Isotope analysis provides information about bat migration corridors

Nathusius's pipistrelle bat. Photo: Christian Voigt
Nathusius's pipistrelle bat. Photo: Christian Voigt

Show me what you ate and I’ll tell you where you come from. This, to put it simply, is the approach of the latest bat research study conducted by the Leibniz Institute for Zoo and Wildlife Research (IZW) in Berlin, Germany. Based on isotope analyses of several kinds of tissue, scientists headed by Christian Voigt successfully defined the preferred habitats of three bat species. For the first time ever, these results shed light on which habitats bats prefer during their annual migrations.

The present study investigates the killing of large numbers of migratory bats at wind turbines, which account for an annual death rate of about 250,000 bats. In detail, the researchers were interested in the specific habitats bats use for migration in order to establish which areas have to be avoided for wind energy development in Germany. Seventy percent of bats killed at wind turbines represent migrating species that pass through Germany each spring and autumn when they move seasonally from the Northeastern to the Southwestern of Europe, where they try to find adequate hibernation places in climatically more favourable areas. “Bird migration routes have been studied intensively, yet barely anything is known about bat migration”, behavioural ecologist Christian Voigt explains. “We wondered if there are areas where wind turbines should rather be avoided because of existing bat migration corridors”.

The IZW researchers analysed the carcasses of three bat species that had been killed on their migration southwards on account of wind turbines: the non-migrating common pipistrelle bats (Pipistrellis pipistrellus), the common noctule bats (Nyctalus noctula) - a "medium-distance flyer" that covers a few hundred kilometres - and the Nathusius's pipistrelle bats (Pipistrellus nathusii), which covers up to 4,000 kilometres per year. The scientists examined the bats’ fur, flying membranes, muscles, liver and blood, and determined the ratio of the stable isotopes of the elements carbon (12C/12C) and nitrogen (15N/14N).

The isotopic composition of a local food web features a structure that is characteristic of the respective habitat. It virtually is its fingerprint, influenced by soil, fertilisation and climate. This signature is passed on through the plant-insect-bat food chain. "The analyses showed us which habitats the bats used most during their migrations, before dying at wind turbines", Christian Voigt says. The question how quickly the isotopic structure manifests within the bat tissue depends to a large extent on the type of tissue investigated. Blood and liver cells quickly regenerate from the components the animal consumed during the past two to three weeks. The fur, in contrast, is already formed in the summer areas before migration and provides information about the animal’s hunting behaviour in its original habitat. Tissuses of muscles and wing membranes are somewhere in between. "This enables us to examine various time slots in retrospect and trace back the habitats in which the bats would prey - from their summer territory all the way to the time when they were killed", Voigt explains.

 The results show that the isotopic values of Nathusius' pipistrelles are fundamentally different from that of the other two bat species. While the common pipistrelle and the common noctule bat feature patterns typical for terrestrial areas, Nathusius' pipistrelles primarily preys on insects that have spent their larval period in aquatic habitats such as ponds, lakes and rivers. "From this we conclude that there are species-specific migration strategies among bats and that waterways are highly valuable corridors for certain species. Wind farm sites nearby waterways or waterbodies are thus unfavourable", Voigt emphasises. He presumes Nathusius' pipistrelles to move “along a string” of nutrient-rich rivers and lakes from the Baltic States to the south of France, following the waterways or stopping over for extended periods. So far it is not clear whether or not common noctules use specific terrestrial systems, such as distinctive landscape structures like mountain chains, for orientation. Studies with bats that have been equipped with GPS devices for their long trip are now about to follow.

While they migrate, bats unfortunately fly almost exactly at the height of rotor blades. US researchers suspect them to even aim for the wind turbines deliberately, mistaking them for trees while they are searching for a roost for the day. Whether or not a potential wind turbine site is located amidst a bat migration corridor can easily be determined beforehand by using acoustical bat detectors, which record the species-specific high-frequency calls of the nocturnal flyers.

Not only in Germany are bats strictly protected animals. In accordance with the Habitats Directive, this also applies to the entire rest of the European Union, as respective populations decreased drastically in the 1960s to 1980s - partly this was due to the insecticides and pesticides used in agriculture. As bats are extremely long-lived animals - some species get up to 40 (!) years old - and (opposite, for instance, to mice that are about of the same size) only give birth to one or two young ones per year, the populations need a long time to recover. Wind turbines now represent a new death trap for bats.

However, species conservation and environmental protection can well be combined if existing wind turbines in bat migration corridors are starting to operate only at higher wind speeds. This is because the bats’ flight activity drastically decreases at wind speed of 8 metres per second or more, which is exactly the range at which the net energy production of wind turbines starts to increase. "Wind energy companies will lose less than one percent of their revenue if they switch the plant off within this range during the nocturnal migration periods", Voigt explains. "It makes no economic sense at all to let wind turbines run at low wind velocities - that is up to 5 metres per second - as they do not produce any energy anyways. However, what they produce is a large number of dead bats". In the US, wind turbine operating companies already switch off the plants voluntarily in such conditions, in order to reduce the number of bat casualties. Implementing this approach in Germany, would mean a significant contribution to protecting bat species.

Bats are the only mammals that can fly actively and they are extremely useful creatures. By preying on insects and eating pests off from plants, they measurably contribute to the worldwide ecosystem. "According to studies, for example the corn industry saves billions of dollars in pesticides every year", Voigt reports. Wherever there are bats, there also is less pest damage in forest trees. That also applies to Germany. "We will be eliminating these so-called ecosystem service providers if we do not manage to implement more efficient bat conservation in these times of the ‘Energiewende’." Apart from this, bats as migrating animals are also protected by an UN convention that Germany as signatory has to follow. "Jurisdiction, reason and international conventions thus require the consideration of species protection interests, if we are willing to promote and support climate protection."



Voigt CC, Lindecke O, Schönborn S, Kramer-Schadt S, Lehmann D (2016): Habitat use of migratory bats killed during autumn at wind turbines. ECOL APPL; DOI: 10.1890/15-0671.



Leibniz Institute for Zoo and Wildlife Research (IZW)
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