ee 
. 
130 THE FLIGHT OF BIRDS 
these Rooks never for more than an occasional half- 
second lost commarm@ of their movements. The 
Starlings were no less at their ease, so that, to my 
thinking, this curious little problem of flight with a 
gale blowing from behind is settled. Birds are 
capable of such flight, and much enjoy it. But to 
migrate in such wild weather would be a different 
matter. It is no wonder that they do not choose 
a dark, stormy night for a long oversea voyage. 
Undulating Flight without Movement of 
the Wings. 
At one time I believed that this was possible when 
the wind was blowing horizontally, but I now feel 
sure that an upward trend is necessary. Take an 
example that is often to be seen. The wind is 
blowing at something like a right angle to the course 
of a steamer, and the Gulls, which are following to 
pick up any scraps that may be thrown into the sea, 
soon recognise that there is a chance of saving their 
muscles. There is an up-current of air on either 
side of the ship, for the wind is, so to speak, bent 
upward some little distance before it reaches the 
obstruction. To start with, then, the Gull obtains 
some slight elevation by means of a few strokes, then 
he glides down a gentle incline in the direction in 
which the ship is travelling. When close to the 
water he turns and faces the wind, which, having 
an upward trend, lifts him to the level from which 
he has just descended. He then glides downward 
again, and so the process goes merrily on. 
Were the wind a horizontal one it would, no doubt, 
help the Gull to rise, but it would not make all 
