Bird migration predictive modelling and threshold fot large-scale bird migration
A lot of knowledge of bird migration patterns is needed to come up with efficient measures. Each spring and autumn, millions of birds migrate over the North Sea. When do birds start their journey over the sea, precisely; which route do they take; how high do they fly? Knowledge of that type forms the basis for a bird migration predictive model. There must also be a clear procedure to follow: when are measures of this kind triggered, which parties are involved, how long does a particular measure last, how are those involved notified? All of these are questions that have to be formulated clearly. Indeed, the Start/Stop project has many facets.
Bird migration predictive model
The bird migration predictive model predicts which bird migration can be expected at a height of between 25 and 300 metres above sea level on the North Sea and whether the migration will exceed a specific level (the 'threshold'). The model uses data that has been collected by special offshore bird radars before being subjected to various analyses. In addition, the predictive model also uses real-time weather forecast data.
The model is used to predict nights on which large-scale bird migration around offshore wind farms will take place. The periods in which the Minister of Climate Policy and Green Growth can announce a decision to stop the offshore wind turbines have been set for 15 February - 15May and 20 September - 30 November respectively.
It might seem more logical to prevent bird collisions by shutting down the turbines in real time: i.e. powering down wind farms as soon as the radar detects bird migration. However, if that were to happen at various wind farms at the same time, this would create a situation of instability in the energy grid. That must be prevented. So the option chosen in this project is a forecast model, in which a window of 48 hours is used to predict a potential migration. The owners of the wind farm thus have 48 hours' notice of the fact, at which point they can pass on their energy production to transmission system operator TenneT. That 48-hour window is the amount of time that TenneT needs to guarantee stability of the energy supply.
Bird experts
In addition to the predictive model, the Start/Stop project uses a group of bird experts who make daily predictions during the bird migration period. The bird experts predict the likelihood of large-scale bird migration per part of the day. In other words, the bird experts use their predictions to validate the outcome of the prediction model.
Threshold value for number of birds per kilometre/hour
The starting point for the prediction model is a flexible threshold value that indicates when there is mass bird migration. The threshold value for spring 2026 has been set at 115 birds/km/hour. This means that when the bird migration prediction model estimates that this value will be exceeded, the Start/Stop procedure will be initiated. If the bird experts and TenneT also give a positive recommendation, the Minister of Climate and Green Growth can decide to shut down (= less than 2 revolutions per minute) the offshore wind turbines.
In order to strike a balance between bird protection and sustainable energy production, a maximum number of hours for which a shutdown can be declared is determined.
The number of hours of shutdown is set in the current site decisions at a maximum of 60 hours per year, starting from the autumn migration (Government Gazette 2026, 2036). During the autumn migration period of 2025, a shutdown of 36 hours has been announced. For the spring of 2026, the maximum number of hours for which a decision to shut down can be taken is therefore 24 hours.