24 INCUBATION MDIFICATION. 



shell. Upon the membrane of the yolk, or yellow matter, we 

 perceive a whitish point, which, in the course of its developement. 

 becomes the young animal, for the nourishment or protection of 

 which, all the other parts of the egg are destined. 



3. In order that the young bird may be developed in the in- 

 terior of the egg, it must be maintained at a certain degree of 

 heat ; in very warm countries, the heat of the sun is sometimes 

 sufficient to bring about this phenomenon, and there, certain birds 

 abandon their eggs; but in most cases it is altogether different, 

 and both parents or the mother alone, maintain the necessary 

 heat by sitting on them. 



4. The duration of inc+balion, (or the time required by the 

 young bird to become developed in the interior of the egg) varies 

 in different species, but it is the same in all birds of the same 

 species ; it is from forty to forty-five days for Swans, twenty-five 

 days for Ducks, twenty-one days for Hens, from twelve to 

 fifteen days for domesticated Canary birds, and only twelve 

 days for the Humming-bird. 



5. Almost al! birds construct a nest, to receive their eggs and 

 to serve as a dwelling for their young, which, during the early 

 period of life, are unprovided with feathers, extremely delicate, and 

 incapable of moving, and of feeding themselves. Generally there 

 is displayed in these structures an art, an adroitness, and an 

 elegance which excite our admiration ; and one thing not less 

 surprising is the regularity with which all the successive genera- 

 tions perform the same tasks, and build nests exactly alike, even 

 under circumstances which prevent these animals from seeing 

 and taking lessons from their parents ; a wonderful instinct guides 

 them, and induces them to take many precautions, all the utility 

 of which they cannot anticipate, or appreciate before hand. 



6. The form, arrangement, and placing of the nest, vary for 

 almost every species of bird. That which is constructed by the 

 largest birds of prey rests upon a flat surface afforded by some 

 part of a rock, or on the platform of some tall tower ; its extent 

 is very considerable, and every year contributes to its increase, 

 for it is rare for these birds to abandon their first monument of 

 maternal tenderness ; those that leave it, return periodically to 

 lay their eggs. This nest is frequently composed of such stout 



3. What circumstance is necessary to the devclopement of the egg? 



4. Is the same period of time required by all birds for incubation ? 



5. What means are provided for the reception of the eggs, and of the young 

 oirds which escape from them ? 



6. Are the nests of a ! l birds alike in their structure ? What is the struc- 

 ture of the nests of birds of prey ? 



