Troglodytes troglodytes (Eurasian Wren)
As I was crossing from the overflow car-park I saw a wren flit down onto the access road. I walked another step, looked again, and HEY WHAT? it had completely disappeared. Then I realized that it had landed more or less on a grating in the road surface.
It can't have gone down there, can it? I thought. I stopped and watched, and sure enough, about 15 seconds later it popped back up and skittered into the undergrowth.
I've since discovered that this caving habit is well-known (except to me, apparently) and it is the explanation for the generic name Troglodytes.
It also helps to explain why wrens can live high up on the slopes of Snowdon in winter. Wrens are insectivores, but crevices are good places not only for shelter but for finding insects and spiders out of season.