226 THE STRUCTURE AND LIFE OF BIRDS cuHap. 
shoulder. A gull as he travels keeps his wing slightly 
bent at the wrist, as photographs show, and probably 
also at the elbow, and does not flex them any more. 
than this for the upstroke: they are merely rotated 
(2e.aturn at the shoulder raises the front margin), 
and then lifted for the downstroke again (frontispiece). 
The birds which take long strokes have long 
pectoral muscles arising from long breastbones: those 
which take short strokes have the pectorals corre- 
spondingly short. I have above explained the rule 
that the amount of contraction possible to a muscle 
depends upon its length (see p. 142). If we bear this 
in mind it will be very interesting to set side by side 
the skeletons of a Frigate Bird and a duck. The breast- 
bone of the former is extraordinarily short, but deep, 
showing that his typical wingstroke is a very short 
but powerful one ; the duck’s has a great superiority 
in length, but not in depth, suggesting the quick long 
strokes of his far more laborious flight. 
Some small birds cross wide seas, and we are apt to 
think of them as having no special qualification for 
such voyages. Golden-crested Wrens come to us 
in flocks from Norway, and sometimes, wearied out, 
cluster in flocks on the rigging of fishing smacks. 
But the wings are, for such tiny creatures, long and 
fine ones. 
Upward Flight. 
Watch a lark as he is mounting. He holds his 
body inclined steeply upwards, often at an angle of 
about 60 degrees with the horizon. While he is in 
this attitude his wings are set so that their under 
