It's tough catching your dinner in icy waters; every time a cormorant fancies a snack it has to plumb the depths. Manfred Enstipp from the Centre d'Ecologie et Physiologie Energétiques in France explains that quite a lot is known about the energetic costs of shallow dives close to the surface,but no one had measured the cost of deep dives. Knowing that birds are less buoyant at depth when the air trapped in their plumage is compressed, the team wondered whether the decreased energy costs of swimming at depth were greater than the cost incurred keeping warm after losing their insulating layer(). Enstipp,David Grémillet and David Jones measured cormorant oxygen consumption when returning to the surface after shallow horizontal 1 m dives and deep vertical 10 m dives, and found that the bird's metabolic rate was 22% greater after a deep dive than a shallow dive. The team suspect that the metabolic advantage of swimming at depth is far outweighed by the cost of maintaining their body temperature in the chilly depths after losing their thick layer of insulating air.
