Tracing the wake of a flying bird

STUDY of the classical problem concerning “the way of an eagle in the air”, when applied to flapping bird flight, offers many difficulties. All dynamic interactions between an object and a fluid medium in which it moves are ciphered in the structure of its wake, but the wake behind a flying bird is difficult to work with because it is an invisible and very short-lived formation; moreover, its structure must be complicated as the bird's wings are working in a constantly changing manner. I report here the results of wake visualisation experiments carried out during short flights of a chaffinch ( Fringilla coelebs ) and a brambling ( F. montifringilla ) in enclosures. The general configuration of a wake is given, but no measurements have been made: in general, to extract some quantitative information from the wake structure, a more precisely documented picture of the dynamics of its formation is needed.