Home for the predators of the night

Natural forests are essential for bats. They offer both varied hunting grounds and old trees with numerous living cavities. Even in small natural forest areas, many species can be found.

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Bats are fascinating because they find their way in a world that is almost completely closed to humans – darkness. Their sense of hearing and locating is so fine that they can detect insects only a few millimeters in size during flight. The different bat species differ in the technique they use to hunt prey. The fringed bat, for example, hunts just above the ground for small mosquitoes – gladly near damp areas. The Bechstein’s bat, on the other hand, relies not only on ultrasound but, thanks to its large ears, can hear the crawling sounds of insects on dry leaves – and pick the prey directly from the ground. The long-eared bat, on the other hand, has such high-resolution ultrasonic hearing that it can detect the subtlest of movements, such as the crawling of a beetle. While hovering, they scan the bark of trees or branches for prey and grab the insect as soon as it moves. All in all, bats consume enormous amounts of insects and spiders. In order to become full, a bat must consume a quantity of food in one night, whose weight is about 2.000 mosquitoes.

The most important bat habitat

In Germany, there are more than 20 different species of bats, with some species being widespread throughout the country and others occurring only in certain regions. The different hunting and living habits of the various bats ultimately lead to the fact that most species are found in near-natural forests. Only here they find a colorful mosaic of open and closed areas, and old and young tree stands. Bat experts agree that natural forests in particular are ideal bat habitats. This is because, in addition to the varied hunting grounds, these also offer the tree hollows that are so important for bats as roosts.

Little evening swift

The little evening swift (Nyctalus leisleri) is a typical species of old cave-rich deciduous and mixed deciduous forests. The bat reaches a head-torso length of 48 to 72 millimeters with a tail of 35 to 48 millimeters in length.The lesser evening swift is predominantly nocturnal and feeds mainly on small insects that are active at night and at dusk.

For a long time, little was known about how bats use caves or what kind of caves they prefer. Thanks to the use of bat detectors and small radio transmitters, however, biologists have been able to study bats in the dark intensively for the first time in the past 20 years and to study in more detail what demands they have on tree hollows. Thanks to this research, we now know that a single cave is far from enough for a bat. Because bats change their cave quarters extremely often. This also means that bats are most interested in forests with a large number of hollows in a small area, especially in old-growth forests.

One cave is not enough

There are several reasons why bats change their roosts so frequently. They leave a cave, for example, when parasites get the upper hand. In cold weather, bats prefer sunlit caves; in hot weather, they tend to prefer shady places. Females that live with their newborn young in mother-child colonies, the maternity roosts, change their roosts particularly frequently in spring. For example, they prefer different caves when giving birth than during the nursing period. Here, too, warmth plays a major role in their choice of cave. For the Bechstein’s bat, females have been found to change roosts on average every 2.7 days in spring and summer. Males are somewhat more constant, but also look for a new dwelling after three weeks at the latest. Old tree stands with many caves are therefore a guarantee that bats will become native in large numbers.

Greater mouse-eared bat, bat at night

The greater mouse-eared bat (myotis myotis) is, with a wingspan between 35 and 43 centimeters, the largest European Myotis-Species. It flies around the forest edge or clearings between trees, listening to the rustling sounds of prey running on the ground.

Even small areas make it big

Surprisingly, the different bat species prefer different types of caves. While Beichstein’s bats inhabit up to 95 percent of woodpecker cavities, the pug bat spends its days in cavities behind the flaking bark of oaks, ashes, elms or even pines. The Greater Bearded Bat, on the other hand, prefers crevices along the trunks. In short, the more caves there are, the more species-rich the bat forest is. In this respect, even relatively small-scale natural forests can have a high attraction for bats. In the natural forest reserve "Kinzigaue" near Hanau, biologists were able to detect twelve bat species in an area of only 0.2 square kilometers during the summer months. Experts therefore classify such a concentration of species on such a small area as in the "Kinzigaue" as "outstanding nationwide".

There are several other reasons why natural forests are so attractive to bats. A decisive factor is the unique microclimate of the near-natural forest sites. On the one hand, clear areas allow a lot of solar heat to penetrate the forest, which is especially important during the hibernation period, when the naked young are growing up. On the other hand, the canopy of the tall trees protects the forests from excessive cooling at night. Many bat species avoid cool areas and prefer areas that remain comparatively warm. In this respect, the stable warm microclimate promotes the settlement of bats.

Reliable home ranges

Many small mammals, such as mice and hamsters, live only a few years. In this respect, bats as small mammals reach an almost old age of up to 20 years. Like other species that grow older, bats are especially site-faithful. Although many species migrate from their summer roosts in the fall to seek frost-free winter roosts such as rock cavities, they return to the old forest site in the spring. Old forest sites therefore offer advantages to bats due to their constancy. The bats do not have to spend time searching for new roosts, which is time-consuming and energy-consuming. A "high forest constancy" thus also affects the reproduction and long-term development of a bat population. In addition, in near-natural, richly structured forests, hunting grounds and roosting trees are usually located in the immediate vicinity. In less natural, fragmented areas, on the other hand, bats often have to travel longer distances of up to several kilometers between home roosts and hunting grounds.

Energy-sapping long-distance flights

This is problematic because bats have a particularly energy-consuming hunting behavior. Pregnant or lactating females have to consume about two thirds of their own body weight as food in one night. Long flights between quarters and hunting grounds can weaken the animals. In this respect, too, less fragmented natural forests offer ideal living conditions. Especially old deciduous forests with numerous caves offer perfect bat habitats. If one wants to protect bats, it is therefore essential to preserve cave trees.

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