Flight heights of Scopoli s shearwaters Calonectris diomedea in the context of offshore wind farm developments

Seabird face new risks through collisions with offshore wind turbines. Wind energy projects are emerging in the Mediterranean Sea, and unbiased knowledge on seabird flight altitudes are scarce in this area. Indeed, previous flight height observations were carried out from boats during daytime, and only in good weather conditions. We measured flight heights of 13 Scopolis shearwaters, Calonectris diomedea, one of the three endemic shearwater species in the Mediterranean, using highly precise barometric data loggers. The birds were equipped at the largest French colony of Scopolis shearwater in the Marseille archipelago. Birds from this location routinely visit forthcoming wind farm areas, within 50 km of their breeding site. We found that Scopolis shearwaters flew at very low altitudes with a mean flight height of 1.8 {+/-} 2.7 m ({+/-} SD) above sea level. Birds therefore rarely (<0.02%) flew within the vertical envelop of collision risk with wind turbines. Ours results are coherent with field observations and the dynamic soaring flight technique used by shearwaters. Nevertheless, collision risk may increase following behavioural modifications close to wind turbines.