Birds tend to shed their tail feathers when stressed. If they are too stressed, they will die because of it. Maybe your bird, for some reason, was just really upset about something. There is a person who answered before me who said to clean the cage with bleach. Do not, I repeat DO NOT CLEAN IT WITH BLEACH. The fumes are not safe for birds at all and WILL kill your birds. I'm not exactly sure why your bird died, and I am very sorry that he did, but do not clean their cage with bleach unless you want all the others to die.
I don't have anything to add. Moggie is completely right. But...
He bred with his own MOTHER!? And underaged into the bargain?! Inbreeding is NEVER recommended. Budgies MUST not breed under the age of 12 months!
He was sick. Unfortunately the signs were there even though they were subtle and you missed them. Lives get busy sometimes and we miss things. He wasnt eating and he was sleeping a lot and he drank from contaminated water.
TRhe water thing can happen to anyone. As budgies drink sometimes seed on their beak goes into the water and sours it within half a day. That can give them an ecoli bacterial infection and it can knock them down very quickly. I have had water drinkers go off real quick in the same way.
He didnt have psittacosis.
As far as mating and breeding with his mother and someone said it was OK. It isnt. Underaged budgies should never be anywhere a nestbox is. And if you see a related pair bonding and mating.......seperate them,.
Most likely not a genetic mutation. If they're parakeets called Budgies, look them up, they really don't have a long life span. Same thing happened with my birds, I used to have 2 at a time, and then 2 died unexpectedly. It could be a draft you have in your room, don't put a fan directly on them, or even in the same room. They have smaller nostrils and just one whaft can cause your bird to get overwhelmed and die. It could also be that he got overexerted and had a heart attack. Whatever it is, with the tail feathers falling off, it might just be a disease...might get a new cage, or check your other birds out one by one.
Hope that helps!
Good luck to you.
With out autopsy it is impossible to say. The bird could have had some genetic mutation, there could have been a bacterial, viral, or fungal infection. You need to wash the cage with bleach to kill anything that could have caused the infection.
My parakeet died also, I dont know why either. What I do know DO NOT USE BLEACH ON THE CAGE OR ANYTHING THE BIRDS TOUCH. I think mine died from pesticides that pest control sprayed 4 days ago. Birds are very sensitive to aerosol/air fresheners/candles/chemicals.