These are the famous Swallows of San Juan Capistrano that return faithfully to nest on the walls of the Mission each year within a day or two of March 19. They spend most of the fall and winter in western South America, especially the grasslands of Argentina, but then spread out to breed over almost all of North America, including northern British Columbia and Alaska.
Their gourd-shaped nests are constructed entirely from mud and saliva of the many Cliff Swallows in the colony. This happened to be a small colony, but Cliff Swallows are a very gregarious species, so there might be as many as 3000 nests in a particularly insect-rich environment.
There was a lot of activity at this colony with birds flying in and out of the nest holes. Sometimes it got a little chaotic, with individuals in adjacent nests almost colliding on their entry or exit.
They are handsome birds with their russet colored cheeks and throats, white foreheads, and blue-black heads and backs. They might be mistaken for Barn Swallows, but have squared-off rather than forked tails, and pale breast and belly feathers rather than russet colored.
Like Barn Swallows, Cliff Swallows construct their mud nests under overhangs, like the eaves of buildings, bridges, or overpasses on roadways. Flying in and out of their nests can put them directly in the path of moving vehicles, and has been a significant source of mortality for these birds. However, ScienceNow reported that wings of Cliff Swallows that successfully nested near roadways were significantly shorter than those of birds killed on the roads. Shorter wings might increase the energy cost of flight, but it would improve aerial mobility, allowing birds to avoid collisions. This species, at least, seems to be successfully adapting to human selective forces.