Grey schizochroic Robin (Erithacus rubecula) in Doñana, December 2019
José Luís Anguita found an unusual bird at La Rocina stream, Doñana, Huelva, Spain the last 28 December 2019. It recalled a Robin (Erithacus rubecula) but it lacked its usual reddish bib and looked overall greyish-bluish. Initially the observer went shocked when the found the bird:
Lo encontré en el arroyo de La Rocina, Huelva, el pasado 28 de Diciembre de 2019. Podrás imaginar el susto. .../...
The bird remained at its site from 28-30.12.2019. And beyond an aberration in a Robin coloration, he wondered if those bluish tones may have something to deal with some kind of hybridization with another species.
Indeed, as he pointed out regarding a previous observation of a bird in the UK this bird is a ‘Grey’ schizocroism mutation affecting one of the species pigments, the phaeomelanin. And the bird is indeed a Robin (Erithacus rubecula).
As Van Grouw (2006) -excerpt below- stated in his clàssic reference on colour aberrations (Not every white bird is an albino: Sense and nonsense about colour aberrations in birds,Dutch Birding 28), this lack of reddish tones are related to the lack of phaeomelanin which formation is genètic based. Hence, a mutation – rare- causes these cold-looking birds:
Not every reddish coloration is a result of carotenoids; for the orange-brown breast in European Robin Erithacus rubecula are caused by a melanin.
Two types of melanin are present in birds: eumelanin and phaeomelanin. In a high concentration, phaeomelanin is responsible for reddish-brown feathers,
The melanin formation process is determined genetically. Any aberration in the process has a potential influence on the colours of a bird.
Phaeomelanin is responsible for red-brown pigments. The diferent shades of this pigment as the human eyes see it from deep red-brown to yellowish-cream, also depend on the concentration.
Schizochroism is defined as the absence of only one of both melanins. Two forms of schizochroism can be distinguished,
Grey is defined as a complete reduction of phaeomelanin (non-phaeomelanin schizochroism). In this mutation, the plumage only contains the black/grey and brown eumelanin. When phaeomelanin is absent (grey), only black-grey and dark brown colours will be visible, the red-brown to yellowish-cream colours having disappeared.
In any case, a very rare mutation and a very nice-looking bird, a Robin coming from the cold! All photos © José Luís Anguita.
Anguita, J.L. 2020. Grey schizochroic Robin (Erithacus rubecula) in Doñana, December 2019. Birds in Spain Blog. https://birdspain.blogspot.com/2020/01/grey-schizochroic-robin-erithacus.html