If I could place the RX100 sensor in A500 for bird shooting, .........

I was going to chime in, but you said it better than I could.

Bottom line... whether you use the entire area of a smaller sensor or a crop to the same physical size area on the larger sensor, f2.8 in either system delivers the same light to the same size area.

DOF is based on actual focal length, F stop, distance to subject regardless of sensor size.

With all that said, something like the the old Panasonic FZ50 with the RX100 sensor and a 15x zoom (28-420mm equiv.) could be a nice little walk around wildlife camera at a measly 2lbs.