Proteins are complex compounds made up of smaller units called amino acids. After a bird consumes protein, the digestive process breaks down the protein into amino acids. The amino acids are then absorbed by the blood and transported to cells that convert the individual amino acids into the specific proteins required by the animal. Proteins are used in the construction of body tissues such as muscles, nerves, cartilage, skin, feathers, beak, and so on. Egg white is also high in protein.
Amino acids are typically divided into two categories: essential and nonessential. Essential amino acids are those that cannot be made in adequate amounts to meet the needs of the animal. The nonessential amino acids are those that the body can generate in sufficient quantities as long as the appropriate starting material is available. There are 22 amino acids commonly found in feed ingredients. Of these, 11 are essential and must be supplied in the feed. Poultry diets typically contain a variety of feedstuffs because no single ingredient is able to supply all the necessary amino acids in the right levels.
Most feed tags indicate only the percentage of crude protein in a given feed. This information does not tell you about the quality of the protein used. Protein quality is based on the presence of the essential amino acids. For poultry, methionine and lysine are the two most critical amino acids. Deficiencies of either of these will lead to a significant drop in productivity and the health of the flock. Commercial poultry diets typically contain methionine and lysine supplements. Because of these supplements, the feed can contain less total protein; without supplements, the feed would have to contain excessive amounts of the other amino acids in order to meet the methionine and lysine requirements.
The main sources of protein in poultry diets are plant proteins such as soybean meal, canola meal, corn gluten meal, and so on. Animal proteins used include fishmeal and meat and bone meal. Fishmeal can be used only in limited quantities (less than 5% of the total composition of the diet) or it will give poultry meat and eggs a fishy flavour.

Protein sources and limits of feeding.

Protein source Recommended level in the diet Effects of feeding excessive amounts
Soybean meal Broilers: Up to 25% in the starter feed and 20% in the finisher feed. Layers: 12-15% Large amounts of soybean meal may cause troubles from pasting up in which excreta sticks to the vent. This can lead to renal dysfunction due to retention of the urates which interferes with passing of the droppings. Soybean meal may also produce goiter
Cottonseed meal May replace up to 50% of the soybean meal in the diets of broilers, but should not be fed in excess of 5% of the layer's diet. The high content of carotenoids will impart a deep yellow pigmentation of skin which is not desired by consumers
Soybean meal 2-3% for broilers and up to 10% for the layers. When large amount of cottonseed meal is fed to the layers, the gossypol found in greenish cast to egg yolks. Cyclopropenoic fatty acids, also present in cottonseed meal, will impart a pink color to egg whites on storage.
Sunflower meal 3-5% for broilers and layers. Sunflower meal contains anti-nutritional components such as pectin's and arabinoxylans, which affect digestibility and bioavailability of protein. This product is also remarkably low in essential amino acids, and is high in chlorogenic acid content which
Fishmeal 2-5% for broilers and layers. Feeding of fishmeal in excess of the recommended level will lead to fishy flavor in meat and eggs. Also, the high cost and the low palatability of the fishmeal are important factors restricting