On my dinner break this evening, I made a quick visit to Baylands Nature Preserve, where Cliff Swallows are building nests along the wall of the small building that controls the outflow from Casey Forebay into the flood control basin.
Yesterday, I watched as swallows flew down to get a bill-full of mud in the forebay, then up to the building where they clung to the wall with their feet while placing the mud on the growing nest. They worked for about twenty minutes then stopped, presumably to let the mud dry: you could see the layer of wet mud sitting on top of the previously dried mud.
This evening the swallows were again picking up mud and placing it on the nests. The nest have not increased all that much in size since yesterday; this appears to be a fairly slow process.