The open sea is a natural obstacle on the migratory map of landbirds, which cover hundreds of kilometers without stopping to rest. When the route includes wind farms with their turbines in operation, such as those installed in the southern North Sea, off the west coast of the Netherlands, the situation becomes more complicated. On May 13, more than 200 wind turbines at two locations in that area reduced their rotations to two per minute for four hours to allow the safe passage of thousands of birds. Although this is already done at other onshore wind farms in other parts of the world, this case is novel because it involves offshore wind turbines and, above all, because the migratory passage was predicted two days in advance. This prediction, achieved by combining meteorological data with information from the migration calendar, makes it possible to improve the management of these encounters, both for the animals and for the energy companies, which have to slow down or stop machines without destabilizing the supply. From autumn onwards, the Dutch government plans to apply similar braking more frequently to the rest of the offshore wind farms.
The wind turbines involved in the exercise were located next to the towns of Borssele and Egmond aan Zee. In the first, 173 windmills were involved. In the other, there were 40, say spokesmen for Dutch Climate and Energy Minister Rob Jetten. They add that in the future it will be mandatory to adapt this control model for companies in the offshore industry. "All parties involved have worked well together and we want to minimize the impact of wind farms on nature," said Jetten. On land, wind turbines tend to be smaller and lower in height and are more dispersed. According to the ministry, there are currently no similar official plans for this environment.
In spring and autumn, millions of birds cross the North Sea on some nights. The two-day gap gained with this study allows the companies involved and the grid operator to generate elsewhere the electrical power thus lost. The prediction model was developed by researcher Maja Bradari? and her colleagues at the University of Amsterdam, who recorded the movement of the birds with the help of radars installed in several offshore wind farms. Together with her team, they have calculated the number of birds based on a unit of measurement called the migratory traffic rate. "It represents the number of birds that cross an imaginary strip of land one kilometer long in one hour," he says in a telephone conversation. On the night the turbines were slowed down, the same ministerial sources point out that the birds that passed through the area were the black-tailed godwit, gray plover, black-throated sandpiper and sandpiper.
The Bradari? analysis attempts to establish seasonal patterns, and shows that the greatest migratory flow takes place on a handful of nights in spring and autumn, associated with specific weather conditions. "Like other research, we have observed that they are related to the passage of high-pressure systems, which bring stable, precipitation-free weather with less strong tailwinds. The birds save their strength," he explains. By choosing these nights, they can migrate faster. Sometimes, they fly anyway if bad weather conditions do not improve for a long period. Measurements suggest that to conserve 50% of the birds passing through the turbine zone, "rotations need to be reduced by 18 hours in spring and 26 hours in autumn over several nights". He clarifies that this figure may end up being higher or lower "depending on political decisions" about the percentage of seasonal migration that will be saved, and recalls that the study has focused on the night period, "which is when they migrate the most". In the calibrated North Sea area, most birds pass through in April and October, "and there may be thousands per hour". Those flagged by the Dutch radars came mainly from the UK (in spring), but also came from Africa to cross the Netherlands on their way to breeding grounds in northern and western Europe. "In autumn, they come from Scandinavia, mostly from Denmark and northwest Germany."
The Dutch NGO Vogelbescherming (Association for the Protection of Birds) supports renewable energies such as wind power, but does not believe it should be implemented at the expense of nature. While they appreciate efforts such as the current one, their representatives point out that "these turbines not only pose a shock risk when they are located along important migration routes. They can also end up in the wrong place, "such as nature reserves or meadows, because the birds will avoid such an obstacle at the cost of not being able to breed or rest". According to this organization, there are detection systems in Portugal and Spain that combine radar with human surveillance. "Unfortunately, the radar system cannot be deployed independently, without the support of observers [for diurnal species], because it is not yet good enough." In the Dutch province of Zeeland, video cameras have been installed on some turbines planted near eagle-eagle eagle (similar to eagles) nests. They are in places where they fly often and an accident can occur. "In Norway, on the other hand, a test was done by painting part of a turbine blade black. We would like to know if something like that works [for daytime bird flights], and a group of energy companies will do a similar test later this year, although this time in Eemshaven [northern Netherlands]."
Maja Bradari? admits that it is difficult to assess at this stage the impact on migrations of more offshore wind farms forming a kind of barrier. The response of different species will not be uniform, "and we believe that the number of collisions and the overall mortality of birds will increase". Over time, even the routes themselves could change, "and that could lead to an unsynchronized migration that could affect reproduction: if they take longer routes to their breeding sites, they may not be able to find a mate and have offspring," he says. "However, these are speculations because we don't know what other effects it may have."
In Spain, it is not new for wind turbines to be stopped due to the passage of birds, although it cannot be predicted in advance. "Here we have no experience in offshore wind, but we do have experience in onshore wind," explains Heikki Willstedt, director of Energy Policy and Climate Change at the Spanish Wind Energy Association. He says that given that one of the pioneering provinces in the development of this technology was Cadiz - one of the most important migratory bird crossings between Europe and Africa - almost from the beginning, systems were implemented to monitor people with binoculars to give the order to stop or slow down the turbines when flocks of birds were detected. Over time, such measures have been automated, with cameras and radar, and have become more widespread. And, according to Willstedt, he knows of no environmental declarations for wind farms that do not now include the obligation to incorporate prevention systems to avoid bird strikes. "It is a novelty that it can be foreseen days in advance, because it allows you to better plan the operation of the plant," he says. "If one day you foresee that you are going to produce 100 at a certain time, but just at that time a flock appears that forces you to stop the machines, well, of course, you have a problem," concludes the representative of the Spanish wind power organization.