Some really great images here.
The trick to getting better from this great starting point is attention to detail (I see it in my own work too). But one never has enough time with birds, so the initial camera setup has to be good, as there will normally be little time to change.
If trying for birds in flight, I recommend Manual mode with Auto-ISO. Set up the shutter speed for the size of birds you are shooting (maybe 1/1000th for the large birds shown here, 1/2000th for pigeon-sized birds, 1/3200th or 1/4000th for the small and fast birds and I have been told that for humming birds 1/8000th is not quite fast enough).
I normally feel that the aperture should be fairly close to wide open, while others prefer a larger DOF. But I would not go smaller than f8 for birds.
Using the D7000 and now the D750, I have/had one of my "user modes" set up to Manual mode, 1/2000th, f5.6 or nowadays f5 (I use f4 primes) and Auto-ISO, with ceiling set to maximum native ISO. I also use AF-C with back button enabled, focus delay set to 0 and 9 points active (with the D7000 I used a single point as it was so much slower). I am not sure if this can be easily achieved with the D500.
This will keep your ISO to the lowest possible level while keeping the other factors suitable. Keeping to a maximum ISO of as little as 800 is, in my opinion, not practical. I say this because I find a lot of the best sightings (and action/interaction) happens very early in the morning, or on rainy days.
My cheat: I have found that on bird feathers, noise can actually look like feather detail. If I am forced to photograph at high ISO's and have resulting noise problems, I will "paint" the entire background (except for the bird and possibly it's perch) and reduce noise, sharpness, clarity and sometimes even contrast and saturation in those areas. For reference, all my editing is done in Lightroom. Since I have no experience with other programs, this is not necessarily a program recommendation, I am only trying to say that this editing process works in LR, but I have no idea whether it is possible in any others.
