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ON THE CLASSIFICATION OF 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 bear 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 



mandible (fig. 33.), which spreads over the front of the 

 head like a casque : nevertheless this frontlet, although 



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 



