Ph.D Thesis

Ph.D StudentRibak Gal
SubjectHydrodynamic Aspects of Cormorant Underwater Swimming
DepartmentDepartment of Biology
Supervisors PROFESSOR EMERITUS Zeev Arad


Cormorants are piscivorous birds foraging by means of submerged swimming. When underwater, they propel themselves by synchronized motions of the webbed feet while their wings are folded next to the body. Their primary adaptation for flight conflicts with adaptations for low cost, underwater swimming, resulting in high buoyancy during diving. To study how cormorants cope with the conflicting mechanical demands of submerged swimming and flight, I combined morphometric measurements on carcasses with the use of live cormorants trained to swim in a controlled experimental setup. The birds were filmed while swimming horizontally at 1 m depth. Their 3-dimensional kinematics was analyzed to model and measure the forces they produce during swimming. I then conducted experiments to test predictions emerging from the model. The cormorants produced large vertical hydrodynamic forces as they swim. These forces counter 85% of the buoyancy, as estimated from cormorant carcasses. The tilt angle of the body during swimming controls the magnitude of these vertical forces. The tail, serving as a control surface, controls the tilt of the body. The thrust supplied by the feet results in a large vertical component of thrust assisting to counter buoyancy. The paddling cycle includes glides between consecutive strokes and results in a "burst and glide" swimming pattern, thus lowering energy expenditure during swimming by 24% compared to constant paddling. These mechanisms to counter resistance and buoyancy allow cormorants to forage in a wide range of foraging sites and explain the success of these flying birds as aquatic predators.