The numbers of birds that collide with wind turbines offshore is one of the major unknowns concerning the ecological effects of offshore wind energy. This number is very much dependent on the avoidance behaviour of birds around the wind farm and around the individual turbines. Birds that strongly avoid wind farms will have a far lower risk of colliding with the turbines than birds that are indifferent to the wind farms or that are even attracted to them. On the other hand, birds that show strong avoidance of wind farms will have a higher risk of suffering barrier effects and displacement effects, which can result in habitat loss and potentially a lowered carrying capacity for local populations.
Because bird collisions with offshore turbines cannot be measured directly yet, impact assessments are currently based on collision risk models (CRM's) such as the SOSS Band model (Band 2012). These CRM's are heavily reliant on avoidance rates, and therefore it is crucial to have accurate figures for avoidance rates of the different individual bird species flying at offshore wind farm sites. CRM's do not take into account the effect of the wind farm configuration, such as spacing between the turbines, orientation in relation to the coast and the size of the wind farm, mainly because it is unknown how wind farm configuration affects avoidance behaviour. The number of collision victims among birds as well as potential barrier effects can possibly be reduced by accounting for the local species composition and the main flight paths of these birds in the planning phase, and by adjusting the configuration of the wind farm to this.
Since the first offshore wind farms became operational, several studies have been carried out internationally to measure their effects on birds. Some of these have focussed on flight paths of individual bird species, while others aimed to determine displacement effects on local populations. Combined, these studies may provide insight in general behaviour patterns of individual bird species around offshore wind farms. In addition, because wind farms vary in size and configuration, the results we have obtained thus far may shed some light on how these factors affect avoidance behaviour.
In this overview we distinguish two types of avoidance, being micro- and macro-avoidance. Macro-avoidance is avoidance of the entire wind farm, micro-avoidance is when a bird does fly into the wind farm, but avoids flying into the rotor-swept zone of the individual turbines.
This report was presented at a symposium at Naturalis in Leiden on September 8, 2015. A complete report containing brief abstracts of all studies presented at the symposium can be found here.