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The burial of cables would, moreover, need to be
weighed against the disturbance caused by the
dredging and ploughing activities, including the
risk of re-suspending pollutants (See Section 6).
The transmission system can, further, be constructed
so that magnetic fields are reduced or to some
extent cancel out each other (see Gill et al. 2009,
VRD 2009), although the costs involved makes this
unlikely to become a standard approach.
9 Impacts on birds
9.1 Collision risks
The interaction between birds and power plants
is the most thoroughly investigated environmental
concern relating to wind power. Early considerations
included the extent of bird collisions with
the turbines and subsequent effects on population
dynamics and migration (Winkelman 1985; Ivanov
& Sedunova 1993, Gill et al 1996; Richardson 1998,
Langston & Pullan 2003, Desholm & Kahlert 2005,
Kunz et al. 2007). Including both on- and offshore
facilities, estimated rates of mortality for different
bird species range from 0.01 to 23 mortalities per
turbine per year (Drewitt & Langston 2005), with
an average across bird species of 1.7 collisions/
turbine/year according to an ongoing scientific synthesis
(M. Green, personal communication on synthesis
in progress 2009). Raptors seem to be more
sensitive than other species according to studies of
land based wind turbines (e.g. de Lucas et al. 2008),
and the average collision rate for raptors was estimated
to 0.3/turbine/year (M. Green, personal
communication on synthesis in progress 2009). For
raptors around onshore wind farms, fatality does
not seem to be dependent on the number of birds,
but varies with species-specific flight behaviour,
weather and topography (Langston & Pullan 2003).
It is important to note that both collision rates and
impacts of increased mortality on populations vary
greatly with species (e.g. Fox et al. 2006, Desholm
2009).
Although monitoring at the established offshore
wind farms have only partly involved combined
visual and radar-based observations of behavioural
responses of migrating birds to the structures
experiences of species-specific responses have
been gathered. Least is known about the collision
risks exposed on the largest component of longdistance
migration: the migration of passerines.
Many studies on collisions on land have reported
that passerines are being killed in larger number
than other birds. Hüppop et al. 2006 reported the
same from the Fino offshore research platform in
the German Bight with several hundred passerines
being killed during isolated events. Still, it’s important
to recall that passerines outnumber other
terrestrial bird species on migration by at least an
order of magnitude, and hence the relative impact
may not be highest for passerines. In fact, the experience
from land-based wind farms point at larger
species as the most sensitive to collision. Frequent
collisions, however, have been reported from only
a few exposed sites with high migration densities
(e.g. at passes, straits and peninsulas) and large
numbers of, for example, soaring resident raptors.
In such worst-case scenarios like the Altamont Pass
and Smöla wind farms (Erickson et al. 2001, Dahl
2008), mortality rates of raptors as a direct result of
collisions with the rotor blades are relatively high
in comparison with the size of the affected populations.
There is an almost complete lack of experience
regarding the behavioural responses of large
birds on long-distance migration, such as raptors
and cranes, around offshore wind farms, as wind
farms have not yet been erected in migration corridors
for these species groups. A worst case scenario
offshore would be a situation in which raptors
were being attracted to an offshore wind farm
along a major migration corridor.
A recent offshore wind farm related study in Germany,
indicated that the majority of collisions
might take place during a couple of days each year,
when migratory birds are hampered by bad weather
(Hüppop et al. 2006). The commonly applied
radar surveys that cover only parts of the migration
seasons, and for which quality decreases with
certain weather conditions, distance and size of
birds, may thus have underestimated the collision
risks for birds passing through wind farms (Hüppop
et al. 2006). The flight altitude of migrating birds
is usually lower offshore than on the coast and
inland (Krüger & Garthe 2001, Hüppop et al. 2004),
limiting the application of data that are collected
on land (Hüppop et al. 2006). For many seabirds,
the flight altitude ranges within 0-50 m (Dierschke
& Daniels 2003), and e.g. most common eiders
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