Abstract
Following the positive results of painting one blade black to reduce collision rates among birds (including White-tailed eagles) on the Norwegian island of Smøla (May et al. 2020), a follow-up study was started in windfarm Eemshaven in the Netherlands. This site has a much more varied species composition and collision rates among birds are known to be relatively high. The study consisted of 14 turbines in a BACI setup (Before-After-ControlImpact). After one year of Before monitoring, during which all turbines had white blades (Control), half the turbines received one black blade (Impact). The After monitoring lasted two years, until the end of 2024. The results of these three study years are described in this report. The effectiveness of a black blade for reducing collision victims is discussed, by looking at the results of the three years of collision victim monitoring, as well as the results of the study ZWEMT. This additional study aimed to better understand the effects of a black rotor blade on bird behavior, by deploying state-of-the-art monitoring techniques, including a Max® 3D bird radar, camera’s, sensors (WT-Bird® system) and sound recorders.
Part 1: Black blade project In three years of collision victim monitoring, a total of 436 collision victims were found, of which 196 under Control turbines and 240 under Impact wind turbines. The species groups that were most often found as collision victims were songbirds and gulls followed by waders. In contrast to the high reduction in collision victims found by May et al. (2020), we found no statistically significant effect of the black blade on the number of collision victims. This result applies to all bird species combined, as well as to all separately tested species groups. Only for diurnally active birds and large gull species a visible though not statistically significant positive effect of the black blade was found. For diurnally active species this effect was probably partly influenced by the presence of a starling roost underneath one of the impact turbines during the null monitoring, but not during the impact monitoring. For large gulls the reduction in mortality at wind turbines with a black blade was only present in the second year of the effect monitoring, whereas the first year was comparable to the null monitoring.
A possible explanation for the rather disappointing results of this study, is that the single black blade might not contrast enough with the (busy) background in the Eemshaven area. A single black blade was meant to reduce motion smear and thereby improve the visibility of the wind turbine for birds. However, motion smear is probably less relevant for modern large wind turbines, due to the lower rotation speed. Therefore, the current hypothesis is that mainly contrast determines the visibility of a wind turbine for birds. Compared to the single black blade, there might be more effective patterns and/or colors to enlarge contrast (both internally and with the background). Alternatively, the large positive effect found by May et al. (2020) could be due to the very small sample size of the Norwegian study. This means that one or two extra collision victims under a wind turbine with a black blade could have led to a significantly different outcome. In this study in the Eemshaven over ten times more collision victims were found compared to the study at Smøla, which makes its results and conclusions more robust.
The fact that the black blade did not work in the Eemshaven, does not mean that increasing the visibility of wind turbines for birds is an ineffective measure to reduce bird mortality in wind farms. After all, the black blade did work at Smøla and good results are achieved in South Africa with red banding on one of the rotor blades. The results of this study in the Eemshaven do show that the single black blade may not be the most appropriate measure for all locations to increase the visibility of wind turbines sufficiently to limit bird mortality. We therefore recommend that follow-up research should focus on the underlying mechanisms. Some preliminary ideas are given below. To successfully increase the visibility of wind turbines for birds as a mitigating measure, we need to understand which patterns and/or colors are effective for which species(groups) and under which circumstances.
In a possible follow-up study, monitoring the head position of birds (specifically gulls) in flight may be promising, as it could test whether they the wind turbines are actually visible depending on their head position in flight. Improving visibility of wind turbines by adding colors and/or patterns can only help prevent collision, if birds are able to see the wind turbines in their flight path. In addition, we recommend studying the flight behavior of different bird species in proximity to turbines with and without a black blade in more detail, as species differences in behavior are still poorly understood. This can yield valuable information about the effectiveness of the black blade for species (groups) that were only found in small numbers during the collision monitoring.
Finally, it would be highly interesting to test a different pattern and/or color, with the primary goal of increasing contrast rather than avoiding motion smear. Before such a study is started in the Eemshaven, we advise to first reach consensus on the most suitable pattern based on other ongoing field and theoretical studies and, if possible, to choose a pattern that can be applied widely in practice. As the scientific field of wind turbine visibility has greatly expanded, there is clear added value in reviewing the results of all ongoing studies on the effects of colored and patterned blades. This can be used to develop a conceptual framework to increase the visibility of wind turbines for birds to reduce the number of collisions.
Part 2: ZWEMT-project The ZWEMT-project revealed no differences in the flight behavior (flight height, flight speed and straightness of the flight) of birds close to wind turbines with and without a black blade. Sadly, the WT-Bird® systems that were installed to measure collisions of birds with the wind turbines only registered two collisions. Due to issues in the installation process, the sensitivity of the sensors was limited. This also meant that not enough data was collected to validate the effectiveness of the WT-Bird® system compared to ‘manual’ collision victim searches. Due to the limited number of collisions registered by the system, the timing of collisions and the conditions under which they occured could not be determined in this project. Eindrapportage onderzoek ‘zwarte wiek’ Eemshaven 9