A swallow used to enter our upper farmhouse room which didn't have windows that time. Day by day,it started to build its mud nest. When I went to the farmhouse after a while, I found it ready! It's unbelievable how the bird made and collected those small balls of mud ,one next to the other using its beak!If you turn the photo upside down, you'd think that nest was built on the ground ,but once you know it was the ceiling, the whole process becomes such a marvel!