The excretory systems of many birds are similar to those of some reptiles. Nitrogenous wastes are removed from the blood by the kidneys, converted to uric acid, and depositd in the cloaca. there, most of the water is reabsorbed, leaving uric acid crystals in a white, pasty form.

What do they eat?

Birds have to eat more food than their proportion size because of their high metabolic rates. As a group birds will consume almost anything, such as; amphibians, carrion, ants, buds,  crustaceans, fish, fruit, grass, insects, larvae, pollen, leaves, mollusks, nectar, other birds, reptiles, rodents, roots, sap, seeds, suet, snails, wax, & worms.  Birds have to have a light mass so that they still are able to carry themselves in flight.  Some birds will starve for hours in order to maintain this small weight.

Digestion System

 BEAK:          The digestion system starts with the birds beak or bill. This is where the food enters the body. This also replaces the teeth or a mouth for the bird.  The size of the beak depends on the birds diet.  
        ESOPHAGUS:    The next step of the digestion system would be the esophagus. The esophagus takes the food from the beak and transfers it down to the stomach. It is a long "pipe" like shaped muscle. Many birds have enlarged esophagus know as crop.  This is temporary food storage.  The crop is also known to soften the food before it enters the stomach. 
      STOMACH:          A bird has two parts of a stomach. One being the a glandular portion known as the proventriculus and the other being gizzard know as the muscular portion.  They have Hydrochloric acid in each of their stomachs to help break down food. 
   INTESTINES:     Next the food would travel through each of the intestines the small and the large. 
   CLOACA:    After it is finished in the intestines it would then be held in the cloaca which is where all the waste products is held.

Why do birds have to maintain a small mass?

Birds have to maintain a small mass because in order to stay in flight.  So to keep this small mass their digestion system helps them.  Plus the more food birds eat, the more heat energy its metabolism can generate.


Why do birds eat stones?

Birds eat stones because with birds being toothless. They eat stones to help breakdown their good in their stomachs.