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- METHODS AND DATA
Worldwide, Germany is the leading country in the use of wind energy. Since sites for the erection of wind turbines became scarce on land, ambitious plans for the offshore regions have arisen. There have been applications for 33 sites within the German Exclusive Economic Zone in the North and Baltic Seas, some of which entail several hundred individual turbines. Eleven pilot projects are approved, and two others rejected. As several hundred million birds cross the North and Baltic Seas at least twice every year, the Offshore Installations Ordinance says that licensing will not be given if the obstacles jeopardize bird migration. Birds are potentially endangered by offshore wind farms through collisions, barrier effects and habitat loss. To judge these potential risks, the occurrence of birds in space and time as well as details on their behaviour in general (migration, influence of weather) and their behaviour when facing wind farms (flight distances, evasive movements, influence of light, collision risk) need to be determined. Furthermore, the influences of construction and maintenance works must be considered. Since 2003, we have investigated year-round bird migration over the North Sea with regard to offshore wind farms. The main objectives were to assess data on the aforementioned aspects of bird migration over sea. These data can contribute to, for example, estimations of collision risks at offshore wind farms, the possible impacts on bird populations and possible mitigation measures. Results from measurements with different techniques, including radar, thermal imaging, and visual and acoustic observations, were compiled. The findings confirm that large numbers of diurnal and nocturnal migrants cross the German Bight. Migration was observed all year round but with considerable variation of intensity, time, altitude and species, depending on season and weather conditions. Almost half of the birds fly at ‘dangerous’ altitudes with regard to future wind farms. In addition, the number of individuals in reverse migration is considerable, which increases the risk of collision. We demonstrated that, especially under poor visibility, terrestrial birds are attracted by illuminated offshore obstacles and that some species collide in large numbers. Passerines are most frequently involved in collisions. Even if the findings regarding collisions at a research platform cannot be directly applied to offshore wind farms, they do show that on a few nights per year a large number of avian interactions at offshore plants can be expected, especially in view of the number and planned area of projected wind farms. We suggest abandonment of wind farms in zones with dense migration, turning off turbines on nights predicted to have adverse weather and high migration intensity, and actions to make wind turbines more recognizable to birds, including modification of the illumination to intermittent rather than continuous light, as the most appropriate mitigation measures. We further conclude that a combination of methods is necessary to describe the complex patterns of migration over the sea. The recordings are to be continued with the aim of refining the results presented here, and of developing a model for ‘forecasting’ bird migration over the German Bight. We expect more information on avoidance behaviour and collisions after the construction of a pilot wind park.
Each year during the migration periods several hundred million birds of roughly 250 species cross the North and Baltic Seas on their journeys between their breeding grounds in northern Asia, North America and especially in Scandinavia and Finland, and their winter quarters, which lie between Central Europe and southern Africa, depending on the species (Dierschke et al. 2003). These two seas are situated at the centre of a global network of migration routes. Furthermore, both seas are used as resting, moulting and feeding grounds for internationally important numbers of waterbirds (e.g. Garthe 2003).
It is current policy in many countries, notably in the European Union, to advance the development of renewable energy sources in order to reduce environmental degradation and anthropogenic climate change caused by the use of fossil fuels (Houghton et al. 2001, Chow et al. 2003). Worldwide, Germany has become the leading country in the use of wind energy. Against the background of limited land resources and especially in view of the greater consistency and force of winds at sea, particular attention is being paid to the production of wind energy in offshore locations. There have been applications for 33 sites within the German Exclusive Economic Zone (EEZ) in the North Sea (27 sites) and in the Baltic Sea (six sites), some of which entail several hundred individual turbines. Eleven pilot projects with 12–80 turbines each are now approved (ten North Sea, one Baltic Sea); two others in the Baltic Sea were rejected because of large concentrations of resting birds in the respective areas. Shoreward of the EEZ, i.e. in coastal waters inside the 12-mile zone, permissions have been granted for further wind farms (for details see, for example, http://www.offshore-wind.de). Even a far more modest development would make the construction of wind farms the greatest human impact in the North and Baltic Seas next to fisheries (Merck & von Nordheim 2000). However, construction works have not yet started at any of the licensed sites in Germany. Potentially, wind farms pose a variety of threats to birds: collision, loss of habitat and barrier effects (e.g. Exo et al. 2003, Langston & Pullan 2003, Hüppop et al. 2004, Zucco & Merck 2004, Gill 2005, Percival 2005).
