90D should be great for birding, Nikon D500 has the reputation but 90D is already as good for birding, I am a happy owner of 7DMKII and when tried to compared to D500 not much of a difference in performance.
Just make sure the used lens has fast enough focus if you intend to shoot birds in flight, in that case you won't be using fast apertures, remember a photo with missed focus isn't usable regardless of the rest.
Attention push-pull on 100-400 isn't obvious to use, make sure you get used to it before.
Just make sure the used lens has fast enough focus if you intend to shoot birds in flight, in that case you won't be using fast apertures, remember a photo with missed focus isn't usable regardless of the rest.
Attention push-pull on 100-400 isn't obvious to use, make sure you get used to it before.