Telemetric monitoring of eagle owls

With an estimated number of 400 breeding pairs, the eagle owl is almost present throughout Schleswig-Holstein. Aim of this telemetry study is to provide a more accurate estimate of the risk for eagle owls to collide with wind turbines. Habitat use and flight behaviour of the nocturnal eagle owl in the vicinity of existing wind farms will be investigated using modern satellite telemetry systems. It is planned to collect data for several eagle owl breeding pairs (tagging of ten adult birds) over a period of two years. The study is the first of its kind in Schleswig-Holstein and is carried out on behalf of the Landesverband Eulen-Schutz Schleswig-Holstein funded by the the Ministry of Energy, Agriculture, the Environment, Nature and Digitalization (MELUND). Cooperating partner is the Bielefeld University.

The GPS/GSM transmitters were purpose-developed for this project. They were programmed to record one GPS location per hour during the day and two GPS locations per hour during night. An acceleration sensor enables to detect flight movements which are recorded with a resolution of one GPS location per second resulting in high resolution, three-dimensional recordings of the flight. Data is transmitted once per day and can be accessed online.

In 2017, four adult birds from three territories were equipped with transmitters in the Schleswig area. Decisive for the choice of these eagle owls was spatial vicinity of the eagle owl territories to wind farms. In case of two of the breeding sites a total of 12 wind turbines are located within the area of possible impact (1 km) and about 60 wind turbines in the range of verification for feeding sites (4 km).

First results

• The home ranges of the investigated eagle owls were large. The 95 % kernel areas (areas the eagle owl uses with a probability of 95 %) were between 14.3 und 20.9 square kilometer in size.
• Currently, a distance of 1 km is recommended between breeding site and wind turbine. In this study, this includes about 61 and 73 % of all recorded locations of the birds.

  

• The results of this study give no indication of avoidance behaviour of eagle owls towards wind turbines. Depending on the flight height, lack of avoidance behaviour towards wind turbines represents a fundamental collision risk. How far eagle owls show smallest scale avoidance behaviour could not be determined based on the data available so far.
• Flight parameters (time with flight activity, covered distance, duration and especially flight height) of eagle owls do not lead to an increased risk of collision with wind turbines. Three quarters of all flight height were recorded below 20 m. Eagle owls only fly for a short time (median 11 to 14 s) and cover short distances (median 90 to 135 m). Frequent stops probably serve to locate prey acoustically.

  

• Eagle owls use hedgerows to a limited extent only as guidance.

  

• Farms are specifically approached. This may be due to corn silage attracting rats.
• The population density of eagle owls and the size of home ranges result in presence of eagle owls throughout the assessment area. This is particularly interesting with regard to predation of common buzzards in the same area (see project “Decline of common buzzards”)