The term 'BIRD'S BUILDING' is a reference to the NEST of a bird, which is a structure built by a bird for its eggs and young. The NEST is usually made from twigs, grass, mud, and other materials, and is often lined with feathers or fur. The NEST is typically built in a tree, bush, or on the ground, depending on the species of bird. The term 'NEST' is a four-letter word that is used to describe this bird's building.