296 INTRODUCTION TO EVOLUTION 



searches bark and leaf clusters and bores into wood like a woodpecker. 

 When a woodpecker has exposed an insect it uses its long tongue to extract 

 the insect from the crack or hole. This finch, lacking the long tongue, picks 

 up a small stick or cactus spine, holds the latter lengthwise in its beak and 

 probes out the insect, dropping the stick and seizing the insect as it 

 emerges. This remarkable practice affords almost the only known exam- 



FIG. 13.6. Beaks of two insectivorous tree finches (Subgenus 

 Camarhynchus). (From Lock, Darwin's Finches, Cambridge Uni- 

 versity Press, 1 947, p. 57.) 



pie of the use of a tool by a bird (Fig. 13.8). Clearly we have here an 

 example of an animal which has "improvised" a means of entering an 

 environmental niche foreign to it on continents of the world (see p. 283). 

 This finch is also the only one to climb up and down vertical trunks and 

 branches like a woodpecker. 



The warbler finch (Fig. 13.9) is so much like a warbler that its true 



relationship was formerly not recognized. Its 

 beak is slender and warblerlike. It searches 

 leaves and bushes for small insects and some- 

 times catches the latter on the wing like a true 

 warbler. Nectar and young leaves are also 

 eaten. 



Thus Darwin's finches afford another beau- 

 tiful example of adaptive radiation, made pos- 

 sible in this instance by absence of enemies 

 and competitors on a group of oceanic islands. 

 As Darwin stated in The Voyage of the Beagle, 

 "Seeing this gradation and diversity of structure in one small, intimately 

 related group of birds, one might really fancy that from an original paucity 

 of birds in this archipelago, one species had been taken and modified for 

 different ends." 



FIG. 13.7. Beak of the 

 woodpecker finch (Subgenus 

 Caciospiza). (From Lack, 

 Darwin's Finches, Cambridge 

 University Press, 1947, p. 57.) 



DREPANID BIRDS OF HAWAII 



In concluding our discussion of the light shed on evolu- 

 tion by the inhabitants of oceanic islands we shall cite another remarkable 



