2 78 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



laid in a nest and incubated by the parents. The eggs are 

 fertilized within the body of the female, the mating time of 

 most birds being in the spring or early summer. Some kinds, 

 the English sparrow, for example, rear numerous broods each 

 year, but most species have only one or at most two. The 

 eggs vary greatly in size and color-markings, and in number 

 from one, as with many of the Arctic ocean birds, to six or 

 ten, as with most of the familiar song-birds, or from ten to 

 twenty, as with some of the pheasants and grouse. The dura- 

 tion of incubation (outside the body) varies from ten to thirty 

 days among the more familiar birds, to nearly fifty among the 

 ostriches. The temperature necessary for incubation is about 

 40 C. (100 F.). Among polygamous birds (species in which 

 a male mates with several or many females) the males take no 

 part in the incubation and little or none in the care of the 

 hatched young; among most monogamous birds, however, the 

 male helps to build the nest, takes his turn at sitting on the 

 eggs, and is active in bringing food for the young, and in de- 

 fending them from enemies. The young, when ready to hatch, 

 break the egg-shell with the "egg-tooth," a horny, pointed 

 projection on the upper mandible, and emerge either blind 

 and almost naked, dependent upon the parents for food until 

 able to fly (aUricial young), or with eyes open and with body 

 covered with down, and able in a few hours to feed themselves 

 (precocial young). 



Classification. The class Aves is usually divided into 

 numerous orders, the number and limits of these as published 

 in zoological manuals varying according to the opinions of 

 various zoologists. The rank of an order in this group is far 

 lower than in most other classes. In other words, the orders 

 are very much alike and are recognized mainly for the con- 

 venience in breaking up the vast assemblage of species. In 

 North America most of the ornithologists have agreed upon a 

 scheme of classification, which will therefore be adopted in 

 this book. This classification, together with a complete 

 catalogue of all North American bird kinds, is published by 

 the American Ornithologists Union as a "Checklist of North 

 American Birds." According to this classification the 800 



