RIBAK, G*; ARAD, Z; KLEIN, N; WEIHS, D; Technion – Israel Institute of Technology; Technion – Israel Institute of Technology; Technion – Israel Institute of Technology; Technion – Israel Institute of Technology: Buoyancy resisting mechanisms applied by cormorants in submerged swimming

Buoyancy is a destabilizing force for diving cormorants that forage at shallow depths, increasing the cost of transport underwater. Cormorants are known to be less buoyant than most water birds but are still highly buoyant (~810 kg m-3) due to the adaptations for aerial flight. Nevertheless, cormorants are known to dive at a wide range of depths including shallow dives, where buoyancy is maximal. We analyzed the kinematics of underwater swimming of the great cormorant (Phalacrocorax carbo sinensis) in a shallow pool to study the mechanisms cormorants developed to counter buoyancy while swimming horizontally. The birds maintained a very uniform paddling pattern. Throughout this pattern, synchronized tilting of the body, controlled by the tail, resulted in slight vertical drift of the center of mass around the average swimming path. When the experiment was repeated with reduced buoyancy by attaching weights to the same birds, the tilting of the tail and body was much reduced. We suggest that this tilt serves two purposes: 1) the elongated bodies and long tails of cormorants, tilted at a negative angle of attack relative to the swimming direction, may generate lift to resist buoyancy. 2) during the active phase, the motion of the feet has a significant vertical component, generating a vertical component of thrust which also helps to offset up-drift. The added cost of the drag resulting from this swimming behavior may be reduced by the fact that the birds use a burst and glide pattern while swimming.