As for the spec ..46 mp is not enough pixel density to replace my 24 mp DX birding cameras.
I am using the D500 (which has a higher pixel density than my D800) with its something like 21 Megapixels for birdshot, and I have never found it to have too little pixel density, and never felt that just 24 would bring much more benefit. I would not trade the AF, the 10fps andthe Cache of the D500 against a 24 Pixel DX body, i am finding these features more essential to bird photography.
BTW 46 Mpx on FX is equalling 21 when set to DX, so a fast D850 can take some of the role of the D500 while benefiting from FX advantages if needed.