Protein builds tissue. Animals' bodies are largely composed of protein. Whenever birds are growing, or producing eggs, or sperm as in the rooster's case, they use up protein. The growth phase probably uses it up faster than any other. This protein must be replaced in the diet.
Protein is composed of molecules called amino acids .
There are 22 known amino acids. They all contain nitrogen, which other foods lack. Just as the 26 letters of the alphabet can make billions of words, so the 22 amino acids can make billions of kinds of protein. Most of the 22 amino acids can by synthesized in animals' bodies out of other foods they eat. Different kinds of plants supply different amounts of these Essential Amino Acids.