In any authorization process issues such as mining rights, shipping routes and safety have to be taken into account. Consideration also needs to be given to the interests of the navy, commercial fishing and nature conservation, as well as those of submarine cable and pipeline operators. One of the reasons for rejection explicitly mentioned in the Offshore Installations Ordinance is ‘jeopardising of bird migration’ (Dahlke 2002). Approval may not, however, be withheld in the absence of rejection reasons. But what does ‘jeopardising of bird migration’ mean in terms of, for example, numbers of collision victims or effects and impacts on bird populations?
There exists a comprehensive literature on bird migration over the North Sea from the end of the 19th century onwards (e.g. Gätke 1891), including extensive technical approaches such as surveillance radar studies by Lack and others (reviewed by Eastwood 1967, for the German Bight of the North Sea, e.g. Jellmann & Vauk 1978) or satellite telemetry (Green et al. 2002). Nevertheless, with respect to questions regarding environmental effects and impacts connected with the construction of offshore wind turbines, severe gaps in our knowledge became obvious:
How many migrants of which species cross the German Bight at which time?
What is the proportion of birds flying in altitudes up to 200 m (as high as the future wind energy plants)?
How are migration intensity and flight altitude influenced by weather, namely by wind, precipitation and visibility?
How many birds are involved in reverse migration?
How do migrants react to anthropogenic offshore obstacles?
Are birds attracted by the illumination of these structures?
How many birds will collide?
Can days of high collision risk be predicted?
How can collisions be mitigated?
Which impacts on populations can we expect?
Several research projects were initiated by the German environmental authorities. The majority of the data and analyses presented here are derived from the project ‘BeoFINO’. The primary objectives were to collect data to address questions (1) to (7), based on measurements of bird migration over the German Bight with a variety of techniques, including radar, thermal imaging, collection of collision victims, and visual and acoustic observations. This is the first project to cover migration year-around continuously with such a variety of complementary methods.
METHODS AND DATA
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- METHODS AND DATA
Within the ‘BeoFINO’ project, automatic recording of bird migration on three unmanned offshore research platforms in the North and Baltic Seas was initially planned. However, owing to unexpected problems only platform ‘FINO 1’ (54°01′N, 06°35′E; http://www.fino-offshore.com/) is available. Remote observations including of ‘invisible’ bird migration became possible with the construction of this platform, where we installed two ship radars, a thermal imaging camera, a video camera and a directional microphone from October 2003 onwards. To allow spatial comparisons, we had to modify the sampling scheme, including the addition of human observers on islands.
Although bird migration over the North Sea takes place throughout the whole year, two periods of intensive migration (spring, autumn) recognizably alternate with two periods of minimal migration activity (summer, winter). Although these seasons have no real precise limits, we have defined them as follows: ‘spring’ (1 March to 31 May), ‘summer’ (1 June to 31 July), ‘autumn’ (1 August to 15 November) and ‘winter’ (16 November to 29 February). A meaningful subdivision of the day taking into account fluctuating daylength proved more of a problem, exacerbated by the diurnal activities of the gulls resident on the platform. Automatic video recordings (see below) showed gull activity before civil dawn and after civil dusk (when the sun is 6° below the horizon). Yet using nautical dawn and dusk as a reference (sun 12° below the horizon) also seemed impractical as it made the nights too short in summer. We therefore decided to use the median of these two values. This is equivalent to the sun's position at sunrise and sunset minus 9°. In order to take into account the early morning and evening peaks in migration activity we defined two further subdivisions of the day – each delimited by the value of sunset/sunrise ± 9°. This gives us four periods of the day that in the following text are always referred to as ‘morning’, ‘daytime’, ‘evening’ and ‘night’. Although there is always a change of date during the night period, because complete nights need to be regarded as a unit, we defined the date of night migration as that valid at the beginning of the night in question.
