I was also going to suggest m43, although I understand that the contrast-detect auto-focus (CDAF) used on most models is not the best for bird-in-flight photography. At least that’s what I recall from reading the dpreview forums. OP, it may be worth a try after you do some more research. Don’t limit yourself to Olympus only though. Lumix (Panasonic) also make excellent m43 cameras and lenses.