At some point they chose this bridge.
Which just happened to include a spot primed to become muddy.
Here is one of the crew checking out the natural resources.
And the work began.
As they came swooping down and collecting mud
for the structure and upkeep of their 'Cliff Dwellings'.
"Will you ever call me a Barn Swallow again?"
Not now that I have been educated and I know that they are Cliff Swallows.