Sea-watching and passerine passage counts
Standardized systematic recordings of ‘visible’ bird migration (alternating ‘sea-watching’ covering recordings of waterbirds over sea, and ‘passerine passage counts’ covering recordings of passerines, pigeons, owls, swifts and woodpeckers) were carried out by observers on the offshore island of Helgoland (54°11′N, 07°55′E) and simultaneously on the coastal islands of Sylt (54°52′N, 08°17′E) and Wangerooge (53°47′N, 07°55′E) in the years 2003 and 2004 (for locations see Fig. 1, for methods see Hüppop et al. 2004, Dierschke et al. 2005). On Helgoland, sea-watching was carried out on 233 days throughout all seasons (see Fig. 2), passerine passage counts on 90 days from July 2003 to December 2004 (see Fig. 3). On Sylt, counts were conducted on 156 (sea-watching) and 98 (passerine passage counts) days in autumn 2003, spring 2004 and autumn 2004. On Wangerooge, 90 days were covered by sea-watching and 58 days by passerine passage counts in spring and autumn 2004.
Figure 1. Positions of the research platform FINO 1 and the investigation sites on the offshore island of Helgoland and on the coastal islands of Sylt and Wangerooge.
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Figure 2. Mean migration intensity per 5-day period (pentade) recorded by sea-watching on the islands of Wangerooge (n = 85 538), Helgoland (n = 87 098) and Sylt (n = 238 765) for main species groups (July 2003 to December 2004). Black bars under histograms indicate periods without sea-watching.
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Figure 3. Mean migration intensity per 5-day period (pentade) recorded by passerine passage counts on the islands of Wangerooge (n = 70 302), Helgoland (n = 21 908) and Sylt (n = 67 670) for main species groups (August 2003 to November 2004). Black bars under histograms indicate periods without counts. Note variable scaling of the y-axes.
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One radar system with a vertically rotating antenna recorded bird migration intensity and altitude at the FINO 1 platform (‘vertical radar’, Furuno FR-2115-B, X-band 9410 ± 30 MHz, 12-kW peak power, 24 r.p.m., used range scale 0.75 nm, pulse length 0.07 µs, pulse repetition rate 3000 Hz, with 8-ft antenna XN24AF, horizontal beam width 20°, vertical beam width 0.95°, side lobe attenuation within ± 10° 28 dB, side lobe attenuation outside ± 10° 32 dB). The radar had an open view to the west-southwest (248°), but, not to the opposite direction, which was obstructed by the helicopter deck. We assume that this is not important for the results and conclusions presented. A second radar, horizontally rotating, operated on FINO 1 from 30 October 2003 in order to record flight directions (‘horizontal radar’, Furuno FR-2125-B, X-band 9410 ± 30 MHz, 25-kW peak power, 24 r.p.m., used range scale 1.5 nm, pulse length 0.15 µs, pulse repetition rate 3000 Hz, with 8-ft antenna XN24AF, horizontal beam width 0.95°, vertical beam width 20°, side lobe attenuation within ± 10° 28 dB, side lobe attenuation outside ± 10° 32 dB). This radar scanned the sea north of the platform from 225° to 135°. Another vertical radar was in operation on the premises of the airfield on the island of Sylt (54°55′N, 08°21′E) with a viewing direction west-southwest and north-northeast from 8 June 2004 to 6 November 2004. The aim was to compare migration density and flight altitude at the coast and offshore. We dispensed with a horizontal radar on Sylt as only flight altitude and migration density were to be studied there.
Although both systems on FINO 1 have continued to operate within the follow-up project ‘FINOBIRD’ (see Conclusions), this article confines itself to the processing of the data from 1 October 2003 to 15 November 2004 (vertical radar) and those from 1 March 2004 to 31 May 2004 (horizontal radar). Regular inspection by means of a web camera (http://www.fino-offshore.com) occasionally showed large numbers of resting gulls on the helicopter deck of FINO 1. Hence, we assume that bird echoes outside the ‘night’ are partly attributable to foraging flights of gulls. By contrast, the number of traces recorded at ‘night’ is in agreement with the phenology described by Hüppop and Hüppop (2004) for ‘passerines’ on Helgoland, so that the vast majority of signals recorded in this period are undoubtedly attributable to migration. Therefore, some analyses were exclusively based on echoes recorded at ‘night’ so as to minimize coincidences with other flight activities.
