Mitigation measures for preventing collision of birds with wind turbines

Abstract

Bird collisions with wind turbines is one of the major environmental impacts of wind energy. To decrease the number of avian fatalities in wind farms, several studies have been done to understand, evaluate and apply measures that reduce the risks of collisions. Overall, mitigation measures during the operational phase of wind farms are based on either active or passive procedures, depending on the need (or not) for an external source to trigger an action for preventing a possible collision. The aim of this work is firstly to provide an overview of mitigation measures that have been demonstrated in-situ, including the overall procedures of the tests, results, as well as advantages and disadvantages of the measures. Subsequently, a new patented control concept to avoid collisions (SKARV) is described. Four measures that have been tested at wind farms in Spain, Norway, and USA are summarized: habitat management and painting (passive measures), turbine curtailment and deterrents (active measures). Estimations from these tests indicate an average reduction of 33%-86% in annual bird fatality for specific targeted species. The SKARV concept is not yet tested, but may be a benign alternative.

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Mitigation measures for preventing collision of
birds with wind turbines
To cite this article: Paula B. Garcia-Rosa and John Olav G. Tande 2023
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1
Mitigation measures for preventing collision of birds
with wind turbines
Paula B. Garcia-Rosa and John Olav G. Tande
SINTEF Energy Research, Sem Sælands vei 11, 7034, Trondheim, Norway
E-mail: paula.garcia-rosa@sintef.no
Abstract. Bird collisions with wind turbines is one of the major environmental impacts of
wind energy. To decrease the number of avian fatalities in wind farms, several studies have been
done to understand, evaluate and apply measures that reduce the risks of collisions. Overall,
mitigation measures during the operational phase of wind farms are based on either active or
passive procedures, depending on the need (or not) for an external source to trigger an action
for preventing a possible collision. The aim of this work is firstly to provide an overview of
mitigation measures that have been demonstrated in-situ, including the overall procedures of
the tests, results, as well as advantages and disadvantages of the measures. Subsequently, a new
patented control concept to avoid collisions (SKARV) is described. Four measures that have
been tested at wind farms in Spain, Norway, and USA are summarized: habitat management and
painting (passive measures), turbine curtailment and deterrents (active measures). Estimations
from these tests indicate an average reduction of 33%-86% in annual bird fatality for specific
targeted species. The SKARV concept is not yet tested, but may be a benign alternative.
1. Introduction
The replacement of fossil fuel-based generation to wind power is seen as fundamental to meet the
current targets for reducing greenhouse gas emissions. However, the expansion of wind farms
(WFs) increases the concerns about the impacts on bird populations, which can undermine
future plans for wind power generation. Habitat loss, barriers for migration and feeding areas
as well as fatalities due to collisions with wind turbines are some of the widely discussed effects
of wind farms on bird life [1, 2]. In this regard, many efforts have been made to understand
and measure the impacts of wind energy on bird population [3–13], i.e. to measure how the
reproduction, mortality, and survival of a population are affected [14]. In addition, mitigation
measures to reduce the collision of birds with wind turbines has also been the focus of a number
of studies [11, 13, 15].
The number of bird deaths due to collision with wind turbines varies greatly among wind
farms around the world, with estimates ranging from zero to almost 40 deaths per turbine per
year [11](and references therein). The birds are exposed to collisions with the static structure
and the rotating blades of wind turbines. A number of factors contribute to the collisions,
e.g., wind turbine design and layout of the wind farm, flight maneuverability, vision acuity and
behavioural factors of bird species, as well as weather and light conditions [2,15, 16].
To minimise the impacts, it is common to consider the mitigation hierarchy that avoids high-
risk sites during the planning phase of new wind farms and applies minimization measures and
compensation for unforeseen or unavoidable impacts [15, 17]. A number of post-construction

