Tuesday is bat day for biologist Sander Lagerveld of Wageningen Marine Research and his bat team. In the late summer, a peak moment for bat migration, they go out every Tuesday to tag bats with transmitters. A tiny gadget is glued to the little animals’ backs, enabling researchers to track their movements precisely for a few weeks (see inset).
The venue this Tuesday is the grounds of the Noorderhaven care institution in Julianadorp, at the northern tip of North Holland province. Other members of Lagerveld’s team are Anne-Jifke Haarsma of Batweter consultancy and volunteer Jan Boshamer. His colleague Bart Noort hasn’t joined them today.
We are not so lucky with the weather. It is pouring with rain and the only umbrella we’ve taken along has to keep the equipment dry. In no time, everyone is drenched. All part of the deal with fieldwork.
In the car on the way to Julianadorp, Lagerveld has explained the aim of the bat project. ‘It is part of WOZEP, which stands for the Wind Turbines At Sea Ecological Programme. In the past, it was the responsibility of the turbine operator to do research on the effects of offshore wind energy. Since 2016, the government has taken over that research, to answer ecological questions. Another change is that the research doesn’t just look at birds and marine life, but places a big emphasis on bats as well. We particularly want to find out what impact offshore turbines have on migrating bats. But local, non-migrating bats sometimes go out to sea too.’
The focus of the bat study lies on Nathusius’s pipistrelle (Pipistrellus nathusii), a miniscule creature that is barely five centimetres in length when folded, a pitifully vulnerable sight. The bat has thick fur and a wing span of over 20 centimetres. An adult specimen weighs an average of eight grams. But tiny as it is, the performance of this ‘microbat’ is impressive. ‘It is a long-distance migrator,’ says Lagerveld. ‘It travels 2000 kilometres from its breeding places in Northeast Europe and the Baltic states to its overwintering spots in West and Southern Europe.’ Actually, the females do this more than the males. ‘The females live in the east in breeding colonies, have one or two babies in the summer and migrate with their young in the autumn to West and Southern Europe.’ So the young males only make the long journey once. ‘Yes, the females do most of the work,’ laughs Lagerveld. ‘They are the really tough ones.’
The pipistrelle bats’ migration only takes a couple of weeks. The main migration route follows the coast of the Baltic and North Sea. At Den Helder, that route abruptly veers south. ‘But some of the bats go straight on, over the North Sea in the direction of England,’ says Lagerveld. ‘What proportion of them do that, we don’t know. This project aims to find that out, amongst other things. And they sometimes cross further south too.’ That crossing can be dangerous. Bats collide with rotating wind turbines or die of ‘barotrauma’: the impact of the wave of pressure set off by the turbine blades.
On their way south, the bats pave the way for a new generation. ‘When the females come by, the males start courting,’ continues Lagerveld. Nathusius’s pipistrelle is not monogamous. ‘Popular males sometimes have up to 10 females hanging in their nesting boxes. But I don’t think we shall see very many today. There’s been too strong a westerly wind for the past few days. Nathasius’s pipistrelles prefer to migrate when there is not much wind, or with a light wind in their backs and at fairly high temperatures. We’ll mainly see lone males hanging in their boxes.’
Popular males sometimes have up to 10 females hanging in their boxes
The bat man of North Holland
To do the tagging, Lagerman makes grateful use of the many bat nesting boxes installed around the tip of North Holland. This is the epicentre of bat research in the country, and volunteer Jan Boshamer is the lynchpin. This former teacher is the bat man of North Holland. The popular flat box in which bats can spend the night even bears his name: the Boshamer box. ‘But I didn’t design it. That box has been around for ages. I just simplified it so that it can be opened and is easier to clean.’
