Some bat species are among the world champions of seasonal migration in the animal kingdom. Leisler’s bat, for example, which weighs between 12 and 22 grams, flies from central or Eastern Europe to the Mediterranean every late summer – and back in spring. Until now, it has only been possible to trace these approximately 1,500-kilometre routes on the basis of sporadic findings of ringed individuals. A scientific team from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) has now equipped Leisler’s bats with tiny transmitters as part of a larger investigation and reconstructed in detail a recent flight of a female from Brandenburg in north-eastern Germany to Italy – the route to her wintering grounds led via Munich, the Alps and Milan to Parma.

The seasonal migration of some bat species is a fascinating behaviour in many respects. Measured by weight and body size, for example, the migration of the lesser noctule or Leisler's bat (Nyctalus leisleri) from central and Eastern Europe to warmer regions in Italy or France is one of the most astonishing energetic feats in the animal kingdom. Which exact route the animals take, where they stop along the way and how long they need for each section of their journey was previously unknown. “Previously, we could only record that bats which were tagged with a ring near Berlin in the German federal state of Brandenburg, for example, were found once in a while in a day roost by colleagues in Italy or France”, explains Prof Christian Voigt, bat specialist and head of the Department of Evolutionary Ecology at the Leibniz-IZW. “In the current project, we have equipped Leisler’s bats with transmitters that only emit a few high-frequency signals each night, inaudible to bats and humans, but detectable by mobile phone towers. This strict limitation of signalling made it possible to minimise the weight of the transmitters and thus the extra load for the animals. The location of the cell phone towers that pick up a 'beep' allows us to deduce the position of the bat and track its flight routes.”

In this way, Voigt and his team reconstructed the migration routes of several lesser noctules in August 2024, including that of a 17-gram female that moved from southern Brandenburg to the vicinity of Parma in Italy. Within just five days, it flew via the German federal state of Saxony, the Czech Republic and the Bavarian Forest via Munich and Garmisch-Partenkirchen into the Alps, was then located in the vicinity of Innsbruck in Austria before it crossed the main ridges of the Alps and then was registered at Lake Como in Italy. From there, it reached Milan and Parma in the Po Valley a short time later, where it stayed since then. It is the first individual of a larger set of tagged Leisler’s bats in the project – additional individuals followed afterwards in the same direction as this female. “The female Leisler’s bat flew south with astonishing speed and precision. In Italy it will presumably mate with males that live there permanently and later in winter the female will reduce its body temperature and spend the chilly months in quiet torpor”, reports Dr Carolin Scholz, scientist at the Leibniz-IZW. In the Apennines and the Po Valley, there are populations of resident male lesser noctules that compete for matings with incoming females. In spring, the females fly back to give birth and raise their young in the forests of central or Eastern Europe.

The Leisler’s bat is a medium-sized, forest-dwelling bat dependent on natural deciduous and mixed forests with a large range of tree hollows, crevices and bark roosts. They prefer to build their nursery roosts in these hollows, which may accommodate around 20 to 50 females. Lesser noctules have narrow and elongated wings that are suitable for fast long-distance flights. Also, outside seasonal migration, the lesser noctule is considered a particularly fast and agile flyer. With flight speeds sometimes exceeding 40 km/h, it hunts above tree canopies for moths, dipterans such as gnats and mosquitoes as well as net flies and caddisflies.

“Owing to their dependence on natural forests for nursing and foraging as well as their seasonal migration, lesser noctules are particularly affected by the intensification of forestry and also by wind energy production in forests”, explains Voigt. “’Clean’ monoculture plantations offer neither enough food nor shelter for these bats. Additionally, wind energy production in forest areas may affect their habitat in many ways, for example through additional habitat loss and the risk of colliding with rotor blades in their foraging grounds above the canopy.” In addition to the challenges of habitat loss or alterations, migratory species such as Leisler’s bats are often at risk from colliding with wind turbines located along their migration corridors, notes Prof Danilo Russo from the University of Naples Federico II, Italy. He adds: “We need more precise knowledge of the routes used by lesser noctules during their migration to protect this species. Their seasonal movements, which can span great distances, highlight the importance of international cooperation in conservation efforts. The thoughtful placement of wind turbines is crucial for protecting bats, nationally and internationally.” In the future, the scientists hope to analyse many data from tagged lesser noctule bats to gain insights into the precise long-distance flight routes of bats.