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minimization measures have been proposed to reduce bird collisions with wind turbines, as
indicated by several reviews on the topic [11, 12, 15]. Minimization measures are particularly
challenging since bird species have different sensory faculties, flight maneuverability, and
behavioral aspects [15,18]. Currently, there is not a single solution that can be applied to all sites
and species. The proposed measures are species-specific and tailored to the most collision-prone
species at a certain location [15].
Even though many measures have been proposed, only a few published works describing their
effectiveness in-situ are available in the scientific literature. The first aim of this paper1to outline
mitigation measures that have been demonstrated at wind farms, while indicating procedures
of the tests and results, as well as main advantages and disadvantages of the measures. Thus,
this work provides an overview of common practices, rather than presenting an exhaustive list
of approaches. The measures described here are categorized into passive, when not requiring
external information from an human observer or sensor technology to reduce the collision risks,
and active, when requiring actual information to trigger an action for collision avoidance.
In addition to tests performed in-situ, post-construction measures to reduce bird mortality
related to wind power generation have also been tested outside wind farms, see, e.g. [20, 21].
Furthermore, new measures are under development. The second aim of this work is to describe
the new patented control concept SKARV2, an example of active measure.
2. Passive measures
Passive measures aim to reduce the risks of bird collisions with wind turbines without the need
for a monitoring surveillance system or an human observer triggering an active response to
prevent the collision. The measures may consist of alterations to habitat, and design or visual
modifications of wind turbines, such as painting or lighting.
2.1. Habitat management
Habitat management measures consist of on-site or off-site habitat alterations to reduce the
risk of bird collision with wind turbines. The aim of on-site alterations is to decrease the bird
activity within the wind farm. This can be done, e.g., by clear-cutting forests or reducing the
attractiveness of the vegetation around the wind turbines either for the birds or their preys [15].
In contrast, off-site alterations aim to promote bird activity in areas outside the wind farm. The
measures include the creation of new areas for foraging habitat and breeding sites away from
the wind farm [11, 15].
The effect of modifying the vegetation around wind turbines to make the area less attractive
to lesser kestrel (Falco naumanni) was verified in [22] for three wind farms in Spain. The
procedure consisted of superficially tilling the soil (3-8 cm deep) at the base of wind turbines by
using a plough, tiller or cultivator once a year for two years. To illustrate the tilling procedure,
Figure 1 shows a freshly tilled soil for agriculture purposes.
In [22], the tilling was done at the beginning of the bird breeding season to eliminate the
natural vegetation, and consequently, to decrease the number of preys (insects). In total, the
surrounding of 41 of 99 turbines in the wind farms had the soil tilled. The statistical analysis
considered the number of dead kestrels found in tilled bases and in non-tilled bases as well as
the mortality before and after the mitigation phase, which represented, respectively, ten and
two years of data. For the three wind farms, the results indicated reductions of 75%, 82.8%
and 100% in the annual collision rates, representing a total average reduction of 86% after the
mitigation measure was applied. This was attributed to the lack of prey around tilled bases,
since the birds had to search for food in other areas away from the wind turbines [22].
1This paper is partly based on [19].
2SKARV is the Norwegian name for the Great Cormorant and acronym for (in Norwegian) “Slippe
fugleKollisjoner med Aktiv Regulering av Vindturbiner”.

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Figure 1. Tilled soil. Image by April Sorrow (UGA CAES/Extension) licensed under
CC BY-NC 2.0.
2.2. Painting
The rotational motion of turbines causes an effect known as motion smear (or motion blur) that
can make the blades appear transparent to birds [21]. Based on laboratory tests, painting a
single blade in black has been proposed as a suitable measure to reduce motion smear and risk
of collisions [21].
Figure 2. Wind turbine with rotor
blade painted in black at the Smøla
wind power plant, Norway. Image by
May et al. 2020 [23] licensed under CC
BY 4.0.
Recent studies have tested the hypothesis that
painting in black one of the turbine blades (Fig.2)
to reduce motion smear, and painting the tower to
increase the contrast against the background, would
reduce the collision risk of birds with wind turbines
[23, 24]. The measures were intended for different bird
species: the former targeted species that fly at the
blade height, such as soaring raptors and birds with
aerial display, while the latter targeted species with
poorly developed vision and flight maneuverability, and
species typically flying relatively low heights, such as
galliformes [24]. Both studies followed a before-after-
control-impact (BACI) approach to test the painting
effects in turbines at the Smøla wind power plant
in Norway. In total, eight turbines were considered
for testing the effects of painting one turbine blade
in black, and 20 turbines were considered for the
experiments with the tower. In each experiment, only
half of the turbines were painted, while the other half
were neighboring unpainted turbines defined as “control
turbines” for ensuring similar spatial conditions in the
comparisons.
Statistical analyse included the number of carcasses
found before and after the painting and the number
of searches with trained dogs. The experiment spanned
over eleven years, where seven and a half years consisted
of data before the treatment, and three and a half
years of data after the treatment [23]. For the in-situ
experiments with the blades, a reduction of around 70%

