Dive angle, swim speed and wing stroke during shallow and deep dives in Common Murres and Rhinoceros Auklets

Abstract

To optimize oxygen usage and foraging, alcids diving by wing-propulsion are expected to regulate body angle, swim speed and wing stroke in relation to the change of buoyancy with the current depth. They may also regulate the diving behaviour in relation to the maximum depth of each dive. We measured body angle, swim speed and wing stroke in vertically diving Common Murres (Uria aalge) and obliquely diving Rhinoceros Auklets (Cerorhinca monocerata) using bird-borne data-loggers that recorded pressure and acceleration. During the descent, the body angle became shallower with increasing current depth in both species. Rhinoceros Auklets kept their swim speed within a range during descent to 40 m. Common Murres slightly increased their swim speed during the descent from the surface to 20 m depth, then they descended with a range of swim speeds in deeper current depths. Both species could make a thrust at each of down- and up-strokes. Possibly to regulate the swim speed during descent, both species decreased the frequency of thrusts by reducing the amplitude of longitudinal acceleration at the up-stroke and the frequency of wing strokes by increasing the duration of up-strokes. During the ascent, both species stopped wing strokes at certain depths and then, ascended passively with increasing swim speed. Both species descended with steeper body angles but with a similar swim speed at depth when they made deeper dives, indicating that the birds would reduce the transit time especially in deep dives. These similarities of the regulation of diving behavior in two species that have different foraging patterns imply a strong biomechanical constraint on flying-diving alcids.