178 BIRDS. 



in pairs, construct their nest with great care, and constantly 

 in the same manner; and each species appropriates for this pur- 

 pose certain materials. All possess a kind of instinct which leads 

 them to choose the most convenient places for their nests, such 

 as best afford concealment, or which render them inaccessible to 

 their enemies. 



In Birds the ova exist already formed in the mother before 

 fecundation ; and it is not a rare occurrence to see eggs laid with- 

 out impregnation, similar in every respect to those which pro- 

 duce young. Fecundation in most of the species is accomplish- 

 ed by mere juxta-position. The eggs of birds differ much in the 

 colour of their calcareous covering. They have generally the 

 form of an elongated ball, and one of their extremities is thicker 

 than the other. The fecundated eggs require a certain heat to 

 be hatched ; and the observation of this fact has led to the prac- 

 tice in Egypt and elsewhere, of hatching large broods of chickens 

 by artificial heat. 



The class of Birds, though not so apparently useful to man as 

 the Mammalia, serve important purposes in the general econo- 

 my of nature. Those whose food is chiefly insectivorous check 

 the excessive reproduction of the insect races, and for this pur- 

 pose migrate at certain seasons to places where their food abounds. 

 The indiscriminate destruction of crows and sparrows in some 

 districts has accordingly been found to give rise to an infinitely 

 more prejudicial multiplication of creatures still more destruc- 

 tive. Some families of birds destroy field-mice, snakes, frogs, 

 and lizards"; and others again are led by choice to feed on car- 

 rion, or dead animal matter. Birds are besides extensive agents 

 in the spread of vegetables and even animals. It is well ascer- 

 tained that wild ducks in their emigrations carry impregnated 

 spawn into remote ponds, and thus stock them with fish ; and 

 many by swallowing seeds whole, and subsequently expelling 

 them, are the means of spreading vegetation over an extent 

 of surface which scarcely any other means could accomplish. 

 A great portion of the class and their eggs may be used as 

 food, and the feathers of many form an object of commerce. 



Nothing is more singular in the history of birds than their pe- 

 riodical migrations. That these are connected in some measure 

 with the necessity of a supply of food and the impulse of repro- 



