According to Joseph Corbo, an associate professor of pathology and immunology at Washington University, the genes affecting red coloration belong to a wider family of genes involved in detoxification. Redness may be a sign of a robust, quality mate who can easily cleanse harmful substances from his body.

“In many bird species, the redder the male, the more successful it is at finding mates,” – Joseph Corbo.

For more see http://www.deccanchronicle.com/science/science/200516/researchers-solve-mystery-of-red-colour-in-birds.html