https://archives.evergreen.edu/webpages/curricular/2011-2012/m2o1112/web/birds.html

Consideration in the Conservation of Feathers and Hair, Particularly their Pigments

Enciclopedia de las aves de España

http://www.seo.org

http://www.sgosgo.org

MARTIN, G.R. y OSORIO, D. Vision in Birds:

https://www.researchgate.net/publication/285088367