Yesterday I posted a picture of a Cliff Swallow in a nest like this. These nests were on another, better lit side of the same building, but did not have any birds in them. I have learned that some birds nest in colonies and are called "colonial nesters." These nests are where the wall meets the overhang, and were quite high up. I have seen similar ones under bridges. Taken on Sunday in Missouri.
Gosia, they are made of mud balls. I believe they roll little balls of mud together with their bills and then fly up and stick them on to the existing mud. It takes quite a long time.
