The silent night is full of sound

An hour after sunset, and the expedition can begin. The bats are out on the hunt.

The clock ticks on towards eleven and it’s a clear, nearly windless June night. Woodcocks flit over the treetops. A falcon swoops by, flying low. Mosquitoes and midges feast on our blood.

“This looks promising,” Martin says.

We’re out after bats. After several rainy nights they should be hungry; they can consume a third of their body weight in just one night.

Martin Brüsin is a master’s student in the Ecology and Environment programme at Linköping University (LiU). We follow the twists and turns of the paths in the Lake Hövern region outside Åtvidaberg, one of the 50 locations he’s chosen for his project - a piece of the puzzle in LiU ecologists’ research into how the layout of the landscape on a large scale influences biodiversity.

He has just two months: June and July, the period when bats rear their young and remain within a certain area. Brüsin explains:

“I monitor two locations a night, so we’ll see how much I manage.”

This June has been unusually wet, which has made a mess of the schedule. The optimal night for bats is overcast and warm, dry, and windless.

He’s equipped with an ultrasound detector that will translate the bats’ high-frequency signals into audible sound. The detector can also slow time down, which makes it easier to differentiate between the various species The detector is connected to an mp3 player to enable recording. In doubtful cases, the files can be entered into a computer program that can subsequently identify the calls of every species.

An hour after sunset, Martin begins the night’s first round of stocktaking. It only takes a few minutes before the first signal hits the detector. It’s a soprano pipistrelle, Sweden’s smallest species, flying to and fro only a few metres away. A little later Sweden’s largest species, the common noctula, flies over at treetop height. The frequency of its call is unusually high - 60 kHz. This is one of the two species whose calls a person with good hearing can detect.

We change into our boots and trudge out into the bog, around a small pond surrounded by dense marsh and deciduous trees. Even the mallards are quiet. The darkness falls, and the air is somewhat biting. A pair of Daubenton’s bats zooms low over the water’s surface, on the hunt for dragonflies and midges.

Suddenly something starts go awry with the detector. The bats’ calls are drowned in interference. However once the batteries are changed, everything works again. The night’s results are fifteen recorded animals from four species; apart from the two named above there is the Northern bat, which is the most common of the 19 species known in Sweden.

With barely half the season gone, Brüsin has confirmed the common noctula, the Northern bat, the soprano pipistrelle, the brown long-eared bat; which, with its four-centimetre-long ears can listen for prey instead of using echolocation, the Daubenton’s bat, and the serotine bat, a species previously only noted in southern Sweden.

“The best location so far was by a hydroelectric plant in Finspång, an open area with standing water, old buildings, and large hollow deciduous trees,” Brüsin says.

Brüsin’s supervisor, Karl-Olof Bergman (pictured here imitating a brown long-eared bat) is one of the project leaders in a research group on conservation ecology at LiU. The research focuses on studying the significance of the entire quality and layout of the landscape, rather than individual objects.

“In one completed study, we studied beetles that depend on large hollow oaks. Using statistical methods, we showed that the beetles reacted to the quality of the landscape within a radius of 4.5 kilometres from the study area. This means that we have to work with relatively large areas so that they’re able to continue to dwell in a region over the long term. Yet with bats, it’s a little more complicated. They look for environments rich in insect life, with a combination of old deciduous trees, water, and open land; additionally, they need hibernation spots in caves or buildings.

The group’s research spans from tigers in the jungles of India to beetles and butterflies in the oak forests of the Östergötland region in East Sweden. The studies are directly applicable in practical nature conservation; it’s not possible to preserve diversity only through establishing nature reserves. Bergman clarifies:

“The reserves are entirely too small and sparse. Ordinary landscapes must also be seen to.”

19 species in Sweden

Bats are mammals of the order Chiroptera. In Sweden, 19 species have been observed: Barbastell (or broad-eared bat), Bechstein’s bat, Brandt’s bat, pond bat (very rare), soprano pipistrelle (smallest, weighs 3-5 g), Natterer’s bat, Particoloured bat (audible), Leisler’s bat, brown long-eared bat, grey long-eared bat, whiskered bat, Northern bat (most common), the ‘nymph’ bat (Myotis alcathoe; only heard, not yet spotted), pipistrelle, common noctula (largest, up to 30 g, audible), greater mouse-eared bat, serotine bat, Nathusius’ pipistrelle, and Daubenton’s bat.

Mating takes place in the autumn. In October and November, bats gather in large groups for hibernation in dark, moist spaces with a temperature of between 0-5 degrees C such as caves, mines, earth cellars, attics, and church steeples. They remain there until April or May.

Fertilisation itself takes place in the spring (known as delayed implantation). The pregnant females move in together as part of a nursing colony of up to several hundred, or even a thousand, individuals, for example in the southern wall of a house or a hollow tree. The young are normally born in late June or early July, one or two per female. At the beginning of August the young are ready to fly, and they learn how to hunt.

They use echolocation for orientation and hunting. Sonar sends out ultrasound with frequencies of 15 kHz and higher. The higher the frequency the finer the sensitivity, but the shorter the range. By registering echoes from sound impulses, the bats create an aural picture of their surroundings. The animals also have social calls.

Their hearing is good, even in the low-frequency range. The brown long-eared bat has exceptionally good hearing, which it can use on the hunt instead of echolocation.

Text: Åke Hjelm
Photo: Vibeke Mathiesen, Jens Rydell/N (the picture of Daubenton's Bat)
26 June 2012

Related Links

• Ecology and Environment Master’s Programme at LiU  
• Karl-Olof Bergman homepage