I see these birds in bright sun. Would like to tone the light down,but still see detail of feathers .Used a 500 mm mirror lens on my cannon xs. Any help welcome .
That is another virtually impossible shot, James, particularly from that angle and with a large mirror lens.
The only way to get good detail and lighting in the subject, is by slightly over exposing the shot ( with a little positive exposure compensation) but that is going to make the sky even worse. For good results, you need to find a better angle for the light.
And yes, I know from past experience how difficult it is to persuade wildlife to settle in a better position.
Possibly a bit of very careful masking during editing would allow you to enhance just the bird but you won't get perfect results.
I have to agree with Geoff (again! - this is becoming a habit ) this was an impossible ask.
Although wildlife seldom cooperates, you have to try shoot it with the sun behind you, especially if shooting against the sky, or you will rarely get an acceptable result.
It's still gonna be difficult even if they're not against the sky, as I have proved to myself again today.