Cumulative effects of offshore wind on birds

At Waardenburg Ecology, we have been researching the effects of wind farms on wildlife, both onshore and offshore, for more than 30 years. It has long been known that wind farms can have negative effects on birds through collisions, effective habitat loss and barrier effects. Although the direct effects of collisions are obvious, the impacts through other mechanisms may be less clear. Habitat loss occurs when birds are displaced from an area due to disturbance or reduced habitat quality such as through changes in food availability. This can indirectly result in mortality as birds are forced to move to other foraging areas and competition for prey or prey availability may change.

Since 2015, the Wozep programme has sought to assess the ecological effects of all planned offshore wind farms at various times, under the name Ecology and Cumulation Framework (KEC). The fourth version of the KEC was published in 2022, and includes an assessment of bird mortality as a result of collisions. This mortality was then assessed at the population level using population models.

Collision-related mortality was estimated using a collision rate model based on the stochastic Collision Risk Model (sCRM, developed by Scottish Marine and Freswater Science). Bird numbers were based on density maps developed from counts from both ship-based and aerial surveys. Species-specific flight height distributions were created from GPS data, survey data and the literature. Species-specific characteristics (avoidance rate, flight speed, size) and wind turbine proterties (e.g. rotation speed, rotor diameter, maximum width, number of turbines) provide additional information in the estimates of collision rates.

Mortality estimates were then assessed at the population level using population models. We had developed these models as part of another Wozep project for the 18 species identified as most sensitive to the effects of offshore wind farms. The use of these population models allows different effect scenarios to be compared.
Within KEC 4.0, in addition to the scenario without impact of wind farms (null scenario), a number of scenarios with impact were calculated.
At the national scale, the planned roll-out of wind farms up to 2030 has been calculated (base scenario), as have various expansion variants. In addition, the effects on an international scale has been calculated; this scenario consists of the national base scenario plus the international wind farms with a planned realization until 2030.

The effects of offshore wind farms at the population level were compared against species-specific threshold values. Together with Wageningen Marine Research, Waardenburg Ecology had developed a method to assess changes at the population level: Acceptable Levels of Impact (ALI). The Dutch Ministry of Agriculture, Nature and Food Quality (LNV) has defined these thresholds for the designated species.
Within the KEC 4.0 assessment, two species (herring gull and northern gannet) fell below this threshold for both the national and the international scenarios.

We investigated the effect of increased air gap; the distance between the water's surface and the lowest tip of the rotor. An increase in the air gap resulted in fewer birds flying at rotor height. For both herring gull and northern gannet, this resulted in a clear decrease in the estimated numbers of collisions; and while for herring gull this was reflected at the population level, for northern gannet, the population still declined to under the assessment threshold.

Where uncertainty exists, due to too few data or variation, we adopt the principle of the worst-case scenario. This means that the impacts are over-estimated but ensures that the initial assessment remains on the cautious side. For species that crossed the threshold of acceptable levels of impacts, such as herring gull and northern gannet, we investigated further to refine the assessments based on more realistic data.

Density Maps:
For herring gull, we created density maps specifically for the Dutch Continental Shelf. This resulted in fewer birds within the planned wind farm areas and consequently fewer collision at planned Dutch wind farms.
For northern gannet, density maps were adapted from those previously published (Waggit et al. 2020). This again resulted in fewer birds and hence collisions in Dutch offshore waters.

Avoidance rates for northern gannet:
The avoidance rate is a key factor in collision rate calculations. To reflect potential seasonal and age-related differences in behaviour we used a different avoidance rate for breeding birds and non-breeding birds. This distinction was made on time of year and distance to colony. This resulted in the impacted population remaining above the threshold level.

The energy transition can be seen as a positive development in the fight against climate change. Yet it is also important to limit the effects on birds as much as possible. This is possile through site-selection and mitigation measures. There is, however, still much to learn over the impacts of offshore wind on birds and it is only through continued research and monitoring that we can improve the understanding to benefit both wildlife and the development of renewable energy.