Why does my female parakeet sit at the bottom of the cage for awhile then climb back up and sit on the perch?

Usually they sit at the bottom of their cage when they are ill. You should get him to a vet before it's too late. Parakeet's hide their illnesses. This is one of the things he can't help to hide. It must be too hard for him to perch. I hope he's okay!<3