The Differentiation of Populations I 227 



Geospiza 

 magnirostris 





Certhidea 

 olivacea 



Fig. 10.7 I Schematic representation of the relationship between bill 

 structure and feeding habits in 10 species of Geospizinae from Indefati- 

 gable Island. (From Bowman, 1961, University of California Publications 



in Zoology 58. ) 



there is almost no overlap in measurements, and observations indi- 

 cate that individuals recognize and mate only with members of the 

 proper form. The three forms sometimes take the same food; their 

 feeding habits are overlapping but not congruent, as shown in 

 Table 10.2. 



The two members of the genus Cactospizo, C. pallida (the tana- 

 ger-like finch) and C. heliobates (the mangrove finch), are very 

 similar. Both forms are primarily insectivorous, C pallida having 

 the unique habit of excavating for beetles and other insects with its 



