Once again. Please read ALL of the info provided before drawin' a conclusion. The OP stated that he/she has never seen any sign of rats on the property, let alone a closed garage. And I don't care what anybody says about rats and small openings. It would take a pretty small rat to slither through a 1" opening. And as a rule, it's the larger rats that are a bigger threat to livestock than the juveniles.
So according to you guys, a single juvenile rogue rat, that's never been on the property before, found it's way into a locked garage, then once there, squeezed itself through a 1" hole, ate it's fill, while never harming any other birds, then left the propery without leavin' a trace, like turds, or chewed wood.
Ok, it's either that fairy tale, or the chance that a few birds were involved in over aggressive picking behavior on a possibly injured bird. Gee, I wonder what makes more sense?
Oh yeah, I almost forgot the other option of a 1" weasel that can open garage doors.