What flight heights tell us about foraging and potential conflicts with wind farms: a case study in Lesser Black-backed Gulls (Larus fuscus)

Avian flight heights are currently a focus of interest in terms of assessing possible impacts of offshore and inland wind farms on birds. We therefore analyzed the flight-height distribution in a tracking study of foraging Lesser Black-backed Gulls (Larus fuscus) on the southern North Sea coast during the incubation period. We distinguished between marine and terrestrial, nocturnal and diurnal, straight and tortuous, and outbound and inbound flights. Individuals were equipped with specifically programmed GPS data loggers to ensure accurate flight-height measurements. A total of 89 % of recorded fixes were below 20 m above sea level, indicating an overlap between foraging flights and the rotor area of most operating wind turbines. The gulls flew lower over the sea than over land, and lower at night than during the day. Straight commuting flights were higher than tortuous flights, when the gulls were supposed to be foraging. Outbound and inbound flights occurred at similar heights, and flight height was unaffected by wind. This study provides insights into the individual flight-height distribution in a common seabird species throughout a range of foraging behaviors. These results might prove important for developing a comprehensive understanding of bird movements within and around wind farms, and the potential impacts of such wind farms on foraging patterns.