Yolk colour varies. It is almost completely dependent upon the feed the hen eats. Birds that have access to green plants or have yellow corn or alfalfa in their feed tend to produce dark yolks, due to the higher concentration of yellow pigments (mainly carotenoids) in their diet. Since commercial laying hens are confined, lighter and more uniformly coloured yolks are being produced. Yolk colour does not affect nutritive value or cooking characteristics. Egg yolks are a rich source of vitamin A regardless of colour.