Olympus OM-D E-M1 mk2
If you want to do serious nature photography with an SLR camera, then you will soon be wanting a lens with a 600mm focal length. And a sturdy tripod and a cart to transport your tripod and telephoto lens. Shooting by hand with 600mm is really not an option.
It is with the Olympus 300mm f/4.
Thanks to the crop factor of the micro-43 sensor, the field of view of this 300mm lens is equal to the field of view of a 600mm lens on an SLR camera with a full-frame sensor. Thanks to the shorter focal length, you benefit from greater focal depth. The high brightness of f/4 remains unchanged, so that even in low light you have a short shutter time. The image quality is very high from full aperture. And the image stabilization is super. That combination means that with this lens you can use a low ISO value more often, so that the ultimate image quality (signal-to-noise ratio, dynamic range and resolution) comes very close to that of the much more expensive and larger telephoto lenses on SLR cameras. And if you do not think 300mm is long enough, then you can combine this lens with the Olympus 1.4x converter, while retaining auto focus.