14 THE BIOLOGY OF BIRDS 



does not increase the impermeability of the feathers, it may 

 increase the flow of saUvary juice which is used in keeping 

 the feathers clean. 



The occurrence or non-occurrence of the preen-gland 

 is rather difficult to interpret, for it may be absent in the 

 near relatives of birds that have it well-developed. It is 

 usually best developed in birds which frequent water, but 

 while it is large in the white stork, it is small in herons. 

 It is present in embryo Ratitas, but seems to persist in the 

 Kiwi only. 



Scales. — Epidermic scales, sometimes small and granular, 

 sometimes coalesced into large shields, usually cover the 

 toes and the (tarso-metatarsal) region between the ankle 

 and the toes. They are reminiscent of reptilian scales, but 

 no one has found any connecting link between scale and 

 feather. In some birds, like the grouse, sand-grouse, and 

 owls, the feathers extend to the tips of the toes. This may 

 be a primitive state, and Davies maintains that the scales 

 on birds' feet are secondary transformations of feathers — a 

 view requiring consideration. 



Claws are specialised scales, and it is interesting to find 

 that they are moulted in the grouse in the autumn, dis- 

 closing new sharp claws beneath the old ones which are 

 discarded. Many birds have a claw on their thumb, and 

 in rare cases, e.g. ostrich, there is a claw on the second digit 

 as well. In the Hoatzin (Opisthocomiis) the young bird 

 makes considerable use of its thumb-claws in clambering 

 on the branches. In old-fashioned birds, like albatross 

 and petrel, the horny sheath or bill covering the bony 

 beak, both above and below, is compound, and its com- 

 ponent parts are equivalent to separate scales in reptiles. 

 It is interesting to find in the puffin an annual moulting of 

 the outermost parts of the bill-scales. In this and in the 

 moulting of the claws of the grouse there is an echo of 

 reptihan moulting, where there is a periodic shedding of 

 the outermost layer of epidermis which covers the scales. 

 The moulting of feathers, where the whole epidermic 

 structure, comparable to a scale, is moulted, is a unique 



