Regulation of wing or foot strokes in deep diving seabirds: a comparison between South Georgian Shag, Common Murre and Macaroni Penguin
In the wild, seabirds dive longer than their theoretical aerobic dive limit (oxygen store divided by oxygen consumption rate). One of the potential explanations for this is that when descending in water seabirds regulate their foot or wing strokes to compensate their upward buoyancy, which decreases with the current depth to reduce oxygen consumption rate. The change in frequency of wing or foot stroke with current depths was compared between South Georgian Shag Notocarbo georgianus (SGSG), Common Murre Uria aalge (COMU), and Macaroni Penguin Eudyptes chrysolophus (MCPN) using biologging techniques. When descending to 60 m depth, the frequency of thrust and foot stroke decreased in SGSG, which make a forward thrust at each foot stroke. While in COMU, the frequency of thrust decreased, whereas wing stroke was nearly constant. COMU made a thrust with each up- and down-stroke in shallow water but only with each down-stroke in deeper water. In MCPN, however, the frequency of thrust did not change with the current depth. The MCPN made strokes at a nearly constant rate, while making thrusts with each up- and down-stroke at any depth. Because of the shallower descent angle of MCPN, compared with those of SGSG and COMU, MCPN should require relatively little thrust in order to compensate for its buoyancy and thus might not need to change the frequency of thrust with current depth. Swimming speeds during descent were nearly constant (1.4–2.2 m/s) in SGSG, COMU and MCPN. These three species appeared to regulate their strokes to maintain a range of swimming speeds. Biologging techniques reveal the biomechanical regulation of movement of birds under morphological and physical constraints. This provides a new approach to understanding the behavioural ecology of birds flying in the air and water.