IV. VERTEBRATA: AVES, RATIT.E 541 



The reason for the dull plumage of the female is due to the fact that she 

 usually sets on the nest, at which time inconspicuous colors protect her from 

 destruction by enemies. In only a few instances is the heat necessary for incu- 

 bation produced by other causes, such as the heat of the sun on eggs buried in 

 the sand, or tlie increase of temperature caused by fermentation of decaying 

 vegatation (Megapodes) The rule is that both sexes build the nest. When 

 the clutch of eggs is complete the female (rarely the male) begins the incubation, 

 at this time in some instances losing the feathers from certain regions the better 

 to warm the eggs. Many birds, like hens and ducks, are so far advanced when 

 they leave the nest that they can follow the mother and feed themselves. Such 

 birds are called Pnxcoces — in contrast to the Altrices, which hatch with incomplete 

 coat of feathers and therefore need the warmth of the nest and the protection 

 and care of the parents. 



The migrations of birds possess great interest. We distinguish permanent 

 residents and migratory species which, in order to obtain food, take long journeys. 

 At the approach of cold weather these seek the south, following regular paths in 

 their travels. They cannot hibernate like reptiles and amphibians, wdien 

 insects and fruit are scarce, because their more energetic vital processes demand 

 a more rapid metabolism and a continuous food supply. Hence they, like the 

 mammals, in contrast to the 'cold-blooded' vertebrates, maintain, under all 

 extremes of external temperature, a body heat of 38-40° (44° ?) C. (100-104° F.). 



The classification of birds is in a state of change. The older system based 

 upon adaptive characters is not in harmony with the results of anatomical study, 

 which would divide the whole class into many small groups. Here it has been 

 thought best to retain the old, larger, easily recognized divisions, and to call 

 attention to the differences from later results. 



Order I. Saururse. 



The view that birds are closely related to reptiles received considerable 

 ' support by the discovery of fossil birds with teeth. The most reptilian of these 

 (ArcIwEopteryx) occur in the Jurassic of Bavaria, and only two specimens have 

 been found. In these carpals and metacarpals have not fused, the three fingers 

 are well developed and clawed, and the caudal vertebrEe, although bearing 

 feathers, form a long slender tail like that of a lizard (fig. 2). 



Order II. Odontornithes. 



These forms, from the cretaceous of Kansas and Colorado, also had teeth. 

 The Odontorm^ {Ichthyornis) had a keeled sternum and normal pygostyle. 

 In the Odontoholc^ (Hesperornis) the wings were reduced (only the humerus 

 persisting), the sternum lacked a keel, and the caudal vertebrfe formed a broad 

 paddle. 



Order III. Ratitse. 



Here are included several families, very different in structure, which agree 

 in having the feathers not arranged in feather tracts; and in that, together with 

 the lack" of flight, many structures normally connected with it are absent. 

 The bones are but slightly pneumatic, the sternum has no keel, and a furcula 

 is not formed, the clavicles being rudimentary (DromcEus) or not present as dis- 

 tinct bones. The wings are small and lack primaries and secondaries adapted 

 for flight, for typical contour feathers with close vanes, as well as typical down 

 feathers, are absent. 



Since several structures apparently adapted for flight occur here (fusion 

 of hand bones and often of caudal vertebrae; arrangement of wing muscles), 



