Spring brings the Jackdaws' nest building to Stonehenge. One of the favoured holes is the natural chimney in Stone 60. The two holes arrowed in the picture below (click to embiggen) are connected by a natural winding passageway inside the stone. As this is very visible from the public path it is easy for visitors to see. The Jackdaws need to block the hole with twigs to build their nests. Yesterday some twigs they were dropping in from the top were still falling right through, but they are close to succeeding.
It is worth pointing out yet again that the concrete infill at the bottom of the stone does not extend up into the stone. It was a simple C shaped cavity that was strengthened in 1959 when the stone was straightened up; there is a hairline crack on the north side of the stone which was thought to presage a collapse.