208 
On the Soaring Flight of the Chough. 
presents to the air stream is increased, it is found that the 
resultant uplift decreases, until, about 15 deg., unstable 
equilibrium is reached, and a ‘ stall,’ well known to airmen, 
commences. This is mainly due to the fact that, as the 
stalling angle is reached, the air is deflected downwards at 
an increasingly acute angle above the wing ; the flow of air 
along it is broken, and ‘ eddies ’ commence, which tend to 
fill the vacuum and thus help to destroy the most important 
lifting factor of the wing (see Fig. 2). 
In order to increase the angle at which ‘ stalling ’ occurs, 
and thus arrive at a wing which was safe under low air speeds 
and high angles of incidence, a device had to be found to 
prevent this eddying, or ‘ burbling,’ as it is called. This 
device, in the aeroplane wing, is known as the Handley -Page 
slotted wing, and consists of a small auxiliary wing fitted 
above the main ones, at the front, which— for reasons not 
within the scope of this paper — ensures a smooth flow of air 
over the wing from front to back, and by preventing eddies, 
enables what is known as a ‘ controlled stall ’ to be accom- 
plished (see Fig. 3). 
There is a second factor also, which tends to reduce the lift 
of a wing, by filling up the vacuum above, and that is what is 
known as ‘ air-spill.’ This is due to some of the air piled up 
under the wing tending to slip round its end or tip, and rush 
up into the space above (see Fig. 4). 
Having grasped these facts, we can now proceed to a 
consideration of how they are compensated for in a bird 
capable of a brilliant soaring display as is the Chough. All 
field observers will have noticed that in birds with wide 
rounded wings which indulge in aerial display — the buzzard, 
or the raven, and other corvidae for example — the primary 
feathers are widespread at the tips of the wings, giving a series 
of spaces, or slots, formed by the emargination of these 
primaries. 
In the Chough, these slots are six in number (not counting 
that formed by the bastard wing), and their purpose is exactly 
similar to that of the Handley-Page slot already referred to 
in the aeroplane wing. The first primary in the Chough is 
Fiq. 3 . Stalling ah$l£ reaches. 
But controlled By slot. 
WINCf 
slot. 
(?ELATiV£ 
Aik 
FLO W. 
Fig, !+■ Air spill. 
The Naturalist 
