Any Suggestions for a New Camera?

With that budget and those qualifications I’d recommend a Canon R10 with the Canon RF 100-400mm lens. The crop sensor will give you 640mm equivalent focal range, and that particular lens is also very good at macro. The fast AF and eye detect in that camera will also serve you well for bird photography. If you wanted to save yourself some money you could go with a used 7D Mark 2 and a used Canon EF 400mm f/5.6L USM, which according to some, has the same sharpness as a $10k lens but for 1/10th the price and less weight. Whether you end up going with Nikon, Sony, Canon or MFT it doesn’t matter that much, you’ll enjoy whatever you get.