This may be a "no brainer"? For bif and perched bird photography (assume 50/50) the Nikon D850+Nikon AF-S 500mm f5.6E PF ED VR comes in at $AUD10,900 whereas the Canon 500mm+1DXii is around $AUD18,000. Opinions to include low light/High ISO comparison, auto focus and TC use. Cheers.