66 ON THE CLASSIFICATION OP BIRDS. 
and frugivorous diet is united, as is the case with most 
of the Tanager finches, the upper mandible is notched, 
for the obvious purpose of more firmly securing that 
part of their food, which, like insects, can escape. 
(59.) The degree of strength possessed by the bill 
is in a great degree influenced by the proportion which 
the upper and the under mandible tear in their relative 
size or thickness. The under mandible of the strong 
bill in the falcons, it is true, is much smaller and thinner 
than that of the upper ; but then it is armed with a 
toothed process, and the strong-hooked point of the other 
renders it sufficiently powerful to tear flesh without re- 
quiring much aid from the lower mandible. But in the 
parrots, where the food is prepared in quite a different 
manner, — that is, by breaking, both mandibles are equally 
strong, in order to produce an equal degree of pressure, 
without which it would be impossible for these birds, 
any more than the great-billed finches just mentioned, 
to crack, with facility, the hardest nuts: hence the mac- 
caws and the grosbeaks have the strongest bills of all 
known birds, because the two mandibles are more equal 
in size and strength than in any others. The weakest 
bills, on the contrary, are those where the under man- 
dible is remarkably thin ; instances of this we see in the 
flycatchers and the whole of the 
humming-birds. In the latter 
family this part is so slender as 
to be almost hid, at its base, by 
the folding over of the upper 
mandible ; so that when these 
birds are just killed, their bills 
are almost flexible. It is much 
to be regretted that our total 
ignorance of the economy of the 
plantain-eaters leaves us unable 
to account in any way for the 
great development of the upper 
mandible (fig. 3 S.), which spreads over the front of the 
head like a casque : nevertheless this frontlet, although 
