GEOGRAPHIC ISOLATION AND SPECIES-FORMING 125 



on insects on the trunks of trees. If we assume that Oreo- 

 mystis, or some other of the genera with short and slender bills, 

 represents the original type of Drepanidse, we have two lines of 

 divergence, both of them in directions of adaptation to peculiar 

 methods of feeding. 



Next to Oreomystis. on the one hand, we have Loxops and 

 Himatione, with the bill pointed, a little longer than in Oreo- 

 wijstis, and slightly curved downward. The species, red or 

 golden, of these two genera are distributed over the islands, 

 each on its own mountain or in its own particular forest. 

 Vestiaria, another genus, remarkable for its beautiful scarlet 

 plumage, has the bill very much longer and strongly curved 

 downward. Vestiaria coccinea, the iiwi of the islands, lives 

 among the crimson flowers of the ohia tree (M etrosidcros) and 

 the giant lobelia, where it feeds chiefly on honey, which is 

 said to drop from its bill when shot. According to Mr. S. B. 

 Wilson, the scarlet sickle-shaped flowers of a tall climbing 

 plant (Strongylodon lucidus) found in these forests " mimic in 

 a most perfect manner both in color and in shape the bill of 

 the iiwi '' so that the plant is called nukuiiwi (bill of the iwii). 



The next genus, Drepanis, has the sickle bill still further 

 prolonged, forming a segment of a circle, and covering nearly 

 fifty degrees. Drepanis pacifica, one of the species, has the bill 

 forming about one fourth of the total length. The species of 

 this genus, black and golden in color, were very limited in 

 range, and are now nearly or quite extinct. Still another 

 group with sickle bills, Hemignathus, diverges from Vestiaria 

 in having only the upper mandible very long and decurved, 

 the lower one being straight and stiff. The numerous species 

 are mostly golden yellow in color. The group contains long- 

 billed forms like Hemignathus procerus of Kauai, and short- 

 billed forms like Heterorhynchus olivaceus of Hawaii. In the 

 short-billed forms the two mandibles are quite unlike: the upper 

 very slender, much curved and about one fourth the length of 

 the rest of the body, the lower mandible half as long and thick 

 and stiff. These birds feed chieflv on insects in the dead limbs 



\) 



of the koa trees in the mountain forests. Some or all of them 

 use the lower mandible for tapping the trees, after the fashion 

 of woodpeckers, while with the long and flexible upper one they 

 reach into cavities for insects or insect larvae or suck the honey 

 of flowers. 



