THE WAY OF A BIRD IN THE AIR. 175 
these both tend on the downstroke to bend upward, 
and as the air resists them on the downstroke it 
glides out behind the wing and tends, like the blades 
of a propeller, to push the bird forward. This alone 
would be enough for slow motion, but after a bird is 
well up the wings reach forward and strike backward 
as they go down. 
It is well established that a flying bird needs noth- 
ing but propulsion after getting a start, since the un- 
der surface of both wings, the body and the tail, by 
being slightly tilted up front, tend to lift the bird as 
it goes forward, just as a kite rises when drawn rap- 
idly by a string. In this case, as in many others, 
rising or keeping up means going. But a bird may 
so adjust itself as not to go when it flutters, as seen in 
hawks, shrikes, bluebirds, ete. 
There are many other features of a bird’s wing, 
such as are shown by the length, number, arrangement 
and special shape of the individual flight quills, and the 

ZZ 
== SS 


Wing of broad-winged hawk, with notched primaries—another way of 
narrowing the tip of a wide wing to fit it for soaring. 
probable causes that have brought about their short- 
ening or suppression, their narrowing, tapering off, 
etc., but our discussion can not include them. 
There are also some correlations, not well under- 
stood, between the number of primaries (quills in the 
