Flight tracks and speeds of Antarctic and Atlantic seabirds: radar and optical measurements

Abstract

A tracking radar and an optical range-finder, placed on a ship, were used to register the flight of eleven species of seabirds, in waters off the Antarctic Peninsula and in the Atlantic Ocean. Albatrosses under calm conditions used swell soaring, turning and twisting extensively within a width of 300-500 m laterally from the overall direction of movement. Their resulting travel speed was on average 10 m s -1 . In windy conditions the albatrosses as well as giant petrels travelled faster, with resulting speeds up to 22.5 m s -1 , by a combination of wave soaring and dynamic soaring. Shearwaters and the antarctic fulmar proceeded by flap-gliding, along tracks that were only slightly zigzag within 50-60 m from the resulting course of movement. The little shearwater flew faster, with an airspeed about 14 m s -1 , than larger-sized shearwaters and fulmars, using continuous flapping flight to a higher degree than its larger relatives. South polar skuas and Wilson’s storm-petrels were tracked on foraging flights, and flocks of imperial shags on commuting flights between feeding and breeding-roosting areas. The south polar skua was able to accelerate to airspeeds exceeding 20 m s -1 in pursuit flights after shags. Wilson’s storm-petrels showed significantly slower airspeeds in foraging flights as compared to non-foraging flights. Average airspeeds of most species fell between the minimum power and maximum range speeds estimated from aerodynamical theory. Species using gliding or flap-gliding flight showed a mean airspeed close to the gliding speed for best glide ratio. Optimal speeds in foraging flights, as expected for the south polar skuas and Wilson’s storm-petrels, are unlikely to coincide with the minimum power and maximum range speeds. Albatrosses reached the fastest resulting travel speeds when moving at angles 120°-150° from the wind (partly following winds), with strong wind forces. They predominantly travelled with the wind from their left side which, in the southern hemisphere, would lead them away from low pressure centres and towards high pressure areas.