276 GENERAL ZOOLOGY 



The robin is not only well equipped to find and capture 

 food, but is also able to profit by experience to a greater 

 degree than a toad or a lizard. It soon learns to avoid un- 

 desirable objects, and usually hunts in favorable places 

 at the most opportune times. After the food has been 

 captured and swallowed, it is stored in the crop, ground 

 among the pebbles which are present in the muscular 

 gizzard, and then absorbed in the intestine. Digestion, 

 absorption, assimilation, and excretion, are assisted by the 

 digestive glands (liver, pancreas) and by certain ductless 

 glands (pancreas, thyroid, thymus) which act " indirectly" 

 by secreting substances into the blood. 



The circulatory system of birds presents some points of 

 special interest. At one time within the egg a robin has 

 several pairs of aortic arches, but these degenerate or are 

 diverted to various parts of the body as development pro- 

 ceeds, and an adult bird has only one great aortic arch, 

 which curves to the right after leaving the heart and then 

 runs backward as the dorsal aorta (Fig. 105). The heart 

 in all birds has four chambers the left auricle and ven- 

 tricle collect and pump blood to the lungs for aeration; 

 the right side receives the "pure" blood from the lungs 

 and forces it through the body. Because of the active life 

 a bird leads, metabolism is very rapid. The body tem- 

 perature is even higher than that of mammals, and respira- 

 tion is accordingly accentuated. A bird breathes through 

 its lungs into air sacs beyond. 



The excretory products of a robin pass into the cloaca 

 from the kidneys and are eliminated through the anus 

 with the faeces. They are chiefly in the form of uric acid, 

 which is a solid and therefore well adapted to an aerial 

 animal. A robin must be light and could not carry a 

 great quantity of water, which would be necessary to 

 dissolve urea. 



Self-protection. A robin's keen eyes and ears usually 

 give adequate notice of approaching danger; any shadow 

 which indicates a prowling hawk causes a speedy retreat 



