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ON THE SOARING FLIGHT OF THE CHOUGH. 
STUART SMITH, B.SC. (LEEDS). 
The Chough. [(Pyrrhocorax pyrrhocorax) (L.)] has for some time 
now been classed among the rare and disappearing birds of 
Britain. There is little doubt that formerly it bred on the 
cliffs of the Yorkshire seacoast, although never so common 
in the east of the country as in the west. Now it seems 
confined to the cliffs of Cornwall and Devon, with possibly 
a pair here and there in Wales. Lt. -Colonel B. H. Ryves 
estimates that the number of breeding pairs in Cornwall is 
as low as six or seven ( Bird Notes and News, 1931), figures 
which compare tragically with those of the Kite ( Milvus 
milvus L.). That the loss of such an attractive bird is a 
real one will be agreed by all who have seen the Chough 
abroad, where, in such places as the Savoy Alps, and along 
with the smaller, but equally attractive, Alpine Chough 
[(P. graculus) (L.)] it abounds. One of its most striking 
characteristics is the supreme ease and buoyancy of its flight, 
and particularly its ability to gain great heights from ground 
level with no flapping flight, but merely by soaring. At this 
it can scarcely have an equal among birds, and it is the object 
of the present paper to investigate the structure of the Chough’s 
wing, with a view to attempting an explanation of this soaring 
ability in the light of modern aerodynamics. 
It is first necessary to investigate the relative flow of air 
over and past a bird’s wing, in flight, and for this purpose it 
is best to imagine the wing as stationary, and the air as 
flowing past it in relative motion. The action of a bird’s 
wing in flight is a cutting or shearing one, so that the air is 
divided, and while some is piled up underneath the wing, 
the rest is deflected above the wing, and by reason of the 
speed of the passage of the wing through the air, is unable to 
regain immediate equilibrium, and tends to form a vacuum 
above the wing. Both these effects contribute to the sum 
total of the uplift felt by a wing, but, contrary to what might 
be expected, the uplift derived from the vacuum above is 
nearly twice that derived from the piled up air underneath, 
(see Fig. 1). The resultant force acts at an angle of 90 deg. 
to the wing surface. As, however, the angle which the wing 
1933 Sept. 1 
