Cormorants are superb at catching fish. Diving near the surface, cormorants easily outpace most fish. However, cormorants fight a constant battle against buoyancy caused by air trapped in their lungs and plumage. They overcome the tendancy to float by swimming fast, but how do the predators manage to execute tight turns when pursuing a tasty treat? Gal Ribak explains that most creatures slow down before turning, but a loss of speed would place a cormorant at risk of bobbing to the surface. Ribak and his colleagues Daniel Weihs and Zeev Arad from the Technion, Israel, decided to see how cormorants overcome their buoyancy while diving through a submerged obstacle course().
According to Ribak, the team had no problems finding cormorants to work with. He explains that great cormorants regularly raid local tilapia and carp farms, so the team were able to rescue two wild birds tangled in nets above ponds. The rest had been raised in captivity from eggs and had taken to diving with ease.
Encouraging the cormorants to dive though a 1 m deep tunnel in a pool was also straightforward; `They'll do anything for fish,' says Ribak. Having trained the birds to dive through the tunnel, the team introduced three obstacles: a barrier across the top of the tunnel near the entrance, a barrier across the bottom of the tunnel at the middle, and a barrier across the top near the end. The cormorants had to swim under the first barrier, over the second and under the last, making a bell shape as they passed through the tunnel. After several days of practice, the team attached a marker to the birds' wing and filmed the birds from the side as they negotiated the obstacles. How tight a turn could the birds manage? Gradually moving the two outer barriers inwards, the team eventually narrowed the bird's bell-shaped swim from a width of 180 cm to a minimum of 72 cm; the radius of the birds'tightest turn was less than half their length.
After weeks of filming, Ribak began analysing the birds' trajectories and speeds and found that the birds did slow a little as they manoeuvred between the barriers, but not much. Even during the tightest manoeuvre, the birds only reduced their speed by 12%. So which forces were driving the divers through their high-speed rollercoaster ride?
Measuring the angles that the cormorants' tails, bodies and necks made relative to horizontal, and calculating the turning forces they generated during the tight vertical turn, Ribak found that the birds took advantage of their buoyancy to bob upwards as they approached the second barrier. But which part of the body was generating the forces needed to dive down? According to Ribak it's the tail, which pushes the body into the correct orientation to generate sufficient downward force to overcome buoyancy and allow the bird to dive down again. Ribak adds that the cormorant's long neck contributes little as the bird overcomes its buoyancy, but the team suspect that the neck's flexibility could allow the bird to snap up fish that might otherwise out manoeuvre them.
