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A giant of the seas
The Great Cormorant, as its name indicates, is a sea bird of very large size : it can measure up to 1 m in length and have a wingspan of more than 1.5 m. It lives on all the continents except South America and the Antarctic. A protected species since the Amoco Cadiz catastrophe, the Great Cormorant has proliferated in France. It is estimated that numbers have increased from 7 000 birds in 1977 to 91 000 in 2001.
The Great Cormorant is a massive consumer of fish : it eats between 400 and 700 grams a day. Sometimes, it is even accused of emptying fish farms and destocking rivers frequented by anglers. The bird is an excellent diver and can swim to a depth of 20 metres or so to seek out its prey.
A permeable plumage …
Most diving birds, such as penguins, have impermeable feathers. When they penetrate below the surface of the water, a layer of air trapped under the plumage acts as insulation. As a consequence, they can spend their time in very cold water.
The Great Cormorant’s feathers are not impermeable. Nevertheless, some of these birds spend the winter in the icy waters off Greenland. Their secret : a plumage half permeable and half impermeable.
… but efficient
The semi-permeable feathers of the Great Cormorant allow air to leave and water to penetrate the plumage. The end result is that the bird retains a thin layer of air beneath its plumage. This is sufficient to enable it to retain its body heat during the dive. Furthermore, the thinness of the air layer allows the cormorant to swim more efficiently. Indeed, the thicker the layer of air, the greater the tendency of the bird to come back up to the surface, an effect that would hinder swimming performance.
Since the plumage of the Grand Cormorant is not totally water-resistant, the bird is obliged to dry itself off when it comes out of the water. That is why it perches on a rock or post and spreads its wings in the wind and the sun. |