turn off outside lights and limit light usage at night in order to combat light pollution. It might not sound like much, but light pollution can have a major effect on nocturnal wildlife, in particular bats. Studies have shown that excess artificial light at night can have a serious impact on bat behaviour. Artificial lighting near a bat roost can disrupt or even prevent emergence from roosts, causing the bats to miss their key feeding window just after dusk when most insects are out. Slower flying species will avoid certain foraging areas or pathways if they are well lit which can reduce their feeding potential and faster flying species will favour feeding under streetlamps (which attract large numbers of insects) making them more visible and therefore more vulnerable to predation.
As part of Dark Skies Week, Friends of Barnes Common will be running our first batwalk of 2022 on 28th April around the Leg o’ Mutton Reservoir which is an excellent site for bats. Last year’s walks were very popular with many interesting sightings including noctules, pipistrelles and even a Tawny Owl. Tickets are available on Eventbrite but there are limited places so book early to avoid disappointment. There will be further walks each month until late Autumn and details of these will be online nearer the dates.
Rhinolophus ferrumequinum – Greater Horseshoe bat, photo by Nils Bouillard on Unsplash