|Cliff Swallow nestlings. Carlsbad, California. June 8, 2014. Greg Gillson.|
This is called an adherent nest, as it adheres to a vertical face, such as a wall or cliff. How many hundreds of mouthfuls of mud did the parents use to build this nest? The local San Elijo Lagoon Conservancy website has my answer: 900-1200 pellets.
These fully-feathered nestlings are probably capable of flight. But it's so much easier to stay home and be fed by your parents than set out on your own.