In order to record bird echoes, a screenshot was taken of the current radar image every 5 min by the software IrfanView (http://www.irfanview.com) and subsequently digitized by hand. Thus, each system supplied us with the data from 12 images per hour with date and time to analyse. Despite occasional system crashes with concomitant data losses, the vertical radar was running for 67% of the investigation period on FINO 1 and generated a total of almost 80 000 images (69% on Sylt). All images that were more than 20% obscured by rain reflections were discarded. This applied to 7.8% of the images on FINO 1 and 7.3% on Sylt. Temporary short breaks in the radar recording also occurred within a 24-h period. In these cases only those periods of the day were included in the analysis, which provided at least 50% of the total number of images theoretically possible. Altogether, 62% of the time was covered by images suitable for analysis, both on FINO 1 and on Sylt. Of the 412 ‘nights’ in the 13.5-month recording period, 226 were entered into the analysis. Most of the missing nights belong to lengthy recording gaps caused by radar system breakdowns, which affected some annual segments more than others (Table 1). None the less, the radar measurements provide a unique almost continuous account of offshore bird migration throughout the annual cycle.
Table 1. Observation nights with vertical radar on FINO 1.
|October 2003|| 17|
|November 03|| 28|
|December 03|| 19|
|January 04|| 14|
|February 04|| 11|
|March 04|| 28|
|April 04|| 25|
|May 04|| 10|
|June 04|| 14|
|July 04|| 6|
|August 04|| 23|
|September 04|| 16|
|October 04|| 4|
|November 04|| 11|
The echoes of each vertical radar sweep were displayed on the screen and remained there with a different colour for the following ten sweeps. Birds flying parallel to the sweep left tracks of dots whereas those flying perpendicular to the sweep produced a single dot. The coordinates of these dots were manually digitized from the screenshots for further calculations. Migration intensities and flight altitudes could be derived from all echoes, while track inclinations and rough flight directions were derived only from tracks with a length of at least 35 m (equivalent to 10 pixels in an image or roughly five times the spatial resolution of the radar).
Whether or not a bird is detected by radar depends on a number of factors (Eastwood 1967, Bruderer 1997a, 1997b). The volume covered by a radar beam increases with distance, while the energy density of emitted and reflected radar beams decreases. This results in a complex relationship between the distance to an object and the probability of the object being detected by radar. In order to compensate for this distance-related ‘sensitivity’ of radar equipment when undertaking quantitative assessments, the number of echoes recorded had to be corrected for this change in detectability with distance from the radar antenna (Hüppop et al. 2002). Therefore, we adjusted the recorded echoes with the help of the program ‘Distance 4.1’ (http://www.ruwpa.st-and.ac.uk/distance/). This correction was based on the assumption that birds flying over the sea are horizontally equally distributed at such a small scale, and therefore that knowing radar cross-sections of birds, for example, is unnecessary. All values from altitudes between 50 and 150 m and at a distance of over 400 m from the platform were included. This was to eliminate data caused by the inclusion of flight echoes of gulls resident on the platform in order to ensure that data entered into the analysis came predominantly from migrating birds. Only a very few bird echoes were recorded from distances of over 1500 m, and these may have been over-adjusted in the correction. For this reason our calculations involve only values obtained from within a radius of 1500 m (for details see Hüppop et al. 2004).
For most of the time, the horizontal radar on FINO 1 produced images with bird traces largely obscured by wave and rain reflections. Consequently, radar images suitable for analysis were obtained only on a few days of light wind and as a result only 4.1% of the total images produced by the horizontal radar were in fact digitized. Presumably, therefore, the horizontal radar did not render representative results. Subsequently, we have attempted to improve the system by modification of the antenna design.
Thermal imaging, video camera and microphone
We developed the new software IRMA (‘Infra red Registration of Migrating Aves’ using DSPack 2.31: http://www.progdigy.com) that detects flying birds or bats within the real-time images provided by our thermal imaging camera (Zeiss Optronics Opus M: with an uncooled ferroelectric detector, a resolution of 240 lines with 320 pixels each, a spectral range of 7.5–13.5 µm, and automatic adjustment of gain and dynamic range). The field of view of its 75-mm lens was 12 × 9° to the north and with an elevation angle of 60° into the open sky. Therefore, we were able to record movements close to the platform around the clock without any supplementary light necessary, and thus obtain indications of intensity, flock size and flight behaviour under different environmental conditions.
During daylight, a video camera (Panasonic AW-E600E) with motor zoom lens on a pan-and-tilt head was used as our ‘sea-watcher’ on the unmanned platform. Unfortunately, we were not able to observe birds remotely because of insufficient resolution, combined with a limited field of view and internet bandwidth problems.
Bird calls close to the platform were detected and recorded automatically by a directional microphone (Sennheiser ME67) with the specially developed software AROMA (‘Acoustic Recording of Migrating Aves’ based on the audio-processing toolkit ‘Snack’ of Tcl/Tk: http://www.speech.kth.se/snack).