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in annual collision rates was observed; mainly for raptors.
Even though the results were encouraging, the authors recommended to replicate or
implement the measure in a larger number of turbines, given the limited number of turbine
pairs in their experiment. The experiments with the tower indicated that the effect of painting
was most pronounced in spring and autumn, as winter generally has poor light conditions and
tower basis are hard to observe regardless of their appearance [24]. In this case, a reduction of
around 48% in annual collision rates was observed for willow ptarmigans (Lagopus lagopus).
Based on the results obtained in [23], wind farms in Spain [25] and the Netherlands [26]
have recently painted one blade in black for a number of turbines to test the measure in their
locations. At the same location in Spain, vinyl shapes resembling eyes were also applied at the
tower bottom of a few turbines to increase their visibility [25].
3. Active measures
Active measures rely on the detection of birds prior performing an action to avoid a possible
collision. Visual observations by humans, radar, camera-based systems or a combination of
technologies have been proposed for detecting birds in the vicinity of wind farms [27–31].
3.1. Turbine curtailment
Curtailment and temporary shutdown of wind turbines are minimization measures for birds
flying at the rotor swept height. Naturally, such measures can only be effective for birds colliding
with rotor blades and not with the turbine structure. Shutdown or curtailment can be performed
whenever a bird is detected in a high collision risk area or within a perimeter of the wind farm,
which can be referred as “informed curtailment”. Additionally, temporary shutdown can be
done during migration seasons or certain weather conditions [11]. Shutting down during specific
seasons or due to the weather relies on collision risk models, and not necessarily on actual risks
of collision. Since this leads to large periods of shutdown and high reduction in the annual
energy generation, this measure has not been well received by wind energy companies [11]. In
addition, this approach tends to be effective for some soaring raptors, but not for many other
bird species [32]. In contrast, informed curtailment considers human observers, radar and/or
camera-based systems to determine the shutdown or curtailment time of specific turbines [33–42].
The effectiveness of selective shutdown of wind turbines was applied in wind farms in the
southern of Spain, Cadiz area [33,35]. In total, 269 turbines were selectively stopped during the
procedure. A selective stopping protocol became mandatory in 2008 due to the high collision
rates of griffon vultures (Gyps fulvus) with wind turbines in the location - an average of 61
death of vultures per year were observed within the wind farms before implementation of the
measure. In the first two years of the application protocol, a reduction of mortality of about
50% was obtained for the vultures [33]. After 13 years of application of the same protocol, a
reduction of about 93% in the mortality rate was observed when compared with the rate before
the measure was applied (2006-2007). By considering all soaring birds, the reduction was about
65% [35].
In order to estimate the mortality rate, the analysis in [35] considered before-after number
of collisions, statistical methods, and annual counts of soaring birds, griffon vultures, passerines
and bats. The protocol targeted soaring birds, specially vultures, while passarines and bats
were considered for comparison purposes only. The wind turbines were shutdown whenever
a dangerous situation was detected by trained observers A typical dangerous situation was
defined, for instance, as griffon vultures with flight trajectories in potential risk of collision with
the blades, or when flocks of medium to large sized birds were flying within or near the wind
farms. In these cases, the turbines involved in the potential risk were switched off within three
minutes after the observers contacted the local wind farm control office. On average, the turbines
were out of operation for 108 minutes. By considering the number of times the turbines stopped

