74 
ON THK CLASSIFICATION OP BIRDS. 
catchers and warblers must be widely separated; the 
crows and the tom-tits he classed together, and the avosets 
joined to the recurved-bill humming-birds. These are 
only a few of the ridiculous combinations which would 
residt from such a principle of classification, which, 
however simple and inviting it may appear in theory’ 
becomes utterly impracticable when we attempt to work 
out its details. 
(68.) The nuchal bristles of birds are certain setaceous 
hairs, more or less developed, hut always simple, which 
arise from the nape, or back part of the neck of several 
birds, and are concealed among the feathers. They are 
most developed in the bristle-neckeil thrushes, forming 
the genus Trichophortis where they are so much 
prolonged as to be tliree times the length of the surround- 
ing feathers ; and it is generally supposed that they are 
altogetlier peculiar to this group. W'e believe, how- 
ever, that a great many of the perching genera are 
furnished with these appendages, for we have detected 
them in our common thrushes, and even in the robin 
redbreast. It is only, however, in the circle of the 
short-legged thrushes, or the Brachypodinm* , that these 
bristles are lengthened; in all other birds that we have 
yet seen, they are merely the length of the ordinary 
feathers of the nape, from which they are to he dis- 
tinguished by being destitute of any webs, excepting a 
few at their extreme point. Their use is not accurately 
known, but they are probably connected with the sexual 
intercourse. 
» First defined and characterised in North, Zoot vol. ii. Appendix i. 
