A nest is a structure built by certain animals to hold its eggs, its offspring, or occasionally the animal itself. Although nests are most closely associated with birds, members of all classes of vertebrates and some invertebrates construct nests. They may be composed of organic material such as twigs, grass, and leaves, or may be a simple depression in the ground, or a hole in a rock, tree, or building. Human-made materials, such as string, plastic, cloth, or paper, may also be used. Nests can be found in all types of habitat. Nest building is driven by a biological urge known as the nesting instinct in birds and mammals. Generally each species has a distinctive style of nest. Nest complexity is roughly correlated with the level of parental care by adults. Nest building is considered a key adaptive advantage among birds, and they exhibit the most variation in their nests ranging from simple holes in the ground to elaborate communal nests hosting hundreds of individuals. Nests of prairie dogs and several social insects can host millions of individuals. Nest building is often driven by a biological urge in pregnant animals to protect ones offspring known as the nesting instinct. Animals build nests to protect their eggs, their offspring, or themselves from danger. The simplest nests are designed to hide eggs from predators, shield them from the sun or other environmental factors, or simply keep them from being scattered in ocean currents. In some cases, nests also help provide safety in numbers for egg-laying animals.