Hanging it out to dry

Birds such as Australasian Darters (Anhinga novaehollandiae), pictured left, and Little Pied Cormorants (Microcarbo melanoleucos), pictured below right, can often be seen standing with their wings outstretched drying them in the sun. The assumption many people make is that they lack the ‘preening’ gland at the base of the tail that produces an oil type substance used by waterbirds to waterproof their wings. Darters and cormorants do indeed have the gland. The birds use their heads and beaks to spread the oil through the feathers. There is some debate as to whether the oil actually produces the waterproofing or if the feather structure itself does the job and the oil just keeps the feathers in good condition.

Feathers work by trapping air both to insulate a bird in cold weather and to create buoyancy so that birds such as ducks can sit on the water surface with little effort. Darters and cormorants however dive to catch their food. Having air in their feathers impedes their ability to dive. To reduce their natural buoyancy their wing feathers are wettable so that air bubbles can be shed. This means that they can dive deeper with less effort but the downsize is they need to dry these feathers afterwards.

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So what does it mean when you see a non-diving bird sunning its wings? (pictured left). I don’t know. It’s probably trying to keep its tummy warm.