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INTRODUCTION TO EVOLUTION 



We note (Fig. 13.13) that the lower mandible of Hemignathus obscurus 

 is slightly shorter than the upper one. In the related species Hemignathus 

 lucidiis the lower mandible is only about half as long as the upper one 

 (Fig. 13.14) but is somewhat heavier in structure than is the lower man- 

 dible of H. obscurus. The feeding habits of these two species are similar, 

 but the heavier lower mandible possessed by H. lucidus increases its ability 

 to chip and pry away loose pieces of bark in search of insects. This species 

 sometimes eats nectar, but less frequently than does H. obscurus, a fact 

 doubtless correlated with the decreased efficiency of a beak of this type for 



nectar gathering. The next species 

 we shall mention has a still more 

 highly modified beak and is never 

 observed to feed on nectar. 



The culmination of this evolu- 

 tionary trend toward shortening and 

 strengthening the lower mandible is 

 attained in Hemignathus wHsoni 

 (Fig. 13.15). In this species the 

 lower mandible is straight and 

 heavy, enabling the bird to fill 

 a woodpeckerlike environmental 

 niche. Like woodpeckers, and like 

 the "woodpecker finch" of the Gala- 

 pagos Islands, these birds climb up 

 and down vertical trunks and 

 branches. The straight lower mandible is used mainly to open the burrows 

 of wood-boring insects. While holding the curved upper mandible to one 

 side, the bird pounds vigorously with the sharp lower mandible, boring a 

 hole and exposing the insect larva. Then the upper mandible, so slender 

 that its tip is slightly flexible, is used as a probe to remove the insect. The 

 slender tongue, which can be extended to the length of the upper mandible, 

 aids in the process of extraction. The two mandibles also cooperate in pry- 

 ing loose bits of bark in search of insects concealed beneath. Perkins 

 (1903) stated that this species feeds on insects inhabiting dead wood only, 

 since it lacks the stout beak and huge muscles which enable Pseudonestor 

 to excavate borers from live wood. 



It is noteworthy that these three species have retained tubular tongues, 

 thus betraying their nectar-eating ancestry. 



Of the drepanids which have forsaken the ancestral diet of nectar, some 

 have developed heavy beaks useful in cracking solid seeds and nuts. In 



FIG. 13.15. Beak of Hemignaihus wil- 

 soni. (After Rothschild; from Amadon, 

 "Ecology and the evolution of some Ha- 

 waiian birds," Evoluiion, Vol. 1, 1947.) 