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during the last three years of the protocol (6700 times), it was estimated that 0.51% less energy
was generated by the wind farms due to the stops. As discussed by the authors, vultures are
large diurnal raptors, and most of accidents with these birds happen during daylight, from two
hours after sunrise to two hours before sunset. Then, it is expected that the turbines can operate
normally at night, minimizing the energy generation decrease by the wind farms.
Informed curtailment was also verified through BACI experimental studies in wind farms in
Wyoming, Western United States [34]. Curtailment consisted of feathering the turbine blades
to dramatically reduce the speed of rotation. In this case, a computer vision system consisting
of a high-defintion stereo camera and classification algorithm [36,42] was used to identify eagles.
The statistical analysis included the number of carcasses found before-after in the treatment site
and in a reference site with similar characteristics (a wind farm with 66 turbines located 15 km
from the treatment site). In the treatment site, 47 automated curtailment units were dispersed
throughout 110 wind turbines, while in the reference site no curtailment was applied. The before
period ranged over about four years, while the after period lasted around one and a half years for
the reference site. The periods varied for the treatment site, as the installation of the automated
curtailment units occurred at different stages. A reduction of 63% in the number of fatalities was
observed in the treatment site after the automated curtailment has been implemented. Before
the automated curtailment, human observers were responsible for ordering curtailments at the
treatment site. Thus, the effectiveness of automated curtailment was in addition to an existing
minimization measure [34]. An estimation of the reduction in annual energy generation due to
curtailments was not presented in the study.
3.2. Deterrents
Some measures for minimizing the collisions are based on sensory cues, such as auditory, visual
and acoustic deterrents, activated to scare or frighten birds and prevent them from coming
closer to the wind turbines. Long-term use of auditory harassment has been proven to become
ineffective with time as birds tend to habituate to the stimuli [43] (as cited in [11]), but the
effectiveness can be improved by varying firing frequency and direction [15]. Preliminary tests
at a wind farm in Spain indicated that griffon vultures can react to long range acoustic device
(LRAD) sounds [44]. The efficacy of different LRAD sounds in dispersing birds depend on their
distance from the device, their altitudes, as well as the number of birds in a flock [44].
Deterrents in wind farms have been commonly combined with real-time detection of birds
by camera-based systems [45–49]. A number of technical reports have evaluated commercial
technologies aimed at detection and deterrence of raptors at different locations [45–48]. The
most recent study [48] included species-level identification, probabilistic methods, unmanned
aerial vehicles, and seven detection and deterrent systems installed on selected turbines in
an operational wind farm in California, USA. The deterrence module operation was based on
emitting an initial audible warning signal when a flying object was detected (bird or inanimate
object), and a stronger dissuasion signal when the object crossed a closer distance threshold
to scare the bird away from the signal noise and wind turbine. The study was performed over
a nine-month periods to quantify the effectiveness of the measure in reducing collision risk for
golden eagles (Aquila chrysaetos) and other large raptors. It was estimated that the installed
systems potentially reduced golden eagle collision risk by 33–53% for the studied case.
Another study in Canada used a predator owl model deterrent paired with an audio recording
of predator and alarm calls to deter birds from a wind farm in Nova Scotia [50]. However, the
results were inconclusive.
3.3. Turbine speed control
A new active concept, nicknamed SKARV, to avoid collision between birds and wind turbines
is described in the patent [51]. The concept consists of making small adjustments to the rotor

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Figure 3. SKARV’s concept - Illustration of how turbine speed control minimises probability
for collision. The coloured areas indicate where the bird will be with high probability when
crossing the rotor plane. A possible collision (red circle) is avoided by reducing the turbine
speed so that the bird instead crosses the rotor plane between the blades (blue circle).
speed so that birds can fly through the rotor swept area without being hit by the blades. This
requires a system to detect and track birds in proximity to a wind turbine. The detection and
tracking can be based on radar, LIDAR or camera; for example, the Spoor system [52] applies
off-the-shelf cameras in combination with artificial intelligence algorithms to detect, classify, and
trace the presence of birds around wind turbines. Based on the observations, possibly coupled
with machine learning, a continuous estimate of the flight path of the bird can be made. If the
flight path is estimated to be towards the turbine rotor plane, the control system will slightly
adjust the rotational speed of the turbine to minimize risk of collision between the bird and
rotor blades, as illustrated in Figure 3.
The SKARV concept can be applied to modern variable speed wind turbines. In such a
case, the concept would be implemented in a software that estimates in real-time the bird’s
probabilistic flight path and the desired rotor speed adjustment to minimize risk of collision. This
requires the use of a bird-tracking system that follows the bird path. The rotor speed adjustment
would be an input signal to the built-in standard wind turbine control system that governs the
rotational speed of the turbine through blade-pitching and generator torque-adjustment. The
speed adjustment would be only a small perturbation to the actual rotor speed, either an increase
or decrease. Notice this is rather different from curtailment, where the speed of the wind turbine
is significantly reduced.
In SKARV, the adjustment of the rotational speed of the turbine to avoid a possible bird
collision should not lead to any noticeable loss of generation, as only small adjustments to the
rotor speed should be made. However, in case the flight path cannot be estimated with any
reasonable certainty, for example in the case of birds with highly irregular flight paths, turbine
shutdown can be an option. The same would be the case if a flock of birds would be approaching.

