150 THE BIOLOGY OF BIRDS 



membrane up to the claws, and the resulting broad foot is 

 well adapted for running on soft snow or sand. When the 

 sand-grouse half-bury themselves in the steppe they are 

 practically invisible, so well does their dorsal plumage 

 harmonise with the surroundings. In Pallas's sand-grouse 

 (Syrrhaptes) the broad toes are bound together in a web, 

 with a warty sole beneath. In the cold Tibetan steppes the 

 upper surface of the w4iole foot may be covered with small 

 feathers, and in the ptarmigan, which spends the winter 

 among the snow, there are feathers on the under surface of 

 the foot as well as on the upper. 



§ 5. Mountain Birds 



A simple functional adaptation is seen in the heart of 

 the ptarmigan which lives at high altitudes where breathing 

 is more difficult owing to the rarer air. This implies that 

 the blood must be sent to the lungs more frequently. 

 Strohl has found (1910) that the right ventricle of the 

 ptarmigan is noticeably stronger than it is in the nearly 

 related willow-grouse which lives at lower levels. Even 

 the young ptarmigan shows the hypertrophy. 



The ptarmigan shows another adaptation in its change 

 of colour. For the brown and grey plumage of summer and 

 autumn, which makes the bird inconspicuous against such 

 background as lichen-covered rock, is replaced in winter 

 by a white suit. This not only gives the ptarmigan a garment 

 of invisibility against a background of snow, thus saving it 

 from the eagle's eye, but it is physiologically the most 

 economical dress for a warm-blooded animal, with a high 

 temperature, in very cold surroundings. 



An interesting adaptation in the American Ruffed Grouse 

 {Bofiasa umhellata) is described by Austin Hobart Clark 

 (1910). When winter sets in there is a development of 

 " snow-shoes " formed from a row of scutes on each side 

 of each toe. This increases the superficial area of the 

 bird's foot by as much again. " Thus a bird walking about 

 in the winter brings to bear only half as much weight on any 



