Here is something I learned in Arizona. . . around here birds often build nests in cactus. I am not sure if they create a hole in the cactus or if they enlarge an existing flaw, but they hollow out an area large enough for their nest. Because they are workiing with a living plant, the area around the nest sort of scabs over and becomes a separate piece of the cactus. After the cactus dies sometimes the hollowed out part can be found. Randy found one of these recently - everything around the nest area has fallen away, leaving just the nest cavity.
The opening to the nest is smaller than the rest of it, and makes it look like a crumpled paper bag. And it's about as lightweight as a paper bag.