The cliff swallows have built their jug shaped nests under a bridge just up river from here.
Cliff Swallow On Nest
The nests are constructed of little mud clumps with some grass mixed in for to help hold the mud together.
The swallows build their nests where the mud will stay dry.
I remember as a boy walking under the overshot of our bank barn and watching the cliff swallows as they peered out of their nest, just like the one in the picture above. Their nest was on a hand-hewn beam just a few feet above my head.