294 INTRODUCTION TO EVOLUTION 



tity of seeds available would have been limited, as numbers of finches in- 

 creased a premium must have been placed upon ability to utilize sources 

 of food not usual for finches. Absence of competitors for these other 

 sources of food made possible a diversity of food habits which finches do 

 not achieve on continents. For example, continents possess woodpeckers, 

 specialists in boring into bark and wood of trees after insect larvae. Con- 

 sequently no continental finch would be likely to take up this mode of 

 gaining a living; woodpeckers already have a virtual monopoly on it. Yet 

 on the Galapagos Islands, in the absence of woodpeckers, a finch did de- 

 velop this woodpeckerlike method of feeding, as we shall see presently. 



As indicated above, the chief differences between the various forms of 

 Darwin's finches are differences in the beaks. The plumage of related spe- 

 cies of these rather drab little birds is very similar. In fact. Lack presents 

 evidence that the birds themselves depend upon the shape of the beak in 

 recognizing members of their own species and distinguishing members of 

 other species. To a considerable extent, also, differences in the beak are 

 associated with the differing food habits mentioned previously. Small beak 

 differences (e.g., those exhibited by different species within one subgenus) 

 do not seem to be thus associated with food habits, however. The beaks 

 display much variation; perhaps the struggle for existence on these islands 

 has not been sufficiently rigorous to result in the rigid standardization 

 usually characterizing continental species. 



Adaptive Radiation of Finch Beaks 



In the following series of sketches we present some of the prinicipal beak 

 modifications connected with differing food habits. 



The ground finches (Subgenus Geospiza) have heavy finchlike beaks 

 (Fig. 13.3). Seeds form the chief item in their diet, although they are not 

 narrow specialists in the matter. As the three sketches in the figure indi- 

 cate, great differences in size of beak are found among the ground finches. 

 To a considerable extent the large ground finches eat the same food as do 

 the smaller ones, yet Lack presents evidence that the larger ones can eat 

 larger seeds than can the smaller ones. In correlation with this fact the 

 larger finches mainly ignore small seeds, such as those of grasses, which 

 form the staple diet of the small ground finches. Thus competition between 

 the various ground finches is reduced and they are able to occupy the 

 same habitat. 



The cactus ground finch (Fig. 13.4) has a long, somewhat decurved 

 beak and a split tongue. It probes the flowers of the prickly pear cactus 



