He is being plucked raw by the hen. Your cage is much too small for finches and they cannot get away from each other resulting in plucking. Minimum cage for two is 30 x 18 inches bigger is better. There is no condition which results in this pattern feather loss besides plucking so she's doing it when you're not around. They'll rip feathers out then snuggle moments apart. Zebra finches do get nasty in close quarters. A much larger cage should limit the plucking. I use 30 x 18 x 36 inch cages and let my birds out to fly indoors six hours a day. I have no plucking issues.
You also want to set up the cage so the birds have room to fly. No plastic toys they don't play like parrots. Natural branches set front to back in the cage on either side. One high one low. A swing. Thats a good layout for finches. No nest basket! The female will pluck the male for nesting material and eventually they will start having babies and will easily overrun you with birds that need even more space. Keep their food cup very low in the cage or they will try to nest on that too. I use a cat food dish on the cage floor.
And absolutely do not use any of those mite repellent boxes. The chemicals are deadly and will kill birds via liver damage over time.
Hello I have a young finch named tiny. I adopted tiny from a shelter a couple months ago because he was being picked on by the bigger males and had to be kept to himself. We got tiny a female now named Minnie so he would be happy and was and so was Minnie, they would sit by each other and eat and they never showed signs of fighting or anything but today I looked in the cage and he is missing tons of feathers on his back and neck, not only this but in the past month he has been losing a lot of weight. I just put new millet for them and he refuses to eat it but Minnie is eating fine. There is no blood so I don’t think Minnie is attacking him. They have a cage cleaned every week and fresh water everyday in two containers and two bowls of food so it’s not like he can’t get any. I put a tiny bit of vita sol in the water. He seems just as active just skinny and with less feathers. I don’t want tiny to die as he has always been a favorite bird of mine. Any ideas why this is going on, (I’ll add photos)
