522 EVOLUTION OF BIRDS xvm. 7- 



peckers. The tits (Parus, &c.) are a large group of woodland birds; 

 they chiefly eat insects, also buds and fruits. The shrikes (Lanius) 

 are peculiar among passerines in being mainly carnivorous, using 

 their strong bills to eat other birds, amphibia, reptiles, and large 

 insects. 



The warblers (Sylvia, &c.) make a very large group of woodland 

 birds, living in trees or scrub. Related to them are the thrushes and 

 blackbirds (Turdus), the British robins (Erithacus), and nightingales 

 (Luscinia), mainly eating small invertebrates, also fruits. They have a 

 very wide distribution and are among the most recently evolved and 

 successful members of the whole class. The hedge-sparrows (Pru- 

 nella) are small omnivorous passerines, possibly related to the thrush 

 group. The wrens (Troglodytes) are small and mainly insectivorous. 

 The swallows (Hirundo) form a very distinct family of passerines, 

 suited for powerful flight and feeding on insects caught in the air. 

 The very long pointed wings and 'forked' tail allow rapid manoeuvring 

 in the air and the insects are taken in a wide mouth. These features, 

 together with the elaborate migrations, mark the swallows as among 

 the most specialized of all birds. 



8. Tendencies in the evolution of birds 



The bird plan of structure, originating in the Jurassic period, 

 perhaps 150 million years ago, has become modified to produce the 

 great variety of modern birds. In trying to discover the factors that 

 have influenced this modification we are handicapped by the poverty 

 of fossil remains ; it is not possible to trace out individual lines as it is 

 in other vertebrate groups. It is clear that the process of change has 

 been radical, the later types often completely replacing the earlier 

 ones: no long-tailed or toothed birds remain today. 



Our knowledge of direction of the change is largely dependent on 

 study of the variety of birds existing today, which is perhaps more 

 thoroughly known than in any other group of animals. In the reports 

 of those who have studied this variation there are two distinct, indeed 

 opposite, tendencies. Many have observed that adaptive radiation has 

 occurred; birds are found occupying a wide variety of habitats, with 

 modifications appropriate to each way of life. Other workers, record- 

 ing minor differences between races occurring in different areas, 

 have found difficulty in believing that these have adaptive significance. 

 The existence of such 'subspecies' with a geographical limitation is a 

 striking characteristic, especially conspicuous in widely distributed 

 species such as the chaffinch (Fringilla). On continental areas such 



