BIRDS. 409 



feeblest are the secqmlanj. The bastard feathers are 

 those that grow from the tliomb, 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 move- 

 ment 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 num- 

 ber, 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 croj) or pouch, wherein the 

 food is lodged, and with a gizzard or muscular ap- 

 paratus, lined with a thick, insensible skin, that 

 serves to grind and comminute the food. In grani- 

 vorous birds the walls of this gizzard are very thick 

 and strong, but in birds of prey, and mc .e especially 

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



Birds, like reptiles and fishes, are Oviparous ; that 

 is, they lay eggs, from which the young are hatched, 

 and their eggs are always covered with a hard calcare- 

 ous 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 inciihation. 



T 



