A house martin looks out from deep inside its cliff nest; the original site for these birds to build their nests before we built houses with eaves for them - artificial rock overhangs.
This colony built their nests in the cave left of the sea stack, at the back of a sheltered bay on the north sea, near Aberdeen.
A closer view shows that it is more of a large overhang of rock rather than a deep cave.
Their are five completed nests in this photograph, and a few that have either fallen off or are only just begun to be built. There were twelve complete nests altogether. Each nest is well placed in a corner or rock within the whole larger overhang. A perfect safe site for a colony.
Then the bird dropped out of the nest and whizzed by, too quickly to photograph without flash - which I prefer not to use on wildlife.