As land-dwellers, we tend not to worry about drag – but hungry cormorants ploughing through water to snack on fish are all too aware of its effects. Submerged cormorants propel themselves forwards by briskly paddling both feet and then gliding until drag slows them down to their original speed. Gal Ribak, Daniel Weihs and Zeev Arad at the Israel Institute of Technology decided to see if using this burst and glide gait helps the birds save energy while chasing their prey ().
To keep its buoyant body submerged while hunting, a cormorant swims with its body tilted in a `head-down-tail-up' orientation.
But this posture increases the bird's drag, making it an energetically costly solution. To see if cormorants save energy by gliding, the team filmed cormorants swimming underwater. Analysing the cormorants' speed profiles, the team calculated the birds' drag during gliding. Then, using a mathematical model of burst and glide swimming, they calculated the birds' drag during active paddling. They found that the birds' drag during paddling was 2-3 fold higher than the drag during gliding, but this difference was smaller when the birds swam slower and tilted their bodies at a larger angle. So gliding provides the greatest energy savings when the birds swim fast. The team's model predicts exactly the same outcome. But the model also indicates that as cormorants dive deeper and become less buoyant, the burst and glide gait may allow slow swimmers to save energy too.
