Diving angle of great cormorants
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Seabirds can maximize the relative time spent at depths where prey occur by minimizing the commuting time taken to reach these depths. One way to achieve this goal is to modify dive angle, but there are few measures of dive-angle in free-foraging seabirds. In 2003, we monitored simultaneously the swimming speeds and diving depths of great cormorants (Phalacrocorax carbo) foraging off the Greenland coast, and used these data to reconstruct their descent angle. Both males and females dived on average <5 m, but some dives were up to 26 m. Dive angles did not differ between dive types (benthic, pelagic). Angles did not change much for dives <12 m but increased with increasing maximum depth in both males and females for dives>12 m. We suggest that birds are able to reduce their descent time for dives beyond this depth by performing pre-dive leaps that allow them to use the momentum of the fall to descend almost vertically and at great speeds. Such pre-dive leaps in shallower dives would be unsuitable because of the proximity of the seabed and the risk of startling prey. Finally, in contrast with deeper divers, descent angles were not steeper when undulations were observed in the depth profile of the previous dive, probably because birds feed on dispersed prey.
- Polar bioscience
Polar bioscience (18), 54-59, 2005-01-01
National Institute of Polar Research