mapper - the mud nests under my house eves come from a mud puddle at the beginning of my driveway. This year the puddles were full of the gravel I added. They got mud from somewhere else this year.
BTW - those that build nests in the eves of the house are just the overflow from the MASSIVE swallow nesting area down the lake about 850 feet. There is a natural formation in the basaltic lava cliffs where no less than 400 to 500 swallows make their nests. You want to get CRAPPED ON - just go down there. Stand on the edge of the cliff - yell, BOO. It can be so grizzly & icky - you darn near want to jump off the cliff and fall the 40 feet into the lake to wash it all off. I reserve that specific location for people who have chosen to irritate me - ha, ha.
Thank goodness - these swallows don't use dirt from the driveway. There is plenty of good, sticky mud at the outlet of my little lake.