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. In 
many 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 
iis bed hee ee een. Celiaine to. bealiscevercd. 
Pipit. 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 bird 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 
