Indirect head-scratchingScratching the head by lowering the wing and then lifting the leg over the wing from behind, so that the head can be reached. 'Direct head-scratching', on the other hand, simply involves bringing the leg up to the head in front of the wing without the need for any movement of the latter. Each bird group characteristically uses one or the other of these methods, examples of those which scratch their heads indirectly being the nightjars (family Caprimulgidae), swifts (family Apodidae), kingfishers (family Alcedinidae) and most passerine birds.
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From Peter Weaver's Birdwatcher's Dictionary
Copyright © 1981 by Peter Weaver