Saturday, January 30, 2010

Anhingas, oil glands, and corrugated tail feathers

Erika and I saw many Anhinga during our recent Florida trip.  Anhingas are also called snakebirds because of their habit of swimming with only their long necks above water.  (The word Anhinga comes from a Brazilian Indian word for devil bird.)
Before writing up this blog entry, I had always assumed that birds' oil glands water-proofed their feathers, and that the Anhinga's habit of habitually holding their wings out to dry might mean they might lack an oil gland.  I have learned, however, that oil glands do not water-proof feathers--ducks with their glands removed still have water-resistant feathers.  The water proofing is in the feather structure. (Cornell Lab of Ornithology Handbook of Bird Biology)
Anhinga feathers are less resistant to water than other birds such as cormorants, which allows Anhingas to do that neck-above-the-water swimming, with their bodies submerged.  But, compared to cormorants, they must spend more time drying themselves out.
Ornithologists are well aware of the transverse corrugations on Anhinga central tail feathers.  These can be seen on the tail of the Anhinga photographed above (click on the photo for a larger image).  I am unaware of similar structures on other birds (perhaps some hornbills?)  None of my sources suggest a reason for this structure. (Notice that this bird has speared what appears to be a sunfish, and that its head feathers are remarkably wet.)

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