related to an unbalanced diet, for example too high quantity of hemp seeds or mealworms).

By other hands, I have also found other interesting hypotheses consulting some books dedicated to the European Finches, written by Marcel Ruelle, famous Belgian studious, director of "The Ornithologue" and published by the Fèdèration Ornithologique Wallonne (Belgium), in particular:

• LE CHARDONNERET ELEGANT
  (The European Goldfinch),
• LE GROS-BEC CASSE-NOYAUX
  (The Hawfinch).

... as not hereditary aberrations, the professor Maurice Pomarèd cites the melanism, characterized for abundance of the black melanin in the plumage. The bird can become entirely black...
Maurice Pomarèd attributes the cause of melanism to endocrine troubles and in particular to the bad functionality of the hypophysis gland which produces an hormone that takes part in the melanization process, but other troubles may accompany the melanism and in particular the sterility.
The melanism disappears during the next molt, when the normal functionality will be recovered, but that could require more than one molt, that is some years. By other hands stress during the molt can cause the melanism...

and again:

... in the magazine "Journal des Oiseaux" of April 1988 has been published a photography of a melanic European goldfinch. Maurice Pomaréde has commented such photography as follow:

"In such melanic european goldfinch the red

Maurice Pomarède bases his sentence on studies of various birds and in particular the one related to the Gouldian Finch, he esteems that the head colors depend on dominant factors inhibiting the depositing of melanin. Many hybrids, obtained by the European goldfinch, inherit its mask, this is tending to demonstrate that such inheritance is due to a dominant factor whose action is completed by another factor related to the nature of carotenoid.
The conclusion of Maurice Pomarède is that this "Black Headed European Goldfinch" is a recessive mutation for relationship to normal type. Therefore it will be possible to fix the type with proper pairing, in particular with return crossings.

we may read again (comment to another melanic European goldfinch, see photography 5):

Maurice Pomaréde reports us an article and a photography published by "Italia Ornitologica" on February 1992 (see photo 5) and by "Uccelli", on December 1991 and on April 1992 (see photo 6) , commenting:

"this subject is truly black, the head, the back, the wings, the chest and the tail are entirely black, the abdomen is grayish. A light tawny soffusion seems however present at the level of cheeks, but it's very limited. An important fact, it does not remain any traces of yellow band on the wings and the red mask and white color on the head, the tail is completely black as the beak tip as.

We know, and the hybridization shows it to us, that the depositing of melanin is normally inhibited at the mask level caused by a dominant gene.