Having just seen the 'Nightingale' images I'm firmly in the Thrush camp.
If you look at image 4 you can see a 'hint' of malar stripe contrasting against the pale sub-moustachial. Also there are no buff tones!... on the underparts quite unlike CN. and the grey wash to the breast band although not as heavy as it might be... is nevertheless grey and contrasts against the white throat.
Add to that the greyness of the upperparts and the absence of warmth in the plumage, tail apart.. cosmetically it favours TN.
The first primary which can be seen in the first three images falls short of the coverts.
In the 3rd image...if my memory serves me?...following the ' high-lighted' trajectory curve of the first primary, it could only fall short of the coverts!
Nice summation Ken.
I feel the bigger picture is better than focusing on the detail. Image 4 shows, what I feel, to be a grey-blocked breast and a "Malar well defined grey stripe suggestion."
Also heeding the OP's (Doozer's) in-fill of a description? That's also a good point to start.
Harry's comments fit in well with many photos I've now studied of
golzii.
"While I do think that this may yet be a (Rufous) Nightingale, I don't think it's right for a hafizi (now changed to golzii by the Dutch at least): that taxon should show pale tertial fringes, pale tips to the greater coverts forming a thin wingbar, perhaps be less warm-toned above and have a more obvious supercilium."
In 5 Summer's constant contact with Sprosser in Poland, I only managed 1 Common Nightingale. As they are at the edge of their range where I was.
Most TNs I stalked were very similar to Doozer's bird. Many had unmarked UT Coverts. And the ones that were marked were subtle. All the focus Western European birders put on obvious dark malar stripe, thrushy streaking, boldy marked UT coverts I found to be an odd approach.
Repeated views of many subjects of this, common East Polish, bird gave me a strong impression that it was a
much more subtle proposition. And these features had been emphasised, as though they would stand out and be well marked. Experience taught me that not to be so.
Most birds I saw looked like Doozer's photos!
It sounds crass, but my personal take on TN is that, "...they look like CN rolled around on a dirty gravel road, with a song to match..."
I can see no problem with this bird being a
Slowik Szary, or Grey Nightingale in Polish.