This is absolutely true!
I have a similar problem as the OP.
My bantam Cochin cockerel Frodo used to be very healthy and vigorous.
He crowed and bred with the girls as much as he could.
His comb and wattles were plump and red.
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I gave him to a friend and after a couple of months he began to deteriorate at her house.
She assumed he was depressed because the ONE pullet that he had ended up dying.
She picked Frodo up and was shocked to discover that he was emaciated.
Another bantam rooster I’d given her was perfectly healthy.
Long story short, Frodo is back at my house now.
He’s been here about a week and I’m disturbed by his condition.
Shrunken comb and bad color.
I think he sits with Pippin because he likes her warmth.
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No crowing- no breeding- extremely emaciated- very hungry for protein and greens but he doesn’t eat much feed.
I’ve checked his crop and it doesn’t decrease very much in size overnight so I think there may be a problem there.
I caught him eating a leaf in the coop run and I hope he’s eating the grit I have in there’s because stuff like that is just going to exacerbate the problem.
I feed all my birds 20% flock Raiser with oyster shell free choice.
I massage his crop several times a day and give him protein treats and greens.
I won’t free range him right now because I’m afraid in his condition he won’t be able to defend himself if the others want to fight.