PROJECT ABANDONED!
This is the story of how I started a project, learned some things and then abandoned it, because it's totally okay to change your mind about something with new trains of thoughts. I had originally thought this was a wonderful image of a person with a mirror case with a bird painted on it. Turns out, his may be St John with his symbol, not a mirror.
I got as far as tracing the bird and transferring it to the back of the mirror ready to start painting before reasonable doubts stepped in.

Here are my original thoughts:
I wasn't 100% convinced this was a mirror as the person isn't holding a comb, or it's a poorly drawn one, but the expression and pose are very suggestive of this, so I'm having a go at reproducing it.
It might be the mirror is polished bronze rather than glass, and shows the reflection of a bird, but the other several mirrors in the same manuscript aren't shown this way. The bird could well be painted on the reverse of the case.
Anyway, I won't be telling people it's definitely one thing or another when I do my displays, but sharing the thought process.
Image Luttrell Psalter, tagged repeatedly as folio 29r, but my phone searches keep bringing up the page of the women in their heraldic surcotes, so this folio number may not be correct. Possibly 13v.
Current, additional thoughts: It looks like St John is holding a book, which it certainly looks like, which explains why it didn't look like a comb, but perhaps you can forgive me for thinking that it may have been a mirror. It has mirror vibes all over it. The very long conversation on Facebook also suggested that the mirror is reflecting his symbol instead of his face, which is possible. For me, the strongest suggestion that it's not a mirror, is that the other hand would usually hold a comb, not a book. The image is rather small here, but in bigger versions, like the top image, it's not a comb. It is a book or a box.