Barn Swallows (pictured here) and Cliff Swallows build nests almost entirely out of mud. Historically, these structures were built on cliffs or near the mouth of caves, but are now most often found on man-made structures.
The disadvantage of building nests in buildings is that sometimes the mud does not stick well to the smooth wood. Sometimes people remove the nests in an attempt to prevent a build-up of droppings. At this location, someone decided to give the birds a hand.
A cardboard berry carton was tied to a rafter, and the Barn Swallow has used it as a foundation for her nest. You can see the mud, plant fibers, and feathers sticking out over the top of the carton. Notice how the rafter is nearly covered with mud from previous nests or nesting attempts.
Here is a beautifully formed Cliff Swallow nest. The overhanging metal serves as the top of the structure, reducing the amount of mud needed. Notice how the mud at the bottom of the nest is a different color and includes plant fibers, suggesting that this nest was built on the foundation of an old Barn Swallow nest.