why is my parakeet losing its feathers

I have 2 parakeets my yellow one is very mean i have seen it stop my other one from eating and its very mean to us. the blue one on the other hand is a sweet heart. I have recently seen that the blue ones feathers on its tummy are gone in patches and hes looking kinda skinny, still acting the same though. is it the other bird doing it to him or do you think its sick?? i have no idea. should i seperate them?

I have worked with domestic birds (pet birds) and wild birds for many years.

It is possible that your parakeet who has lost its feathers is extremely stressed since the other one is mean to it.

When a parakeet or bird is stressed they become feather plucker's and will pluck out their feathers as this becomes a habit for nervous or stressed birds.

You also mentioned the parakeet looks skinny. This is also the result that it is unable to eat it's proper amount of seeds due to the other bird being mean or more dominant.

I would highly recommend that you separate these two parakeets immediately. This will allow the one parakeet to grow it's feathers back and gain some weight.

A good tip: If you think one of your birds are under weight, use your index finger and palpitate around it's breast bone. when you locate the breast bone, move your finger to the sides just below the breast bone. If you feel the actual soft bone plate and no meaty area on either side of it's breast bone, then your bird is very under weight.