Estimating flight heights of seabirds using optical rangefinders and GPS data loggers: a methodological comparison
Determining the collision risk of seabirds with offshore wind farms is crucial for the environmental impact assessment of such installations. The collision risk is often assessed by measuring avian flight heights. Therefore, we measured flight height distributions of 15 seabird taxa, abundant in German offshore waters, with an optical laser rangefinder (n = 2508 measurements). For lesser black-backed gulls, we compared these rangefinder measurements to flight heights recorded by GPS data loggers which were attached to 17 birds during incubation. Both methods have specific advantages and disadvantages. Rangefinder measurements are only possible during the day, and rain or fog prevents successful measurements. Data were negatively biased against low and very high flight heights. Since measuring low-flying birds proved more difficult during application of the method in the field, observers should be instructed to dedicate extra effort to measure low-flying birds. Visual observations of low-flying birds can also help to reduce uncertainty and overcome bias problems. Flight heights ranged from – 3 to 431 m above sea level. Most seabirds (70%) flew below rotor level (30 m), but about 30% (mainly large gulls and cormorants) flew in the rotor-swept area. GPS height measurements show a similar general pattern of flight height distribution as rangefinder measurements. However, this method is restricted to few individuals. Both methods complement each other and can provide a reliable estimate of seabirds’ flight height distribution.
We thank J. Kotzerka, B. Mendel, J. Sommerfeld and V. Corman for help with data logger fieldwork, and all observers who measured flight heights during boat-based SAS surveys. N. Markones managed the survey database and provided help with database queries. We also thank the National Park Administration of the Wadden Sea National Park of Lower Saxony (Germany), and J. Dierschke and the Institute of Avian Research on Helgoland/Wilhelmshaven (Germany) for logistic support during fieldwork. G. Peters gave technical support with the data loggers. S. Furness provided linguistic support. Major parts of the fieldwork were funded within the frameworks of the projects TESTBIRD (Federal Ministry for the Environment, Nature Conservation, Building and Nuclear Safety; Grant no. 0327689A/FTZ3), WINDBIRD (Federal Ministry for Economic Affairs and Energy on the basis of a decision by the German Bundestag; Grant no. 0325281) and the Marine Biodiversity Monitoring Programme of the German Federal Agency for Nature Conservation (BfN).
Compliance with ethical standards
Conflict of Interest
The authors declare that they have no conflict of interest.
All applicable national and institutional guidelines for the capture and handling of animals were followed. All birds were caught, ringed, and equipped with data loggers under licenses issued by the State Agency for Agriculture, Environment and Rural Areas Schleswig–Holstein and the Ministry of Energy transition, Agriculture, Environment and Rural Areas Schleswig–Holstein, Germany (file numbers: V 312-72241.121-37 (34-4/11), V 312-7224.121-37 (80-6/13)), the National Park Administration of the Wadden Sea National Park of Lower Saxony, and the Lower Saxony State Office for Consumer Protection and Food Safety (file number: 33.14-42502-04-11/0666). All birds were handled in strict accordance with good animal practice. Handling time of all birds was reduced to a minimum to minimize stress during capture.
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