Eels for breakfast

· by · in Wild Attraction. ·
Two birds on a peg

Two birds on a peg

Walking along the local lake early in the morning is a good time to watch all the water birds feeding. One of rarer birds is the great Cormorant Phalacrocorax carbo. This waterbird is easily recognised by its size as it can grow up to 90cm with a wingspan of up to 160cm. It has an entirely black plumage and a white and yellow chin. The bill is grey and the legs and feet are black.

The great cormorant feeds mainly on fish but wouldn’t turn down crustaceans, aquatic insects or even nestling birds. The average adult cormorant requires around 400g – 500g of food a day but that can vary considerably on any particular day. Although they feed mostly on their own they can sometimes be seen fishing in flocks. The birds would move slowly forward with a group of them diving almost synchronously in successive waves to drive the fish before them towards the surface.

oops, all gone

Eels are occurring naturally in our waterways. They are considered a delicacy not only by humans but also by birds, like the great cormorant. While most of the fish they catch are on the smaller side it is quiet a treat to watch them having an eel for breakfast. It takes quite an effort to get the struggling fish into the right position before swallowing it, while fending off competitors.

Cormorants, like most animals are naturally buoyant. This buoyancy has to be overcome to be able to dive for food. Their plumage does not lack the waterproofing oils but is fitted with a special microstructure that releases the air out of the plumage under the pressure of the water while diving without allowing water to penetrate in. When the birds come up from their dive the pressure is reduced and with that the water is ejected from the structure. This ability of the microstructure of the feathers is adapted to the necessary diving depth of each bird species to be able to hunt for their type of food and also avoids competition between them. Nonetheless, there is always a small amount of water retained in the plumage. This increases with every dive but the birds will limited their dives so that the added mass of retained water will not exceed a certain amount.

This sophisticated technology however, only works in water. If the feathers are immersed in oil, as it can happen after an oil spill, they are fully wetted. The microstructure collapses and water cannot be released again. The best results for removing such oils have been with special detergents.

Reducing buoyancy by increasing the wettability of feathers comes at a price. It compromises insulation. A thin layer of waterproof feathers still traps air so that water never comes into direct contact with the skin. The consequence of such poor insulation means the birds must warm themselves in the sun after dives, some more than others. The dark plumage helps by absorbing the heat from the sun and some birds will keep their wings outspread even after they are dry. That is why we often see those birds hanging out to dry on logs near the water.

Little pied cormorant Microcarbo melanoleucos

References:

  • Olsen P and Joseph L (2011). Stray Feathers: Reflections on the Structure, Behaviour and Evolution of Birds. CSIRO Publishing. ISBN: 0643094938
  • Ribak G, Weihs D and Arad Z (2005) Water retention in the plumage of diving great cormorants Phalacrocorax carbo sinensis. Journal of Avian Biology, Volume 36, Issue 2, pages 89–95