Ecology of tern flight in relation to wind, topography and aerodynamic theory

• Published in: Philosophical Transactions of the Royal Society B: Biological Sciences
• Main Authors: Hedenström, Anders, Åkesson, Susanne
• Format: Text
• Language: English
• Published: The Royal Society 2016
• Subjects: Articles; Arctic; Southern Point
• Online Access: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4992720/; http://www.ncbi.nlm.nih.gov/pubmed/27528786; https://doi.org/10.1098/rstb.2015.0396

Flight is an economical mode of locomotion, because it is both fast and relatively cheap per unit of distance, enabling birds to migrate long distances and obtain food over large areas. The power required to fly follows a U-shaped function in relation to airspeed, from which context dependent ‘optimal’ flight speeds can be derived. Crosswinds will displace birds away from their intended track unless they make compensatory adjustments of heading and airspeed. We report on flight track measurements in five geometrically similar tern species ranging one magnitude in body mass, from both migration and the breeding season at the island of Öland in the Baltic Sea. When leaving the southern point of Öland, migrating Arctic and common terns made a 60° shift in track direction, probably guided by a distant landmark. Terns adjusted both airspeed and heading in relation to tail and side wind, where coastlines facilitated compensation. Airspeed also depended on ecological context (searching versus not searching for food), and it increased with flock size. Species-specific maximum range speed agreed with predicted speeds from a new aerodynamic theory. Our study shows that the selection of airspeed is a behavioural trait that depended on a complex blend of internal and external factors.