GENERAL ZOOLOGY 



Fig. 18.28. Representative modern birds. A, a pair of 

 northern blue jays, Cyanocitta cristata, with nestling youne;. 

 S, South American condor, Vultur gryphus. C, great white 

 heron, Ardea ocadentalis. (All photographs courtesy New 

 York Zoological Society.) 



extinct aquatic, toothed birds, Hesperornis (Fig. 18.26) and Ichthyornis; the 

 Palaeognathae are various flightless birds such as rheas, emus, cassowaries, 

 ostriches, and others (Fig. 18.27); the Impennes are the penguins (Fig. 18.26); 

 and the Neognathae include the more common and famiUar birds, forming the 

 majority of existing species (Fig. 18.28). 



As we mentioned previously, a group of reptiles, the pterosaurs or pter- 

 odactyls, also evolved the power of flight, flourishing and then becoming 

 extinct. These were contemporaneous with the primitive birds and sprang 

 from the same reptilian line, although representing an independent offshoot. 

 Characteristics of selective advantage, evolved by the ancestors of the birds, 

 were their temperature-regulating ability, their nesting habit, the greater 

 care given the young, and their feathers, which form a strong, light, and 



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