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

It's called a Nikon 1 V2, only 16 megapixels not 20, but 1" sensor and you can just get an adaptor for any DSLR lens (simplest with Nikon as you get full coupling). You can go even further and get a Pentax Q body with a truly tiny sensor.

The A500 does not give you what the V2 does, a full electronic finder view and focusing from your 'super tele' lens at eye level.

David