As previously mentioned, the first question I would ask is what is your budget?
Other points to consider:
1) Would you consider a APS-C size sensor rather than FF sensor?
2) Do you like to crop your photos a lot? Many bird photographers do.
3) What lenses do you plan to use with the camera? Do you already own lenses or plan to buy new?
4) What is the largest print size you produce?
If you are starting out new in bird photography you might also want to consider Nikon a viable option. The Nikon 600 mm f6.3 PF and 800 mm f6.3 PF lenses are excellent though do get pricey. The Nikon Z8 is popular among bird photographer that use Nikon products.
Good luck with your decision(s)!
Thank you John for your answers.
1. Budget i don't know, not decided, it is open and limited at the same time, means i can go up to $4000-8000 but not up to $12k-20k, if i am very rush then $2000-3000 is what i can afford very soon, so i will take time and save to afford.
2. I don't mind APS-C as long it will deliver quality.
3. In sports i crop a lot, so i assume in birding i will crop even more, so cropping is a big option in my choice.
4. I do have old lenses that i can still use with adapter, but along the time and journey i might replace them one by one, and keep others that doesn't need to be changed, also depends on how often i will image.
5. No prints in my plan yet, mostly for birds it is maybe A4 maximum or A3 if i like, most galleries if i submit ask up to A3 too, magazines A4 or less according to press/media Corportation requesting my photos.
I am new to bird photography because i didn't do it since very long time, but i am an old photographer in sports and landscape and else, and for that i could use my skills for birding and learn new things, i am a Canon use, and added only one Sony mirrorless, that is decades ago, i started photography in 2006 i believe and kept buying until 2014 which i bought my last camera back then which is Sony A7R, then i stopped, now i want to be back, i want to go with Sony or Nikon fast cameras, but i already have Canon lenses, Sony is my resolution slow camera for landscapes and portraits or still photography, while i depends completely on Canon for action and moving subjects, so sports and birding i will stay with Canon or moving to Nikon if necessary and buy only 1-2 lenses, so i am still asking around about that.