226 THE STRUCTURE AND LIFE OF BIRDS CHAP. 



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 

 {i.e. a turn 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 FligJit. 



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 



