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Understanding the risk of birds colliding with offshore wind turbines


The potential for birds being killed after colliding with wind turbine blades is often cited as one of the key environmental risks posed by offshore wind farms. This view was reinforced when an unacceptable risk to a Sandwich tern breeding colony was stated as a reason to refuse planning consent for Docking Shoal Offshore Wind Farm.

Collision risk models are used to predict the number of birds likely to collide with turbines during the Impact Assessment for wind farms. The industry standard is the Band collision risk model, developed by Bill Band for SNH and first used for the Islay onshore wind farm, but recently updated to better reflect data collected offshore. The model combines the characteristics of a flying bird, such as size, shape and flight speed, with those of a turbine, like size and rotation speed, to estimate the probability of a bird and a blade occupying the same space at the same time. To estimate collision risk, this probability is multiplied by the number of birds passing through the wind farm, at heights overlapping with the turbine blades, and the proportion likely to take action to avoid collision (referred to as the avoidance rate).

The well documented sensitivity of collision estimates to the avoidance rate means selecting appropriate values is source of continuing debate between stakeholders. Despite this, the evidence base for avoidance rates is relatively weak as collecting the necessary data is extremely challenging. With this in mind, Marine Scotland Science commissioned BTO and UHI ERI to review avoidance behaviour in five key species – gannet, kittiwake, lesser black-backed gull, herring gull and great black-backed gull.

A key difficulty was identifying and defining the spatial scales at which avoidance behaviour occurred. Birds may avoid entering a wind farm completely, avoid passing close to turbines within a wind farm, or pass close to turbines taking last-second action to avoid colliding. These scales are referred to as macro, meso and micro respectively. As evidence suggests birds may be attracted to a wind farm, or even turbine bases, it was felt that it may be more appropriate to refer to a behavioural “response” rather than avoidance when considering the macro and meso scales. Our review found evidence that whilst gannets showed strong avoidance of offshore wind farms, gulls showed no consistent evidence of attraction or avoidance.

Whilst several methodologies have been trialled to collect micro-avoidance data at offshore wind farms, none have collected sufficient data to derive robust avoidance rates. However, using data collected from coastal wind farms in the UK and northern Europe, we were able to estimate within wind farm (meso + micro) avoidance rates for gull species. As may be expected, these were very high, with more than 99% of gulls estimated to take action to avoid collisions.

The review recommends that avoidance rates of 98.9% be used for gannet, 99.2% for kittiwake and 99.5% for lesser black-backed gull, herring gull and great black-backed gull. This review represents a significant step forward in understanding avoidance behaviour in seabirds and has been welcomed by a range of stakeholders.

Author: Dr. Aonghais Cook; Research Ecologist BTO