Owing to the lack of offshore wind farms in German waters, investigations on collisions with man-made offshore structures had to be confined to the FINO 1 platform. On each of 44 visits to the facility by helicopter (more or less equally distributed over the period October 2003 to December 2004) all bird carcasses found were documented. The bird remains were generally taken to the laboratory, where measurements were taken and an attempt was made to establish the cause of death. This was done by thoroughly examining each individual for external injuries, contusions and fractures. Additionally, a few birds were X-rayed.
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- METHODS AND DATA
Our findings confirm that large numbers of diurnal and nocturnal migrants cross the German Bight, with considerable variation of migration intensity, time, altitude and species, depending on season and weather conditions. The enormous variability makes precise analyses very difficult and requires further investigations. Dierschke (2003) estimated from systematic visual observations that in 18 species significant proportions (> 1%) of the respective biogeographical population pass Helgoland during migration, with more than 10% of the population in Red-throated Diver, Pink-footed Goose, Greylag Goose, Brent Goose and Little Gull Larus minutus. From the radar data presented here it can be concluded that also large proportions of nocturnally migrating species fly across the German Bight, although the species composition is partly unknown. With regard to future wind farms it is important to emphasize that almost half of the birds fly at ‘dangerous’ altitudes. In addition, the number of individuals in reverse migration is considerable, which increases the risk of collision. Normally, migrating birds seem to avoid obstacles, even at night (Isselbächer & Isselbächer 2001, Schmiedel 2001, Desholm & Kahlert 2005), which diminishes collision risk but increases flight costs. However, we were able to demonstrate that under poor visibility, caused by drizzle and mist, terrestrial birds in particular are attracted by illuminated offshore obstacles. We documented with our thermal images that disorientated birds flew around the platform repeatedly, so that both their risk of collision and their energy consumption increased (see also Hope Jones 1980).
Even if the findings regarding collisions at the research platform FINO 1 cannot directly be applied to offshore wind farms, they do show that on a few nights per year a large number of avian interactions at offshore plants can be expected, especially in view of the number and planned area of projected wind farms. Owing to the methods used, previous studies at Swedish or Danish offshore wind farms have been able to investigate diurnal collisions only in good weather conditions, which are expected to be few in number (e.g. Pettersson 2005), or refer only to large species such as geese and ducks (e.g. Desholm & Kahlert 2005), but not to small bird species, which according to our findings are most frequently involved in collisions.
Regardless of the existing knowledge gaps, some mitigation measures can be recommended:
Abandonment of wind farms in zones with dense migration.
Alignment of the turbines in rows parallel to the main migratory direction.
Free migration corridors of several kilometres width between wind farms.
Avoidance of construction of wind farms between, for example, resting and foraging grounds.
Turning off turbines in nights predicted to have adverse weather and high migration intensity.
Refraining from large-scale continuous illumination.
Taking measures to make wind turbines more recognizable to birds.
In particular, the penultimate of these measures urgently requires appropriate experiments involving the brightness and colour of wind farm illumination in order to minimize collision rates. Perhaps the most effective solution would be to use lighting that is adjusted to the weather conditions, e.g. flash-light with long intervals instead of continuous light in fog and drizzle. During the very few nights in which a high frequency of bird strikes is expected, in predicted adverse weather conditions with high migration intensity, turning off turbines and adjusting the rotor blades to minimize their surface relative to the main direction of migration could be helpful in reducing collision extent.
Our findings lead to the further conclusion that a combination of methods is necessary to describe the complex patterns of migration over the sea. However, even with virtually non-stop recording as on FINO 1, the wide variation in bird migration and in weather (together with its effect on the former) lead to an insufficient number of samples per weather situation. The funding of further research in the follow-up project ‘FINOBIRD’ (financed by The German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety, grant no. 0329983) is a response to this problem. The recordings are to be continued with the aim of refining the results presented here. Furthermore, we plan to develop a model to ‘forecast’ bird migration over the German Bight with the aid of weather forecasts, for example to establish a basis for mitigation measures. However, as long as no investigations at existing plants are carried out to provide reliable data on collisions and avoidance behaviour, the actual scale of these problems will remain a matter of speculation. We expect more information on avoidance behaviour and collisions with the construction of a pilot wind park close to the FINO 1 platform (not before 2007).