368 



THe Bird 



curved, and in a neglected aviary I have seen birds which 

 were prisoners on their perch, unable to untwist their 

 claws from it. When wild, birds wear down these struc- 

 tures by constant rubbing, and if given plenty of rough 

 bark and wood in their cages, their claws will remain of 

 usual length. 



Although the tarsus, or that portion of the foot which 

 we usually call the leg, is, in almost all birds, covered with 



horny scales, yet these vary con- 

 siderably in different groups. la 

 man}' the scales are small, six- 

 sided or oblong, as in plovers. 

 In some of the higher song-birds 

 these scales have become joined 

 together until, as in our robin, 

 the front of the leg is covered 

 with a long ^'boot" of horn. 

 The cause of this coalescence yet 

 remains to be discovered.. 



A considerable degeneration of 

 the legs and feet is found among goatsuckers, humming- 

 birds, and chimney swifts; but, small as are the feet of 

 the latter birds, they make frequent use of them to break 

 off the short twigs which are used in the construction 

 of their nests. One may take a young swift and place it 

 against the vertical surface of an ordinary brick (Fig. 326), 

 and the ])ird will hold fast without slipping a fraction 

 of an inch. The slender nails fasten in the slightest 

 irregularity of the surface and hold the bird safely. 



Some species of swifts have all four toes pointing 



Fig. 289. 



-Foot of American 

 Pipit. 



