3 i2 BIRDS. 



the air, and move through it with rapidity, is in proportion to the 

 expanse of its wings. The feathers that contribute most to the 

 extent of the wing, and are most useful in flight, are those attached 

 to the hand, and consequently farthest from the body. They are 

 always ten in number, and are called primaries. The feathers of 

 the fore-arm are called secondaries, and those which are attached 

 to the arm humerus, and the feeblest are the terliaries. The bas- 

 tard feathers are those that grow from the thumb, and the coverts 

 those that spread over the bases of the quills. 



It is evident that the greater the surface of the wings, all things 

 being equal, the greater will be the power of flight ; hence it 

 follows that birds with long wings are not only able to fly with 

 greater rapidity than birds with short wings, but they are also 

 able to support themselves for a longer time in the air, because 

 they are not obliged to repeat the movement of these organs 

 so frequently, and therefore do not become so readily fatigued. 

 Thus all birds remarkable for rapid and long-sustained flight 

 have large wings, while those that have short or moderate wings, 

 compared with the volume of their body, fly less swiftly, and 

 require rest more frequently. The feathers of the tail likewise 

 assist in flight, and are used as a rudder. They are usually twelve 

 in number, and are called the tail-feathers : the name of tail- 

 coverts is given to those feathers that cover their base. 



Birds, instead of teeth, are provided with a strong horny beak ; 

 their aliment is generally swallowed without being divided ; on 

 this account most birds are provided with a crop or pouch wherein 

 the food is lodged, and with a gizzard or muscular apparatus, 

 lined with a thick insensible skin, that serves to grind and com- 

 minute the food. In granivorous birds the walls of this gizzard 

 are very thick and strong ; but in birds of prey, and more espe- 

 cially in such as feed upon fish, it is thin and membranous. 



Birds, like reptiles and fishes, are oviparoiis that is, they lay 

 eggs, from which the young are hatched, and their eggs are always 

 covered with a hard calcareous shell. To secure the maturation 

 of the enclosed young, it is necessary that the eggs should be 

 kept at a certain degree of heat. In very warm climates the heat 

 of the sun is sometimes sufficient, but in most cases one or both 

 parents maintain the necessary temperature by sitting or in- 

 cubation. 



The time required for the development of the young varies in 

 different races ; but it is the same in all birds of the same species. 

 Thus, it is from forty to forty-five days for swans, twenty-five days 



