Upstroke Thrust, Drag Effects, and Stroke-glide Cycles in Wing-propelled Swimming by Birds1

SYNOPSIS. A number of bird species swim underwater by wing propulsion. Both among and within species, thrust generated during the recovery phase (upstroke) varies from almost none to more than during the power phase (downstroke). More uneven thrust and unsteady speed may increase swimming costs because of greater inertial work to accelerate the body fuselage (head and trunk), especially when buoyant resistance is high during descent. I investigated these effects by varying relative fuselage speed during upstroke vs. downstroke in a model for wing-pro-pelled murres which descend at relatively constant mean speed. As buoyant resis-tance declined with depth, the model varied stroke frequency and glide duration to maintain constant mean descent speed, stroke duration, and work per stroke. When mean fuselage speed during the upstroke was only 18 % of that during the downstroke, stroke frequency was constant with no gliding, so that power output was unchanged throughout descent. When mean upstroke speed of the fuselage was raised to 40 % and 73 % of mean downstroke speed, stroke frequency declined and gliding increased, so that power output decreased rapidly with increasing depth. Greater inertial work with more unequal fuselage speeds was a minor con-tributor to differences in swimming costs. Instead, lower speeds during upstrokes required higher speeds during downstrokes to maintain the same mean speed, resulting in nonlinear increases in drag at greater fuselage speeds during the power phase. When fuselage speed was relatively higher during upstrokes, lower net drag at the same mean speed increased the ability to glide between strokes, thereby decreasing the cost of swimming