How do cormorants counter buoyancy during submerged swimming?

Abstract

Buoyancy is a de-stabilizing force for diving cormorants that forage at shallow depths. Having to counter this force increases the cost of transport underwater. Cormorants are known to be less buoyant than most water birds but are still highly buoyant (ρ=~0.8·kg·m -3 ) due to their 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 discover and evaluate the mechanisms countering buoyancy while swimming horizontally. The birds maintained a very uniform cyclic paddling pattern. Throughout this cycle, synchronized tilting of the body, controlled by the tail, resulted in only slight vertical drifts of the center of mass around the average swimming path. We suggest that this tilting behavior serves two purposes: (1) the elongated bodies and the long tails of cormorants, tilted at a negative angle of attack relative to the swimming direction, generate downward directed hydrodynamic lift to resist buoyancy and (2) during the propulsive phase, the motion of the feet has a significant vertical component, generating a vertical component of thrust downward, which further helps to offset buoyancy. The added cost of the drag resulting from this tilting behavior may be reduced by the fact that the birds use a burst-and-glide pattern while swimming.

Regulation of stroke and glide in a foot-propelled avian diver

“…Buoyancy in cormorant species of similar mass to European shags has previously been estimated to be between 2.2 and 8·N at the surface, to have decreased markedly by 20-40·m, and to remain positive down to depths of at least 100·m (Lovvorn et al, 1991b;Wilson et al, 1992a;Riback et al, 2004). In our study, in which shags did not dive deeper than 40·m, birds ascended passively from this depth, indicating that they had positive buoyancy down to at least 40·m.…”

“…In cormorant species the power stroke is generated by sweeping the webbed feet backwards and upward to make dragand lift-based forward thrust (Johansson and Norberg, 2003). In great cormorants Phalacrocorax carbo swimming horizontally in shallow water, the body is accelerated downward and forward during the power stroke of the feet and decelerated during the glide or recovery phase (Riback et al, 2004). For shags we also found that when the body accelerated downwards, it simultaneously accelerated forwards (Fig.·3A,B).…”

“…Great cormorants swimming horizontally in shallow (<1·m) water have a negative tilt (angle of body axis against swimming direction) of 6-15°, which gives negative lift, presumably to compensate high buoyancy (Riback et al, 2004). When the shags have a body angle of -70°during descent, the maximum error of estimated speed is 15% allowing 15°of tilt.…”

“…With the exception of penguins that swim by producing lift during both upstrokes and downstrokes (Bannasch, 1995), diving seabirds generate forward thrust mainly by the contraction of major muscles in order to make a propulsive downstroke with their wings or push their webbed feet backwards (hereafter referred to as the power stroke; Alexander, 1992;Rayner, 1995;Riback et al, 2004). Birds decelerate (hereafter glide) either during the upstroke of the wings or during the forward movement of the feet.…”

“…To maximize feeding time, bottom-feeders should minimize transit time, and hence descend and ascend vertically (Houston and Carbone, 1992;Hansen and Ricklefs, 2004). Although shags and cormorants are less buoyant than many other diving birds, in general, they are positively buoyant over the range of depths used for foraging (Wilson et al, 1992a;Husler, 1992;Riback et al, 2004). Shags and cormorants make forward thrusts by pushing both of their webbed feet backwards simultaneously, i.e.…”