What Is a Nest Made Of? Choosing Materials for a Structure That Must Not Fail

A nest is an engineering problem with several requirements that pull against each other. It must carry the weight of eggs, of growing young and of an incubating adult without collapsing, which needs structural material. It must retain heat, since eggs and nestlings cannot regulate their own temperature, which needs insulation with trapped air. It must shed water or drain, since a waterlogged nest chills its contents fatally. It must attach securely to whatever supports it against wind. It must be inconspicuous to predators. And it must be built within the time and energy the bird can spare, which limits how elaborate it can be and why many species build a rough structure quickly rather than a good one slowly.

Different parts of a nest use different materials for different properties:

• •Twigs, stems and grasses for the structural outer layer, chosen for stiffness and interlocking
• •Mud, used by several groups as a binder or as the whole structure, which sets hard
• •Spider silk, used to bind and to attach, and unusual in being both strong and elastic
• •Moss and lichen for the outer surface, which camouflages against bark
• •Feathers, down, hair and plant fluff for the lining, all of which trap air
• •Saliva, which some swifts use as the entire building material

Experiments have established that selection is not random and involves more assessment than expected. Birds offered materials of different stiffness choose appropriately for the structural role, and zebra finches given the choice select stiffer material when building in a situation that demands it. Birds prefer materials of a particular length and abandon pieces that do not work, showing something that looks like trial and adjustment. Some species incorporate aromatic green plant material into the lining, which reduces parasites, and the selection is specific to plants with the relevant properties. Species nesting near people incorporate paper, string, plastic and in several documented cases cigarette ends, which contain residual nicotine and have been reported to reduce parasites while carrying costs of their own.

Construction varies enormously between species in effort and in who does it. A pigeon assembles a few sticks in a day and loses eggs through the gaps routinely. A weaver bird ties knots in grass to build a suspended enclosed structure over about a week, and a male builds several in a season for females to inspect and reject. Swallows and martins carry mud pellet by pellet over hundreds of trips. Eagles add to the same structure across years until it weighs a tonne and breaks the tree. In many species the female builds alone, in others both work, and in a few the male builds while the female judges. Some species build nothing at all, laying on bare rock or in a scrape, which is a viable strategy where predation pressure and climate allow.

Nests are used as evidence in several ways. Structure and material are diagnostic enough that a nest can identify the species that built it, which is the basis of field guides devoted to them and of survey methods that count nests rather than birds. The materials record what was available locally, which makes historical collections a source of information about past vegetation and about the arrival of synthetic materials in the environment. Contaminants accumulated in nest material have been analysed as a pollution record. And changes in nest composition over decades track changing land use, with several studies documenting the replacement of natural fibres with plastic and the ecological consequences that follow, including entanglement of nestlings.

A nest must carry weight, insulate, drain, attach and stay hidden within the energy a bird can spare, and different materials serve each requirement. Stiff stems build the structure, moss and lichen camouflage the outside, and feathers and fluff line it. Experiments show birds choosing stiffness and length deliberately rather than gathering whatever is nearest.