Shags take advantage of headwind to take off

Abstract

During the nesting season, the pressure is on for avian mums and dads to provide as much nourishment as possible for their chicks. For seabird parents, this can mean costly foraging trips across the waves. Katsufumi Sato from the University of Tokyo, Japan, explains that it was thought that seabirds may improve their flight cost efficiency by flying into headwinds to increase their air speed (the speed relative to the prevailing wind). However, it was not known whether birds actively choose to fly into the wind to reduce their costs and few studies had recorded seabird flights in enough detail to find out whether the seabirds take advantage of headwinds. So, Sato and his colleagues Yukihisa Kogure and Yutaka Watanuki added tiny accelerometers to the new generation of lightweight GPS tracking systems, which enabled them to use the equipment to follow foraging seabird flights with sufficient accuracy to assess whether the wind helps improve their efficiency.Travelling to Scotland to work with long-term collaborators Francis Daunt and Sarah Wanless from the Centre for Ecology & Hydrology, UK, Sato and his colleagues were ready to track the flights of foraging European shags. Over three consecutive breeding seasons, the scientists caught the boat to the Isle of May off Scotland where they carefully attached accelerometers and GPS trackers to the necks and backs of 14 male shags before returning the birds to their nests. Then, having measured the wind speed at the highest point on the island and after successfully retrieving the data loggers, Sato and Kogure were faced with the gargantuan task of reconstructing the bird flight paths from thousands of GPS coordinates, in addition to measuring the birds' wing beat patterns from the acceleration traces, before calculating the birds' flight and air speeds.Having analysed 147 flights, Sato and Kogure could see that the birds flapped their wings harder during take-off from the water than when cruising through the air, suggesting that they were expending more energy as they became airborne. Also, all of the birds took off into a headwind regardless of their final destination, to take advantage of the lift generated by the wind rushing towards them. And on two occasions – when the wind speed was in excess of 8 m s−1 – the birds were able to become airborne without the usual exertions by simply flapping their wings as if they were cruising.However, once they were aloft, the wind direction seemed to have little effect on the shags' choice of course, with the birds flying in all possible directions. And when the team estimated the birds' air speed, by subtracting the wind speed from the ground speed, they found that it was relatively stable at around 15 m s−1, unless the birds were flying into a strong headwind, when they flapped their wings harder and increased their air speed. The team suspects that instead of flying at the lowest power that allows them to remain aloft, European shags fly at a speed that allows them to travel the greatest distance for the least energy consumed during the flight. They also suggest that the birds are able to adjust their flight strategy to take the greatest advantage of any assistance that the wind can give them. However, they warn that the stronger winds that are predicted as a consequence of climate change could have beneficial and detrimental effects on foraging shags, giving the birds a helping hand to take off while hampering them as they battle headwinds once airborne.