A male spanish Sparrow showing erythism. Sparrows usually have black bibs but sometimes a reddish-brown coloration occurs, replacing black. Plumage colour in birds depends on two groups of pigments: melanines and carotenoids. The chemistry is very complex. Melanines are determined genetically while carotenoids are taken in with food. The deposition of the pigment occurs at the start of feather growth. Black depends on melanines, but melanines are also responsible for grey, dark brown and reddish-brown coloration and mutations can occur with reddish-brown partially replacing black. This is called erythrism and is one of the more frequent colour abnormalities in sparrows. Like all abnormalities, it is not very common though.