[Arthur Macmillan]

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.

This is actually a friend of mine from Flickr (the photo sharing site, which I suppose everyone knows).  She is a very good birder, which is to say, better than I am by a wide margin.  She really loves her 5D Mk IV used with the original "push-pull" 100-400L. 

She gave her Canon 80D to her husband (I gather more of an old school birder: Binoculars, and spotting scope) when she bought the 5D Mark IV.  That is what you call a good wife! 

You would think why ask me?  I have been trying to get clues from her on how I can improve!  But we somehow fell into discussing what is next on the camera horizon for us.  We each had our eyes on a different system, and have graciously shot each others dream cameras down.

I only have hands on experience with Canon, and Olympus.  I have become something of a raving bird fanatic over the last couple of years, marked by my relegating my Canon 5D Mark II aside exclusively for macro use, and discovering the 90D as a pretty capable birding camera.

I think Canon 7D Mark II is a good choice.  I think the 80D might be as good if it had decent water sealing.  I doubt that it does.  I think the IS one the 100-400 Mk II really makes it an order of magnitude better.  I was able to get a good shot of an owl yesterday at 1/40th of a second, and ISO 1250.  So maybe..

-Pairing the 100-400 Mk II with, the already owned 80D and 5D Mk. IV.  or...
-With the very robust 7D Mark II.

It might just be my own bad luck, but I had an instance apiece with the 70D, and the 90D where just very light sprinkling rain was enough to stop the camera functioning.  That is scary!  In both cases I immediately removed the battery as a precaution, and dried them at home.  They both came back to life, but it was scary!

Thanks for your reply Toni-a!

-Mac