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Table 1. Summary of mitigation measures demonstrated in wind farms.
Measure On-site habitat
alteration
Painting
(one blade)
Painting
(tower)
Turbine
shutdown
Sound
activation
Type Passive Passive Passive Active Active
Targeted species lesser kestrel raptors
galliformes/
willow ptarmi-
gans
soaring birds/
griffon vultures
eagles
raptors/ golden
eagles
WF location Cuenca (ES) Smøla (NO) Smøla (NO) Cadiz (ES)
Wyoming (US)
California (US)
Analysis of re-
sults consider
Before-after
deaths
Before-after
deaths
Before-after
deaths
Before-after
deaths
Measured bird
activity
Duration of the
measure
2 years 3.5 years 3.5 years 13 years
1.5 years
9 months
Reported effec-
tiveness
86% 70% 48% 65% 33-53%
Shortcomings Birds are dis-
placed. Im-
pacts in other
wildlife.
Less effective in
low light. Vi-
sual impact.
Less effective in
low light. Vi-
sual impact.
Loss in power
production and
revenue.
Impacts in
other wildlife.
Disturbance
to nearby resi-
dents.
4. Discussion
Table 1 summarizes the demonstrated mitigation measures described in this paper. The on-site
habitat alteration has the highest reported effectiveness, but resulting in displacement of birds
and negatively impacting the environment. The painting of a turbine blade is reported to be
the second most effective measure, while turbine shutdown is reported to be third most effective
measure. All the mitigation measures have some shortcomings that need to be balanced against
their effectiveness in reducing bird mortality.
As discussed in [15], the efficacy of on-site habitat alterations relies on the importance of the
habitat for a certain species. If the area is not dramatically modified, it may still be revisited.
In any case, habitat alterations might affect other wildlife that was not previously affected by
the wind farms. Additionally, the creation of artificial breeding for raptors or building perching
towers for offshore birds have been proposed as off-site habitat alterations. Still, the measures
do not prevent the birds to move through the wind farms and utilise the area for foraging [15].
Painting the turbines to increase visibility is a relatively simple and cost-effective measure.
However, when possible, it is recommended to paint rotor blades before construction of the wind
farm, as in-situ painting has high costs and requires specialized personnel [23]. A limitation of
visual cues based on painting is that the measure becomes less effective in low light levels during
poor weather conditions or at night. In addition, reducing the motion smear by painting the
rotor blades is also less effective for species that constantly look downwards when flying [11]. In
either way, the birds might not see the wind turbine ahead.
Informed curtailment is also effective in reducing the risk of collisions of soaring birds with
rotor blades. However, the measure requires an efficient monitoring system to limit the number
of turbine shutdowns, or curtailment, and to limit the loss in annual energy generation. When
the curtailment is automatized, the results are further improved [34], but at the same time,
the number of false positives might increase with an automated system, which also leads to
unnecessary number of shutdowns. The automatic curtailment of wind turbines relies on the

