Finally got the big bad lens

 I did it, I found a good deal on a 400mm f/2.8!  It is a 1985 model and very heavy, but it works!

Shot with my new to me 400mm f/2.8 on a D500

I was able to hold my 400mm on the monopod in my left hand while getting closer shots like this with my 105mm in my other hand.