Shallow productive near shore waters are important habitats for resident and migratory seabirds. These areas are also ideal locations for offshore wind farms, which in order to meet growing renewable energy demands, are increasing in size and number every day (http://en.wikipedia.org). Consequently, interaction between seabirds and wind farms is high. Potential effects include injury or mortality from collisions, avoidance, interruption to migration or feeding flights, and habitat modification.
Impacts are site specific, so each wind farm is assessed separately. For example, those located on feeding or migration routes are likely to have greater impacts than those placed in less inhabited areas. Similarly, those positioned close to breeding colonies have potential to cause high levels of disturbance.
Risk is also species specific. In order to assess sensitivity of seabirds to wind farm disturbance Garthe and Hüppop (2004) devised a Wind farm Sensitivity Index (WSI). Nine factors deemed to influence seabird sensitivity to wind farms were assessed. These included manoeuvrability, flight altitude, percentage of time flying, nocturnal flight activity, sensitivity to ship and helicopter disturbance, flexibility in habitat use, population size, survival rate, and conservation status. The WSI varied considerably between 26 species studied. Black throated diver (Gavia arctica) had a WSI of 44.0 and was considered most vulnerable, whereas Northern fulmar (Fulmaris glacialis) was least at risk with a WSI of 5.8. When applied to the Exclusive Economic Zone (EEZ) and national waters of Germany, values were higher in coastal areas, and varied on a seasonal scale. Consequently, seasons must be considered separately (Garthe & Hüppop 2004).
To understand how wind farms affect seabirds, information on distribution, abundance, and population status of species is required. Studies that assess populations before and after construction are ideal because they can provide insight into how habitat use has changed. For information on how seabird data are collected see www.seabirdsurveys.co.uk
SEABIRD COLLISIONS WITH WIND TURBINE STRUCTURES
Wind farm related mortality or injury of seabirds is a result of collisions with turbine blades, or associated structures, such as meteorological masts or power lines. Exact numbers of fatalities is difficult to estimate, especially since many corpses sink, are carried by currents, or taken by scavengers. Consensus is that levels of mortality are low, but seabirds are long-lived, reach maturity late, and lay small clutches once a year; thus, small changes in adult survivorship can have population level impacts.
To assess the impact of manmade structures on seabirds Hüppop et al. (2006) (http://bit.ly/OGm01N) carried out bird monitoring from the FINO 1 research platform in the North Sea (http://bit.ly/1fLhwAx). A total of 442 dead birds were discovered between October 2003 and December 2004. Of these, 245 had obvious injuries linked with collisions, including bleeding bills, skull contusions, and broken legs. Over 50% of the deaths occurred on just two nights when visibility was poor. Thermal cameras revealed disorientated birds flying around the illuminated platform. Results indicate that at times of poor visibility, illuminated structures can cause significant collision risk to birds. Considering the research was not conducted on wind farms, few conclusions about their effects can be drawn, but wind turbines are illuminated, so it highlights the fact that at times of poor visibility, birds maybe at heightened risk of collision. To reduce the risk, Hüppop et al. (2006) suggested research into bird-friendly lighting that minimises the possibility of attraction.
Collision risk is defined as the probability that an individual will collide with a wind turbine, and can be estimated using Collision Risk Models (CRM). Assuming most birds detect and consequently avoid wind turbines, it is essential to factor avoidance into these models. For example, using radars, Desholm and Kahlert (2005) observed geese and common eiders (Somateria mollissima) around the Nysted wind farm in the western part of the Baltic Sea (http://bit.ly/1jDgRC2). Results indicated that less than 1% of migrant eider ducks and geese that entered the wind farm area flew close enough to the turbines to be at risk of collision. Desholm (2006) produced a collision prediction model for common eiders in the Nysted wind farm. Model variables included migration volume, proportion entering the wind farm, proportion within horizontal and vertical reach of rotor blades, proportion not displaying avoidance behaviour, probability of individuals passing rotor blades safely, and the number of turbines passed. A collision risk of 0.020–0.021% was estimated, which equates to a mean of 46.2–48.1 individuals colliding with turbines during a single autumn season.
EFFECTS OF WIND FARM CONSTRUCTION ON SEABIRD HABITAT
Few studies have examined seabird habitat use before and after construction, therefore level of effect is primarily unknown. Avoidance and displacement because of wind farm construction and operation could potentially represent loss of aerial habitat. Obstacles in flight paths increase flight time, and energy use, leading potentially to avoidance or displacement, although natural variations in weather conditions also cause seabirds to alter flight behaviour. Conversely, at sea, increase in underwater prey abundance as a result of potential ‘reef’ effects of the turbine subsea structure may also be a possibility. In general, however, positive or negative wind farm-related habitat loss impacts upon survival or fitness of birds is difficult to elucidate.
Petersen et al. (2004) (http://bit.ly/1py08DD) compared the presence of seabirds in the Horns Rev wind farm area (http://bit.ly/1cbVLLp) before and after construction. Lower numbers of divers, common scoters (Melanitta nigra), guillemots (Uria aalge), and razorbills (Alca torda) were present within 2–4 km of the wind farm after construction. In contrast, numbers of gulls and terns were higher than expected after construction, highlighting a possible preference for the wind farm.
Desholm and Kahlert (2005) (http://bit.ly/1cq5et1) reported that the diurnal percentage of birds entering the Nysted wind farm area decreased from pre-construction to initial operation (1 year). Masden et al. (2009) (http://bit.ly/MyyHtF) also studied effects of the Nysted wind farm on birds. A Before After Control Impact (BACI) study assessed the flight paths of eider ducks prior to and following its construction. Use of the wind farm area declined, and distances from the wind farm area increased from 30 m up to 224 m. Results were significant, in that eider ducks travelled ca. 500 m further post-construction, but it was considered unlikely to have substantial biological consequences and wind was deemed to be more influential (Masden et al. 2009).
Avoidance of a single wind farm may have minimal effect, but if multiple wind farms are located along a single migration route disturbance may be cumulative. Consequently, the entire flight route needs to be studied as a whole to understand the level of effect wind farms will have on migrating seabirds.
WIND FARM SEABIRD HABITAT MODIFICATIONS
Introduction of wind farms modifies the marine environment. For example, wind turbines can act as artificial reefs (http://bit.ly/1o8Py2l), and aggregate marine life, including prey species of seabirds. An increase in prey could subsequently attract seabirds to the area, which could mean increased foraging success or some species, or increase the risk of collisions of others.
Noise created during wind farm construction at Scroby Sands in the North Sea (http://bit.ly/1fWghuG) during the herring (Clupea harengus) winter spawning season was thought to be responsible for a reduction in herring abundance (Perrow et al. 2011). This decline coincided with a reduction in feeding success of little terns (Sternula albifrons). Increased egg abandonment, and decreased hatching success were also reported (Perrow et al. 2011).
van Deurs et al. (2012) (http://bit.ly/1lnDpcX) carried out a BACI study to assess the effects of a wind farm on three species of sandeel (Raitt’s sand eel (Ammodytes marinus), lesser sand eel (A. tobianus), and greater sand eel (Hyperoplus lanceolatus)). A short-term positive effect on adult and juvenile greater sand eel was observed up to one year post construction. This coincided with a decline in silt and clay in the sediment, which improved habitat quality for sand eels. A long-term negative effect on juvenile populations was observed but it was not possible to say with certainty that positive or negative effects were down to the wind farm alone. The study did not address how these changes may impact other populations, but sand eels are important prey species for some seabirds, so an increase in numbers would likely attract seabirds.