Ivo — Instead of looking in the books, I started with its rather strange posture, not typical of most birds, and immediately thought of the Nuthatch,
Sitta europaea, as we had had one frequent the oak tree under which we pitched our tent this summer. Ours had walked up the trunk, down the trunk, round it one way, round it the other, and then it did the hokey cokey, turned itself around, and that's what it's all about!
Everyone together now:
Oh, the hokey cokey,
Oh, the hokey cokey,
Oh, the hokey cokey,
That's what it's all about.
Then it did it again, around a branch.
... and again:
Oh, the hokey cokey,
Oh, the hokey cokey,
Oh, the hokey cokey,
That's what it's all about.
It's the only British bird that habitually and quite normally walks down tree trunks and along the underside of branches, so more or less any posture is OK. Incidentally, the Treecreeper,
Certhia familiaris, will do the same, but not very often, preferring to fly to a new location and work upwards (at least those I saw on the same Purbeck campsite did).
If you take into account a very limited colour palette, artistic licence, and that it was probably painted from a drawing, it's a fairly good likeness. See the Royal Society for the Prevention of Birds
web page.
I hope you all joined in and enjoyed that little musical interlude.
Bernard C.

ps — In The Netherlands
Sitta europaea has the wonderful name
Boomklever — see
here for other names.