Merry Christmas, Ian, some thoughts.
Fifteen percent is not an awfully high amount of protein and if the mug you're talking about is a coffee or tea mug, that's not much for three standard birds. They might be leaving it because once their crops are full, they won't eat anymore. But that crop empties into the stomach providing fuel for the bird. If there's a sheet of ice and rain, chances are your chickens won't be able to find enough bugs and nutrients free ranging to give them what they need.
There's a school of thought, one that I follow, that the protein should be increased to at least 20% in the winter to help offset the lack of bugs and things. Even layer feed here in the U.S. has 18% protein, more than the 15% you have been feeding. Remember that chickens don't eat when it's dark and since you're in Ireland, your days are even shorter than mine in Colorado this time of year. Right now I have about eight hours of light.
Chickens don't overeat so it's really best to let them free feed on their food. I have six birds and buy a 50 lb bag of crumbles every two months for them. I usually have to take about five pounds of it to the park for the birds there because they don't even finish that. They have food out for them all the time, a mix of 20% protein and I'll supplement with bits of ground beef or an egg for a protein boost once or twice per week. They're healthy and are laying eggs about every other day. It can't hurt to leave oyster shell and grit out for them as well. It's not expensive and it's good to have it available for them. They'll regulate themselves.
At approximately 20 weeks, my chicks underwent a "soft" moult; in which they lost their baby feathers and grew in some adult ones. It wasn't an actual moult in which birds replace all their feathers and have bald spots. Know that during this time they need even more protein because growing in more feathers requires it.
Feather loss can also be attributed to mites and/or lice so it might be a good idea to check them. At night, when they are roosting, take a torch and shine it on their little bottoms, around the vent. You'll see black spots moving around if they have mites or lice. If they do there's poultry dust available. You'll need to do the birds, the house and dark corners of the run. Then repeat in a few days because it won't get the eggs that will hatch in the meantime.
Good luck and, once again, Happy Holidays from the U.S.
Mary
