Feather sexing is usable, but imprecise without very careful breeding. Practically speaking, for the home chickeneer, it is near useless since it is both breed and strain dependent. This is where it differs from vent sexing.
It is mentioned later on in your quoted post that the breeding information given is not sure, so we still don't know for certain what breeds make up the Amberlink. I have learned that they originated in S. Africa, but not what particular breeds were used. Whether it was Sussex on RIR remains unsure, although there is reason to suspect that.
Feather sexing became possible in 1969 thanks to genetic research by the Tegels Poultry Breeding Company. This method to determine the sex of newly hatched chicks is possible if a female from a slow-feathering breed (dominant K alelle) is crossed with a male from a fast-feathering breed (recessive k allele).
The sex of the chicks produced from this cross can be determined during the first 48 hours after hatching by looking at the primary and secondary feathers located on the chick's wings. The primary feathers will be noticeably longer than the secondary feathers on a female chick. On a male, the primary and secondary feathers are the same length.
As I recall, feather sexing is possible with only a few of today's common breeds, primarily those among the Mediterranean group. It is known that the Mediterranean lines were introduced into Britain by the Romans, and they have mingled over time to eventually become what we call the Sussex and Dorking breeds. So using a strain of Sussex where the fast feathering trait was retained makes sense for breeding feather-sexed Amberlinks. However, that remains unconfirmed.
Its certain that all breeds of chickens have feathering genes (K/k), which feather sexing relies on. But the fast feathering trait has been receded in nearly all of them, so the feather characteristics of both sexes at birth generally appear identical. Only careful culling and extensive breeding among at least two wholly separate flocks would bring out the fast feathering trait where it doesn't already exist. Since we want to spend our money on slower feathering, larger birds in our domestic industry, vent sexing of such birds has remained the standard in the domestic hatchery business.
Feather sexing remains mostly on the commercial side, since it applies to the many leghorn crosses seen in the egg business. It seems the Amberlink is an escapee from that commercial trade, now being sold into the domestic market. Who'dve thunk it?
