Joe,
It sounds like you are shooting with a Nikon.
I have tested and owned both the Bigma & 80-400mm. The Nikon 80-400mm is very slow focusing and known as a soft lens (not sharp). The Sigma, aka the Bigma is also on the soft side especially out past 400mm and at 500mm your aperture will be 6.3 so you will need good light. Wide open at f/6.3 it is soft, but it will get a
bit sharper if you stop it down to f/8 and beyond but at these f stops you will not get good results on dull days.
If I was in your shoes, and I have been there, buy a used Nikkor 300mm AFS f/4. It is tack/razor sharp and much faster focusing than the above lenses. I think used you can buy one for $800 ish, new they are around $1300. Add a 1.4 Kenko teleconverter and you will be shooting a tack sharp 420mm at f/5.6 and will never look back.
Hope this helps,
Rob
Both of these shots are with the Nikkor 300mm f/4 & 1.4 tele-converter.