Nests built by birds vary in size, location and building material. The location of most nests is directly correlated to the flight proficiency of the animal that has constructed it. A bird that does not spend much time in the air may nest on the ground. Likewise a bird that spends a lot of time in flight tend to nest in treetops or other higher places. Knowing which type of bird nest you are looking for can help discover it quickly.
Look directly on the ground if you are seeking a bird's nest that has been built by a bird that does not typically fly a lot. A nest on the ground is called a scrape and is typically just a small depression in the ground. Sometimes a scrape will include a couple stone, sticks or piles of leaves that can help you distinguish it. Birds that build scrapes include many shorebirds, gulls, and nighthawks, as well as more exotic birds such as penguins and vultures.
Search out cliff faces, river beds and other embankments that have exposed faces if you are looking for a nest of a bird that burrows. If you notice a large number of holes in the side of a cliff, it can be a good indication that nests of burrowing birds are within. Birds that burrow nests include bank swallows, kingfishers and other waterfowl. Since the nests are built within tunnels dug into cliff faces, seeing an actual nest requires disturbing and possible destroying the bird's handiwork. It is not advised to dig into burrows.
Look for holes in trees where a bird may be nesting its eggs or hatchlings. Many birds, including passerines, woodpeckers, owls and parrots will bore holes themselves or find holes in trees in order to build their nest. Birds that construct their own holes are called primary cavity nesters. The holes they create will look unnatural to the tree and should be quite easy to spot. Birds that use natural cavities in a tree are called secondary cavity nesters. These holes many be harder to see as they blend into the natural appearance of a tree.
Look within tree branches or other areas of a tree that will support an external nest. The most common type of nest, one that is used by most songbirds, is called a cup nest. They are, as the name suggests, shaped like a cup and made out of a variety of different natural materials. The nests can be located in the crotches or branches of trees as well as many shrubs. They are almost always supported from below which requires them to be built on a structure that will support their weight.
Search out buildings and other unnatural structures if you are looking for a nest of a barn swallow or other bird that tends to fly in and out of buildings. Nests of these birds, called adherent nests, are typically made of mud and sticks and are plastered to the side or corner of a building's wall or a stone ledge.
Don't investigate a bird's nest to the point of disturbing the bird. Keep your distance from the nest if the bird is currently in it. As well, keep your visit brief if eggs or hatchlings are inhabiting the nest. While the old assertion that touching a bird's eggs will cause the mother to reject your young is false, you want to minimise any fear or infringement on the bird's environment.