xviii. 8 BIRD EVOLUTION 523 



subspecies usually grade into each other (making 'clines') and are 

 interfertile at the areas where they meet. On the other hand, where a 

 group of individuals becomes isolated on an island, or by some other 

 geographical barrier, it may become infertile with the 'parent' species. 

 If the two groups again come to occupy a common area, then either 

 one eliminates the other or slight modifications of habits enable the 

 two to survive side by side as two distinct 'species'. 



Definite cases of formation of new species in this way have been 

 recorded in birds, which are especially suitable for such study. Thus 

 in the Canary Islands, besides a local form of the European chaffinch 

 {Fringilla coelebs) there is the blue chaffinch (F. teydea), which was 

 probably originally an offshoot from the European form. On the 

 mainland F. coelebs inhabits both broad-leaved and coniferous forests, 

 but in Grand Canary F. teydea occupies the pine woods, F. coelebs the 

 chestnut and other woods. On the island of Palma, however, the blue 

 chaffinch is absent and the European form occupies both habitats. It 

 is presumed that the blue form is more suited to the coniferous woods, 

 but this could not be deduced from its specific characters as recorded 

 by a systematist. The adaptive significance of the differences between 

 groups of animals may not be easy to discover, but it is most unwise to 

 assume that it does not exist until a very thorough study has been 

 made. Detailed observation generally shows small differences in habits 

 and behaviour between animals occupying what seems at first to be 

 a single 'habitat'. As Lack, who has studied this question in detail, 

 puts it, 'A quick walk through the English countryside might suggest 

 that there is a wide ecological overlap between the various song-birds. 

 In fact, close analysis shows that there are extremely few cases in 

 which two species with similar feeding habits are found in the same 

 habitat.' The differences may be in the breeding habitat, as in the 

 case of the meadow, tree, and rock pipits (Anthus pratensis, A. trivialis, 

 and A. spinoletta). The spotted and pied flycatchers (Muscicapa striata 

 and M. hypoleuca) catch their food in slightly different ways and the 

 chiff-chaff (Phylloscopus collybita) feeds higher in the trees than the 

 willow warbler (P. trochilus). 



A complicating factor is that birds that occupy similar habitats 

 for one part of the year may migrate to different regions, for instance, 

 the tree and meadow pipits and the chiff-chaff and willow warbler. 



Birds are able to get their food and to breed in many different ways, 

 and when a race finds a situation occupied it perhaps often survives 

 by a slight change of habits, creating a new 'habitat' not previously 

 occupied. This presumably results from the action of certain indi- 



