Hard to dismiss 'mutation' entirely, even though temporary melanism (for whatever reason, and sometimes due to old age??) is common in Red Strawbs & OB's
Has the breeder put 'black to black' and produced like-coloured young
Melanism in parrots is a recessive mutation . . while 'grey-green' and olive (and they are different mutations) are dominant with single and double factor
I suspect that this would also be the case with finches
If the breeder is charging the same for the dark birds as for his normals, it may well be worth a gamble
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And on the subject of temporary melanism . .
Double Bars with a complete black area between the 'bars' appeared some years ago. They certainly seemed to be mutations as there was no other unusual areas of black
But . . alas the black moulted out
And on an aviary visit to the Hunter region many moons ago, I saw Cubans as black as Jacarinis, but still with some yellow . . striking birds. . .
After a few months I decided to purchase some and was delighted to be told that they were the same price as 'normals', so I purchased 3 pair . . when they arrived they were no where near as black as the originals, and were indeed 'normal' to my eye, and I was a bit disappointed but had no real gripe as I had only paid for 'normals' in the first place, as it were
Have heard no more of these 'black cubans" . . so maybe this was another example of temporary melanism