Sauropsida: Class Aves 553 



The heterogeneity of this Order is reduced by segregating the 

 following birds into additional Orders: Owls, Strigiformes; oil- 

 birds, goatsuckers, whippoorwills, nighthawks, Caprimulgi- 

 FORMES; trogons, Trogoniformes; swifts, hummingbirds, Micro- 

 podiformes; woodpeckers, toucans, Piciformes; colies (small 

 finchlike African birds), Coliiformes. 



Order 20. Passeriformes: "Sparrow-like" birds; a very large and 

 homogeneous group in which as many as some 7000 species and sub- 

 species have been recognized. Birds of medium or small size, short- 

 legged arboreal perchers and good fliers; most of them songsters of 

 proficiency ranging from that of crows and blue jays to that of 

 larks and nightingales. Most of the genera are cosmopolitan: 

 Sparrows, thrushes, robins, finches, flycatchers, wrens, war- 

 blers, swallows, orioles, starlings, and many others. 



Relationships 



The geologic evidence clearly indicates that reptiles gave rise to 

 flying animals of two very different types. In the pterosaurs (Fig. 

 439) the wing was a broad lateral web of skin stretching from the 

 pectoral limb, whose fourth digit was greatly elongated to support the 

 web, back to the pelvic limb. The pterosaur was presumably cold- 

 blooded and scaly, but certainly not feathered. It had a low carina on 

 the sternum but otherwise, so far as can be judged from the fossil 

 skeletons, its anatomy was not so highly specialized in relation to 

 flight as to forbid classifying it as a reptile. It was a flying reptile 

 just as the modern bat is a flying mammal. 



Birds also were derived from reptiles — probably from some small 

 terrestrial dinosaur-like animal which had a tendency toward bipedal 

 locomotion. Such reptiles existed. But birds acquired feathers and a 

 complex of specialized features involving all parts of the body. The 

 wing, totally unlike that of the pterosaur, consists essentially of a 

 system of feathers supported by the highly modified pectoral limb. 

 Pterosaurs and birds were probably allied in their remote ancestry, 

 but certainly neither was derived from the other. Birds departed so 

 far from the anatomic pattern of terrestrial reptiles that they must be 

 regarded as forming a distinct Class of vertebrates. But, in view of the 

 reptilian features which birds retain, they are included with reptiles 

 in a "superclass" SAUROPSIDA. 



Between the Archaeornithes and their terrestrial reptilian an- 

 cestors, there must have been a long period of gradual acquisition of 

 avian characteristics. It is highly likely that feathers were acquired 

 before flight was achieved. If feathers make a bird, then there must 

 have been running birds before there were flying birds. It is therefore 



