Nests are structures built by certain animals to hold eggs, offspring, and occasionally animals themselves. Although nests are most closely related to birds, all kinds of vertebrates and some invertebrates build nests. They may consist of organic materials such as branches, grass and leaves, or they may be a simple depression in the ground, or a hole in a rock, tree or building. You can also use artificial materials, such as thread, plastic, cloth or paper. Nests can be found in all types of habitats.

Nesting is driven by a biological impulse called the nesting instinct of birds and mammals. Usually, each species has a unique nest. The complexity of nest is related to the level of parental care of adults. Nesting is considered to be the main adaptive advantage between birds. They show the biggest change in the nest, ranging from simple holes on the ground to exquisite public nests that can accommodate hundreds of people. Prairie groundhog nests and several social insects can hold millions of people.

Nesting is often driven by the biological impulse of pregnant animals to protect their offspring, which is called nesting instinct. Animals build nests to protect their eggs, their offspring or themselves from danger. The simplest nest like structure is suitable for avoiding predators' eggs, protecting them from sunlight or other environmental factors, or just preventing them from spreading in the current. In some cases, nesting also helps to provide a safe number of spawning animals.

Many nesting workers provide parents' care for young people, while others just leave after laying eggs. In birds, they usually hatch (by sitting on eggs). In general, the complexity of the nest increases with the level of parental care provided. Nesting will strengthen social behavior and allow more people to be accommodated in a narrow space, thus increasing the bearing capacity of the environment. Insects with the most complex nest structure also have the largest social structure. In mammals, the naked mole mouse shows a caste structure similar to that of social insects, and has built a wide range of caves that can accommodate hundreds of people.

In general, birds are the most skilled nesters, although not all kinds of birds build nests. Some birds lay their eggs directly on rock ledges or exposed soil without first changing the area. Complex nesting is considered to be one of the main adaptive advantages of birds. Nests help regulate temperature and reduce the risk of predation, thus increasing the chances of adult offspring.

Bird's nests vary from simple depressions on the ground (called debris) to a collection of disorganized branches, to carefully woven pendants or spheres. Giant toes are one of the few groups that do not breed directly, hatching their young in a rotten vegetation. One of them is the giant macro (macrocephalon maleo), which uses hot volcanic sand to keep its eggs warm. There are falcons, owls and many water birds in simple nesting. Weavers may show off the most exquisite nests and complete them with bundles of grass tied together. Most nests are located somewhere in the middle, while most cup-shaped nests are made of mud, branches, leaves and feathers. Some birds, such as flamingos and swifts, use saliva to help hold their nests together. Edible bird's nests are made of saliva only. The reddish brown hornero nests are made entirely of mud and dung, which are placed on branches so that the sun can harden them into usable structures. Tailoring birds sew the leaves together to provide cover for their nests.