While searching for unusual nests, I found the book "Silent Nests" by photographer Viki Topaz via thestylesaloniste.com. The book documents the deserted dovecotes or "columbiers" of Normandy and Britany. Dovecotes were used by landowners to house pigeons for meat, eggs, and their droppings were used for fertilizer. Thousands were built between the 13th century and the French Revolution.
"Lesser structures have been built to worship gods, let alone to commune with birds."-Scott Matin Kosofsky