Bird nests come in many different shapes and sizes. Though many birds build nests on top of tree branches or on ledges, other species attach their nests to walls or build them in a hollow on the ground. Some species make holes in tree trunks and build their nests inside. Some species even hang their nests from tree branches. Nests that hang from the underside of branches fall into two classifications: the pensile nest and the pendulous nest.

The pensile nest is an open cup or oval nest suspended from the underside of the forks of tree branches as opposed to sitting on the top of the fork. These nests hang on the underside of the fork of two or more small tree branches. The sides and back of the nest attach to the branches with spider webs or small grasses. The open cup is where the female lays her eggs.

The olive-backed oriole, Eurasian golden oriole, vireos, reed and fan-tailed warblers and kinglets build pensile nests. They all construct their nests using bark fragments, grass, moss, twigs and wasp nest paper with spider webs to glue it all together. Using spider webs or small grasses, they attach the sides or rim to the underside of the forks of branches. They line the inside with fine grass, rootlets, feathers and animal hair to form a soft, warm nest area.

Pendulous nests usually have a round or oval nest area with a neck that suspends from the tip of a branch. The neck of a pendulous nest varies in length and width depending on the bird. Different birds create entrance holes in different areas of nest. Some entrance holes are located at the top of the neck and some at the top of the round nest area. Some birds add a tail to the bottom of the nest with an entrance hole.

The goldcrest, fire crest, white-eyes zosteropidae, sunbird, oropendola, Baltimore oriole, bushtit and weaver birds build pendulous nests. These birds weave grass, spider webs, twigs and other plant fiber together to form the nest and neck. They line the inside nest area with down, moss and other soft materials. Some of these birds, such as the bushtit, decorate the outside of the nest with flowers and leaves. Weaver birds nest in colonies; it is common to see nests for up to 20 or 30 mated pairs hanging from the branches of a single tree.