Boshamer is a volunteer with North Holland Landscape and nature management organization Staatsbosbeheer. He put up the first box in 1987. ‘A duck run had to be dismantled and it turned out there were lots of bats in it. What to do? We put up boxes.’ And then one thing led to another. His collection now runs to about 250 boxes, which he visits on a very regular basis, as he has been doing for over 30 years. ‘And I want to make it to 50,’ he says. ‘Bats are unbelievably fascinating animals. Back when I started, hardly anyone was working on bats. A whole new world opened up for me.’
Today’s round of checking up on bat boxes in the woods at Noorderhaven only reveals six Nathusius’s pipistrelles, which are weighed, measured and sexed. They are then fitted with a tiny transmitter weighing 0.3 grams, stuck to their backs with surgical glue. Attached to the transmitter is a wafer-thin, roughly 10-centimetre long antenna.
Boshamer and Haarsma do the practical work. ‘They are certified bat catchers,’ explains Lagerveld. ‘That entails quite an intensive training course. Bats are fragile little creatures and this is an animal experiment. Those transmitters bother them.’ But not for long: the transmitters stay on for a couple of weeks at the most and then fall off. Lagerveld comes across one now and then. It happens today: a transmitter is stuck to the flat plank at the entrance to box 21. Lagerveld has already received the signal on his mobile phone. Using a unique code he can trace exactly which bat had it on its back. The transmitter falling off is unfortunate for the study, but finding it is a lucky break, as they cost 150 euros apiece.
‘This is an expensive project,’ agrees Lagerveld. He wants to tag a total of 500 bats, which brings the cost of the transmitters to 75,000 euros. But the biggest expense is the 35 receivers, which cost up to 9000 euros apiece, and the cost of installing them comes on top of that. ‘This study is unique. Nowhere else in the world is bat migration being studied on this scale.’
Turbines at a standstill
Lagerveld expects to present the results of the research next year. Whatever comes out of it, it certainly won’t lead to fewer wind turbines or to changing the planned location of new ones. ‘Those locations have already been decided on,’ says Lagerveld. ‘That is a gigantic puzzle, what with all the other users of the North Sea. But this study will make it possible to predict the conditions under which migration takes place. On those days you could bring the turbines to a temporary standstill.’
There is no doubt that bats fall victim to the wind turbines. ‘The guestimate for the number of victims on land is between five and ten migrating bats per turbine per year,’ says Lagerveld. How many victims are claimed each year by an offshore turbine is not known, and this study should clarify that. ‘The hypothesis is that if the Netherlands realizes its ambitions for offshore wind energy, even one victim per turbine will have a negative impact on the population of Nathusius’s pipistrelles,’ says Lagerveld. Which underlines the importance of this wet day of fieldwork.
Even one victim per offshore turbine per year has a negative impact on the population
Mini-transmitter signals Morse code
For mapping the movements of bats, researcher Sander Lagerveld of Wageningen Marine Research uses a telemetry system called Motus (Latin for movement). This system is used all around the world to detect the flight movements of small birds, bats and dragonflies – animals that are too small to lug GPS transmitters around with them. They are fitted with a mini-transmitter that sends out a Morse code-like radio signal of short pulses every five to eight seconds. Each transmitter has a unique code. The signals reach receivers placed on tall buildings, radio masts or lampposts. Lagerveld has 35 receivers along the North Sea coast, four of them across the sea on the English coast. Each receiver has four to six antennae installed at fixed angles. Lagerveld: ‘By getting locations from two stations you can locate the bat at any given moment, and from that data you can deduce its flight path.’
The showpiece of the collection of receivers is the one on the Grote Kaap lighthouse at Julianadorp, where six antennae monitor the surroundings.
The number of wind turbines on the North Sea is growing fast. According to an overview on Wikipedia, there are currently 3240 turbines, which generate nearly 15 gigawatts (GW). With its 289 turbines, the Netherlands only accounts for a small proportion, but the number of turbines is set to grow considerably in the coming decades. Offshore wind turbines play a key role in the transition to sustainable energy. The government has decided that offshore turbines must generate 4.3 GW by 2023 and 11 GW by 2030. Other countries around the North Sea have big plans for expansion too.