What Is a Nest? Built Structures From Animals With No Tools

The requirements are specific and they conflict. It must hold eggs securely, since a rolling egg is a lost egg, which is why cliff-nesting species that build nothing lay pointed eggs that pitch in a circle rather than rolling away. It must maintain temperature, since developing embryos need a narrow range and the structure insulates against both cold and overheating, with thicker linings measurably correlated with colder climates across species. It must be concealed or inaccessible, since predation is the dominant cause of nest failure. It must be strong enough for the load and the weather, and the weight of a growing brood is substantial. It must permit the parents to enter, sit and feed. And it must be built quickly and cheaply enough that the investment does not exceed the value of the brood, which is why many species reuse structures or build minimally. Those requirements trade against each other, which is why designs vary so much.

Bird nests fall into recognisable types and the variety extends well beyond the familiar cup:

• •Scrapes, meaning a shallow depression with little or no material, used by many ground-nesting birds and relying entirely on camouflage
• •Cups, built of twigs, grass and moss and lined with softer material, frequently bound with spider silk and shaped by the bird pressing its body against the interior
• •Cavities, either excavated as woodpeckers do or occupied as many species do afterwards, which offer excellent protection and are in short supply
• •Pendant and woven nests, including those of weavers and oropendolas, which involve genuine knots and are suspended from thin branches to defeat climbing predators
• •Mound nests, including those of megapodes, which bury eggs in rotting vegetation and let fermentation heat incubate them while the adult regulates temperature by adding or removing material
• •Colonial structures, including sociable weaver nests housing hundreds of pairs in one mass that persists for decades and provides thermal buffering

Whether nest building is learned or innate turns out to be a mixture, which is a more interesting answer than either extreme. Birds raised in isolation from adults and without ever seeing a nest generally build a recognisable species-typical structure on their first attempt, which establishes a substantial innate component. At the same time, experienced birds build faster and better, first attempts are frequently poor, and experimental work has shown that birds learn material preferences, adjust technique after failures and can be influenced by observing others. Studies with weaver birds recording repeated attempts found individual variation and improvement over time rather than a fixed routine. Some species incorporate learning about the local environment, selecting materials available where they are and adapting to novel ones including string and plastic. The general picture is of an inherited template refined by experience, which is the same pattern found in birdsong and in several other complex behaviours.

Birds are the obvious nest builders and far from the only ones. Social insects construct the most elaborate structures, with termite mounds incorporating ventilation systems that maintain internal conditions through passive airflow and with wasp nests built of processed wood fibre, which is paper and predates its human invention. Fish build, with sticklebacks constructing tunnels glued with a secretion and several species excavating pits. Great apes build sleeping platforms fresh each night, which is a genuine construction behaviour and one of the closest parallels to human shelter building. Mice, squirrels and many mammals build nests for young. Some spiders build nursery webs. And several species build structures with no reproductive function at all, most strikingly bowerbirds, whose bowers are display arenas decorated with collected objects sorted by colour, which serve no purpose except to be assessed by a visiting female.

The structure must hold eggs, hold heat, hide from predators, carry a growing brood and cost less than the brood is worth, and those requirements conflict, which is why designs range from a bare scrape to a woven suspended chamber. Cliff nesters that build nothing lay pointed eggs that pitch rather than roll. Birds raised in isolation build a recognisable species nest and still improve with practice.