Maybe Rudie is upset or depressed about something? Possibly inside the household, since he will hunt & eat a bird outside?
Anything new or different going-on for him? It could be something that seems insignificant to you, but cats can be quite sensitive.
My sister has a cat that is depressed and has been on kitty antidepressants.
As far as food goes, my vet has told me that trying baby beef food is good for kitties (human baby food like Gerber).
I had a cat several years ago that had a very touchy tummy, and I fed her that for sometime.
Kedi, Wylie, Rudy, and the dog Scout!