The Evolution of Communities 



223 



(a) 



(b) 



(c) 



(d) 



Fig. 95. Adaptive shifting of beak structure and head pattern in rock 

 nuthatches of the genus Sitta. Above, ranges of the species S. neumaijer and 

 S. tephronota showing area of overhip. Below, heads and beaks of the same 

 two species, (a) S. tephronota from Ferghana; (b) S. neumayer from Dalmatia; 

 (c) and (d) S. tephronota and S. neumayer, respectively, from western Iran 

 where they occur together. Note the beak similarity between the two species 

 where they occur separately, and the much stouter beak in S. tephronota where 

 they occur together. The same displacement of the dark stripe through the eye 

 may be linked with sexual selection. (Above, after Vaurie; below, modified 

 from Vaurie.) 



