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Wind farms, as vertical structures with mobile elements, may represent a risk to birds, both as residents or migratory birds. It is difficult to give a clear opinion about the impacts of wind energy on birds for several reasons:
The types of risks that may affect birds are:
The main factors which determine the mortality of birds by collision at wind farms are landscape topography, local winds characterised by direction and strength, turbine design characteristics and the specific spatial distribution of turbines on the location. (De Lucas, in press). Specific locations should be evaluated a priori when a wind farm is planned. Every new wind farm project must include a detailed study of the interaction between birds' behaviour, wind and topography at the precise location. This analysis should provide information to define the best design of the wind farm to minimise collision with the turbines. Raptors present a higher mortality rate due to their dependence on thermals to gain altitude, to move between locations and to forage. Some of them are long lived species, with low reproductive rates and thus more vulnerable to loss of individuals by collisions.
The mortality caused by wind farms is very dependent on the season, specific site (e.g. offshore, mountain ridge, migration route), species (large and medium vs. small and migratory vs. resident) and type of bird activity (e.g. nocturnal migrations, movements from and to feeding areas).
Bird mortality seems to be a sporadic event, correlated with adverse weather or poor visibility conditions. Results from Altamont Pass and Tarifa on raptors showed some of the highest levels of mortality; however, the average numbers of fatalities were low in both places ranging from 0.02 to 0.15 collisions/turbine. In Altamont Pass this was due to the high number of turbines involved in the area. In Tarifa, the two main reasons were that the wind farms were installed in topographical bottlenecks where large number of migrating and local birds can be flying at the same time through mountain passes, or because of the use of wind by soaring birds to gain lift over ridges. In Navarra, studies of almost 1,000 wind turbines and including all types of birds, showed a detected mortality rate between 0.1 to 0.6 collisions per turbine and year. Raptors were the bird group more affected (78.2%) during spring followed by migrant passerines during postbreeding migration time (Sept.-Oct)
At global level, it can be accepted that many wind farms show low rates of mortality by collision (Drewitt and Langston, 2006). However, even these low collision mortality rates for threatened or vulnerable species could be significant and make it harder for a particular species to survive.
Comparative study over birds' mortality by anthropogenic causes was carried out by Erickson et al. (2002). The next table shows the distribution by human activities:
Table 2.2: Anthropogenic birds' mortality:
|Causes||Bird deaths (per 10,000 bird deaths)|
|High tension lines||800|
A more recent study stated current wind energy developments are only responsible for 0.003% of bird mortalities caused by anthropogenic activities.
Concerning habitat disturbance, the construction and operation of wind farms could potentially disturb birds and displace them from around the wind farm site. The first step in analysing the disturbance is to define the size of the potential disturbance zone.
Wind turbines can trigger flight reactions on birds avoiding the wind farm area. Potential disturbance distances have been studied by several authors giving an average of 300 m during the breeding season and 800 m at other seasons of the year. Approximately 2% of all flights at hub height showed a sudden change of direction in the proximity of wind farm. An indirect negative impact of wind farms include a possible reduction in the available area for nesting and feeding by birds avoiding wind farm installations.
During construction, species can be displaced from their original habitat, but in most cases they return during the operational phase. However, exclusions may occur for other species during the breeding period.
Mitigation measures to minimise impacts vary by sites and by species, but common findings have been defined by reviewing available literature.
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