PLANT-EATING BIRDS, REPTILES, AMPHIBIANS, FISHES 187 



cvhich it is applied, as here explained; but nature appears not 

 to invent organs at once for the functions to which they are now 

 adapted, but avails herself, here of one already-existing structure 

 or instinct, there of another, according as they are handy when 

 need for their further modification arises." 



CROWS (CoRViD^) 



The mere shape of a bird's beak is often a very imperfect 

 indication of the nature of the food. Thus we have seen (p. 58) 

 that in the Woodpeckers this organ is of powerful character, and 

 shaped like a long straight cone, enabling the bird to cleave bark 

 and wood in its search for insects. A very similar beak is found 

 in the crow-like Nutcrackers {Nucifragd), but in this case it is 

 used for breaking open hazel-nuts. The familiar British Nuthatch 

 {Sitta ccssia), though insectivorous in summer, lives on nuts to a 

 large extent in the winter, opening them by means of its strong 

 pointed beak, as in the preceding case. In summer the beak is 

 used in the same way and for the same purpose as that of a 

 woodpecker. 



FINCHES (Fringillid^) 



The small birds known as Finches feed mostly upon seeds and 

 small fruits, and they possess short conical beaks of great strength, 

 well suited to this kind of diet. Common British examples are 

 the Greenfinch (Ligurinus chloris), Chaffinch {Fringilla ccelebs), 

 Linnet {^Linaria cannabina), House-Sparrow (^Passer domesticus), 

 and Tree- Sparrow {P. montanus). It must not be supposed, 

 however, that birds of the Finch kind are exclusively vegetarian 

 in habit, and it is interesting that the food may differ at different 

 ages. It is, for example, well known that house-sparrows feed 

 their young upon caterpillars and other insect larvae, which are 

 no doubt better suited to the youthful digestion than seeds or 

 fruit. The Crossbills are interesting members of the Finch 

 family which possess the unique peculiarity of having the two 

 halves of the beak crossed over each other, as seen, for example, 

 in the British species {^Loxia curvirostrd) {fig. 430), found in 

 northern Scotland. These birds live in forests of pine or fir, 

 and their curious beaks are admirably suited for the extraction 

 of the seeds which are hidden between the woody scales of cones. 

 This happens to be a good illustration of the biological truth that 



