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Wednesday, 22 April 2009

Indoor Meeting - Feathers and Flight

Some things, it seems, are not quite as simple as they sound; or so it appeared to me when listening to Timothy Seller's masterly presentation. Let me explain. Feathers are for flight, you might say. Well, not entirely. Tim told us that millions of years ago they provided warmth to birds by trapping layers of air, and they still have that action, but they are really multifunctional. Feathers are used for display (the peacock is a good example), for signalling identity and territory, for camouflage (note how the nightjar merges into its surroundings), and for what Tim called disruptive colouration (the colour of lapwing chicks, for instance, breaks up their outline so long as they keep still). He also mentioned aggressor camouflage such as is found in seabirds with white undersides flying above the water who are less visible from a fish's viewpoint than if they had black undersides. We learnt also that bodily distribution can vary; 80% of the feathers on the Bewick's swan, for example, are found on the head and neck. And the bird with the smallest number of feathers (the ruby-throated hummingbird) has many more feathers per gram keeping it warm than has the Bewick's swan. But, most marvellous of all, is how the precise, beautiful and at times spectacular feather colouration such as is found on the kingfisher or the head of the teal is produced on each and every bird of the same species, producing the lovely and varied patterns to which we attach so much importance in bird identification. Then we turned to the principles of flight and, hence, migration. Tim explained the physics of lift, the benefits of broad, short wings for soaring in thermals, the efficiency of narrow, pointed wings in seabirds needing to cope with high wind speeds, the difficulties that landing poses for birds with their relatively brittle bones, and much more. Finally, in any discussion of flight, who could fail to be impressed by the feat of the arctic tern with its annual journey of about 25000 miles from within the Arctic Circle to Antarctica and back. Overall, a very informative lecture, full of interest.

Brian Shreeve