First of all, NO bird is perfect, although all breeders are in search of one
I do not like males with small combs--combs are related to virility and lines with males with too small combs have had noted fertility problems. Mis-shapen combs can indeed be difficult to correct--particularly getting rid of spikes at the top. I think I'd much rather breed to a single comb than to a badly mis-shapen walnut comb. But to me the hardest part of correcting comb issues is when the bird has dark skin and a red comb--
that is hard to correct.
Number one issue for me is body type--old time breeders have said that birds get colour from the father and shape from the mother, and there have been some studies that suggest this is correct (not sure why, but there is a lot about the genes of type that have not been well researched).
Toes are easy to correct; toe feathering a bit more difficult.
Many breeders ignore whether a skull is vaulted or not. Some breeders select against vaulted skulls.It is not a breed requirement as it is for polish.
If you have a bird with a super feature, then breed it to those birds where that feature needs work, but don't choose to use an inferior bird when a better one is available. If you have a bird with an oversized crest and one with a too small crest, that is (based only on that feature) an ideal pairing. Likewise pairing an 11 toed bird with an 8 toed bird. But choices aren't usually that obvious. Is it better to use a bird who has perfect type but carries its wings too low or one whose type isn't quite as good, but carries its wings nice and high?
Right now I am finding myself selecting which of two buff cockerels to keep (and 2 months ago I was certain that I wouldn't want either one). Both cockerels have pretty much the same type--still a bit leggy, but much less than 2 months ago--they are gradually growing into their bodies. Both have very nice crests and well-shaped combs. Both have excellent toe spacing, and heavy foot feathering.
One is a gorgeous deep apricot shade of buff: stunning! He has a comb that is lighter than I prefer, but it is dark enough that it shouldn't be a DQ. However, his very full foot feathers are not very silkie, and his shoulders are a shade or two darker than the rest of his plumage.
The 2nd cockerel is a ligher less stunning shade of buff--but very evenly coloured over his whole body. He also has massive foot feathers, but they are very silkie. His comb is much pinker/redder--good chance for a DQ. So who should I keep? (BTW--I have already made up my mind, but what does everyone else think?)