Greetings.
Too bad that you did not write me that by the parent bird . It would also be a good description of the conditions of the behavior.
bird in the picture does not have dark legs , flint, or beak . It is therefore not melanizmus . melanizmus causes darkening of all body parts . There are three options .
first one regards dakr factor - dakr only factor causing darkening of the plumage .
The second option is a disease of some internal organ such as the liver .
third option , bird was reared in unsuitable conditions , and this is therefore a temporary modification.
personally rather inclined to believe that it is a dark factor . dakr factor exists as SF - dakr factor and DF - dakr factor . dark factor has free and heredity is dominant. therefore the offspring revealed on SF shape .
I suggest you wait for further exchanges of feathers . whether the mutations or modifications can be determined by mating of the birds and normally colored individuals , if dakr factor transferred to the offspring. in the case of a modification , you should not pass on to offspring .
Tell occurred, as further exchanges feathers .
Greeting
Marek Buranský