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detection and identification of birds using camera-based systems, which is an ongoing topic of
research.
Overall, the activation of warning and deterrents signals to discourage birds to fly close to
wind turbines also has great potential to reduce the risk of collisions. However, this measure is
also subject to a large number of false positives, as indicated by [46, 48]. In addition, warning
and deterrent signals may disturb nearby residents and non-targeted wildlife [48], and may cause
unpredictable effects on the bird’s flight trajectory. Thus, caution is advised when the deterrent
is applied at a short distance from wind turbines [11].
The new control concept SKARV is a promising solution to reduce the risk of collisions
between birds and wind turbine blades without disturbing other wildlife and causing any
measurable loss of production. However, its effectiveness is still not demonstrated, and there
are a number of challenges that must be addressed prior full development of the technology, for
example:
•Targeting bird species and different flight patterns: Ideally, functional mitigation measures
should be tailored to as many bird species as possible. However, this is a challenging
problem as birds present different behavior and morphology [15]. Thus, it is important to
understand which/how many species can benefit from a collision avoidance control scheme,
and to verify how the control should adapt to different flight patterns.
•Detecting and tracking birds approaching the rotor: To implement an active avoidance
strategy, approaching birds must be detected and their path tracked. This must be done
with equipment that comes at a reasonable cost on a per-turbine basis. Bird detection and
tracking systems are available, e.g. the system of Spoor uses off-the-shelf cameras [52], but
it is still an open question whether these systems provide sufficient accuracy and reliability
for the approach and if modifications will be required for the integration.
•Preventing bird strikes: The idea relies on the ability to predict the birds’ flight paths far
enough ahead of time, so that a small correction to the rotational speed provides an effective
reduction in the probability of collision. A practical control algorithm that implements the
collision-avoidance strategy must be developed. The main challenge is to develop a real-
time algorithm that provides a sufficiently accurate and reliable estimate of the flight path
a few seconds ahead of a potential collision. The turbine speed control would then ensure
that the blades will be as far away as possible from the bird when passing the rotor area.
•Keeping dynamic loads on structures low: The dynamic response of the turbine places
constraints on the type of control actions that are feasible. Abrupt acceleration and
deceleration of the rotor implies large fluctuating forces in the pitch actuators and turbine
structures. Thus, the earlier that the control action is initiated, the more benign will be
the consequences for fatigue of turbine components. Preliminary analysis using a 10 MW
turbine model indicates that a 10 second lead-time would give no significant consequence
on fatigue, while 5 seconds may be the shortest lead-time that can be accepted.
It will not be trivial to accomplish both control objectives of collision avoidance and keeping
dynamic loads low. Where the best trade-off lies must be established through the development
of the control system. In addition, it is acknowledged that certain species (especially small
birds) fly in patterns that cannot be predicted a few seconds in advance. In this case, to prevent
collisions, the turbine might need to be shut down as soon as the bird is detected. The active
control aspect of SKARV is most likely applicable for species which fly in regular patterns.
5. Conclusion
A number of post-construction minimization measures have been proposed to reduce bird
collisions with wind turbines. There is not a single solution suitable to all locations and species;
the proposed measures are species-specific and tailored to the most collision-prone species at a

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given location [15]. To verify the effectiveness of a measure, a suitable experimental study, such
as before-after control-impact experiments, should be performed. Otherwise the results become
difficult to interpret if the number of fatalities before the implementation of the measure is not
known [32]. Alternatively, the current bird activity within the wind farm should be included in
the analysis, as done in [48].
Overall, there is a limited number of scientific works that estimate the effectiveness of
minimization measures in-situ, i.e. within the wind farm. This paper summarized the
experimental results of two passive measures (habitat management and painting) and two active
measures (turbine curtailment and deterrents). Such measures were performed at different wind
farms worldwide and showed great potential to reduce the risk of collisions between targeted
species and wind turbines. The efficacy of the methods were calculated in terms of bird fatality
reduction, but other factors such as loss in power production, disturbance to non-targeted wildlife
and implementation costs are also considered in the development of mitigation measures.
The SKARV concept is an innovative control system to reduce the risk of collisions between
birds and wind turbine blades, without disturbances to other wildlife and significant loss in
power production. The hypothesis is that the concept can be effective for birds with relative
regular flight patterns, whereas turbine shutdown can be applied as a back-up in the case of
bats or birds with highly irregular flight paths. The SKARV concept has, however, not been
demonstrated yet, and a number of challenges must be addressed before any conclusions can be
drawn upon its effectiveness.
Acknowledgments
This work was supported by the Norwegian Research Centre on Wind Energy, FME Northwind,
WP5 “Sustainable Wind Development”, funded by the Research Council of Norway under
Project 321954.
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