442 



COLLEGE ZOOLOGY 



Nervous system 



The brain of the pigeon (Fig. 392) is very 

 short and broad. The cerebrum and cerebel- 

 lum are comparatively large, as are also the 

 optic lobes, showing that birds have well- 

 developed powers of coordination and of 

 sight. The olfactory lobes, on the other 

 hand, are very small, indicating poorly de- 

 veloped olfactory organs. Contrary to popu- 

 lar belief vultures detect the presence of 

 carrion largely by keenness of sight rather 

 than smell. 



Sense organs 



The bill and tongue serve as tactile or- 

 gans. Tactile nerves are also present at the 

 base of the feathers, especially those of the 

 wings and tail. Birds are usually unable to 

 distinguish delicate odors; and on the whole 

 their sense of smell is very poor. The sense 

 of taste is also poorly developed, but it is 

 nevertheless present as can easily be demon- 

 strated if a bad-tasting morsel of food is 

 presented to a bird. 



The cochlea of the ear is more complex 

 than that of reptiles. The Eustachian tubes 

 open by a single aperture on the roof of the 

 pharynx. Birds have acute and discriminat- 

 ing po\^'ers of hearing— a power correlated 

 with their singing ability. 



The eyes of birds are very large, and the 

 sense of sight is remarkably keen. The visual 

 acuity is 8 times that of man in some of the 

 birds of prey. Birds also have a wide field 

 of vision in all directions, and the night- 

 hunting birds are adapted for vision in dim 

 light; an owl's capacity for seeing in dim 

 light is about 10 times that of man. The 

 birds have extraordinary powers of eye ac- 

 commodation, which explains why they can 

 fly rapidly among the branches of a tree 

 without striking a branch or swoop down to 

 the ground from a great height in the air, 

 changing from far-sighted to near-sighted 

 vision in an instant. There is substantial evi- 

 dence of good color vision in birds. 



Endocrine glands 



Birds have endocrine glands such as are 

 characteristic of mammals. The pituitary 

 gland is at the base of the brain, the thyroid 

 in the neck, the islets of Langerhans in the 

 pancreas, the adrenals on the ventral sur- 

 face of the kidneys, and the endocrinal tis- 

 sue in the gonads. The bird hormones most 

 studied have been those associated with the 

 gonads. 



Reproductive system 



The male has a pair of oval testes (Fig. 

 306). From each testis, a duct, the vas 

 deferens, passes back and opens into the 

 cloaca; often it is dilated at its distal end to 

 form a seminal vesicle. The sperms pass 

 through the vasa deferentia and are stored 

 in the seminal vesicle. When copulation 

 takes place, they are discharged into the 

 cloaca and transferred by contact to the 

 cloaca of the female. There is no copulatory 

 organ in most birds; however, a curved penis 

 arises from the ventral wall of the cloaca in 

 ducks, geese, and swans. 



The right ovary of the female usually dis- 

 appears during development so that only the 

 left ovary persists in the adult. The ova 

 break out of the ovary and enter the oviduct. 

 During their passage through the oviduct, 

 the albuminous substance known as the 

 "white" of the egg is secreted in the walls 

 of the middle portion. The double parch- 

 mentlike shell membrane is then secreted 

 about the egg, and finally the shell is 

 added by the posterior part of the oviduct 

 ("uterus") a short time before deposition. 



Fertilization takes place in the upper ovi- 

 duct about 41 hours before the eggs are 

 laid. At the end of the period of incubation, 

 about 14 days, the young have developed to 

 such a stage that they can break through 

 the shell, and hatch. They are at first covered 

 with fine down, but they soon acquire a 

 covering of contour feathers. During early 

 life as nestlings, they are fed "pigeon's 



