Dartford Warbler (Sylvia undata)

found dead in a way

Did you preserve the bird?
Finding a freshly dead passerine is a big luck.
And not-so-fresh ones make good skeletons, apart from complete feather set.

No, I didn't - a birder friend took it for try to obtain the skeleton. I asked him for the feathers but he never gave to me.
We find this bird in a trip in January, maybe died from cold? Also we found a dead fresh skylark in the same trip. What a casuality!

At least it wasn't wasted, as many sensitive people tend to do... be it a find, or a rescued bird that died in captivity.
I found some fresh Waxbills last winter, that got drunk eating mountain ash fruits, and died overnight.
Unfortunately, they weren't mounted as the freezer broken
All that left are feathers & bones.

Dartford Warbler

I think you mean Waxwings.

Thanx for correcting
But real waxbills would be nice too. If they weren't so small and hard to mount.

I got 0_0 about that mountain ash berries that can kill a bird, overall a bird specialized in consuming berries and that have the same natural distribution and habitat than the mountain ash.

We have waxbills in Spain - they formed colonies from escaped individuals. Also red avadavats and golden bishops. I never saw one in the wild, at the moment.