They hollowed out a knot hole to family specifications.
After many trips out to dispose of their excavations they were ready to line the nest in time for the eggs.
They foraged for construction material.
as well as for the long days of brooding.
A promising nook with an overhanging roof drew a closer look.
The phoebes got right to work adhering their nest to the wall with mud. It's just visible at the top of the recess.
Once the family is born the parents have a busy job keeping the chicks fed with their fly-catching forays around the quarry.
Do we devitalize our wonder with the explanation: instinct?