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may fly at altitudes lower than 20 m (Larsen &
Guillemette 2007), well below the rotors of wind
turbines. Nocturnal migrants may, on the other
hand, be attracted to illuminated wind turbines
(e.g. Montevecchi 2006, and see Hüppop et al.
2006 for references, but see Dong Energy et al.
2006). However, for example common eiders seem
to keep a longer distance from turbines at night
compared to in daylight (see Desholm 2009 for references).
For common eiders passing Nysted wind
farm, it was predicted that 0.02 % (45 birds) would
collide each year and impact of this single wind
farm was considered negligible (Dong Energy et
al. 2006). For the 250 bird species migrating across
the German Maritime area, it has been estimated
that increases up 0.5 - to 5% (depending on species)
of the adult mortality would cause no effects
at population scale (see Hüpopp et al. 2006 for
references). Modelling tools for different scenarios
and turbine types are available (Garthe & Hüppop
2004, see Hüppop et al. 2006 for references, Desholm
2009).
In relation to local movements of birds considerations
should be focused on staging birds. The local
movements undertaken by waterbirds and seabirds
in staging areas may be attributed to current
drift, movements between sites in response to prey
aggregation and between sites of different functional
role. No field studies have yet investigated
the frequency of local movements in their staging
and wintering areas, and hence the risk of collision
for these birds cannot be assessed.
Conclusions
Most studies indicate small impacts of bird collisions
on assemblages as a whole for most species
studied and the few areas considered, although
any effects would be long-term. The temporal and
methodological limitations in most studies and variability
among species call for further clarification
though. Certainty: 3.
9.2 Migration barriers
Several bird species avoid wind farms during migration
(e.g. Pettersson 2005, Masden et al. 2009,
Muselears 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 barrier effects exposed on the landbirds,
including large species like raptors and cranes,
whereas due to the Danish demonstration projects
a large amount of information is available on
the behavioural responses of migrating waterbirds
to offshore wind farms (DONG Energy et al. 2006).
Waterbirds reacted to the wind farms at Horns Rev
1 and Nysted wind farms at distances of 5 km from
the turbines, and generally deflected at the wind
farm at a distance of 3 km (Petersen et al. 2006).
Within a range of 1-2 km more than 50% of birds
heading for the wind farm avoided passing within
it. At the Rønland offshore wind farm 4.5 % of all
56 GREENING BLUE ENERGY - Identifying and managing biodiversity risks and opportunities of offshore renewable energy
waterbird flocks entered a zone of 100 m from the
wind farm (Durinck & Skov 2006). At the Utgrunden
wind farm in Kalmar Strait low-flying flocks of eiders
were rarely seen to pass within 500 m of the wind
turbines during daytime, and avoidance behaviour
was observed, with some birds altering direction
3-4 km before reaching the Utgrunden wind farm to
fly around it (Petterson 2005).
For long-distance migrations, the energetic losses
due to migration barrier effects through avoidance
of single wind farms seem trivial, especially considering
the impacts of other factors, such as wind
conditions and visibility, although there may be
potential cumulative effects of several wind farms
in a region (e.g. Petterson 2005, Masden et al.
2009). Energetic costs due to single wind farms are
only likely to be measurable for species commuting
daily within a region, for instance between foraging
grounds and roosting or nest sites (e.g. Masden
et al. 2009). In these cases wind farms could cause
fragmentation of coherent ecological units for the
birds (e.g. Fox et al. 2006, Hüpopp et al. 2006, Stewart
et al. 2007).
Conclusions
The potential impacts on long distance migrating
birds are considered to be small, but for daily
commuting birds, for which long-term habitat fragmentation
extended routes, could have moderate
effects on assemblages. Several published studies
and estimations exist. Certainty: 3.