a carton for store-bought eggs, nest materials help to
cushion, insulate, and keep the clutch together. These
materials may be of little importance for birds such as
terns and murres, which require little more than a resting
spot for their eggs. In contrast, nest materials can be
critical for some species -- as in the case of Bald Eagles,
whose young require a durable playpen that an adult pair may
maintain for decades.
An important function of nest materials gathered by ground-nesting precocial birds is to prevent their eggs from becoming embedded in sand or mud after heavy rains or flooding due to exceptionally high tides. Often only enough material is collected to create a buffer that raises the eggs off the substrate. Such a buffer also helps to guard against cracking when the eggs are rotated for uniform heating during incubation. In the case of many marsh-dwelling birds, the buffering layer may be expanded to form sides and sometimes extended to create a relatively fragile canopy, helping to hide the site.
Because the form of the nest varies from habitat to habitat, and must be adapted to fit a bewildering diversity of supporting structures, it is not surprising that an almost limitless variety of materials (including stones and mud, animal and plant products, and human-made artifacts) have at one time or another been incorporated into nests. Avian products that become part of nests include saliva (the main ingredient in cave swiftlet nests used in Chinese "bird's nest" soup), ejected pellets, feathers, down, and guano. Feathers are highly valued, in part because of their capacity to trap air and provide insulation. Products of other animal species may include silk from cocoons and spider webs, cast snake skins, hair, fur, bits of cow pats, shells, etc. The variety of plant and manufactured products found in nests is enormous, including virtually anything that can be carried.
Some adhesive materials are required to bind and to provide support in adherent and hanging nests. Such materials include mud (swallows), saliva (swifts), caterpillar silk (hummingbirds, vireos), certain plant fibers, and leaf mold (Wood Thrushes). These binding materials can be remarkably durable. For example, cellulose, the major constituent of plant fibers, is waterproof and, ounce for ounce, stronger than steel. Other water-shedding substances used in nests include lichens and spider webs.
Some materials are selected specifically to help sanitize the nest. More than half of our hawk species routinely add fresh green leaves that contain natural pesticides such as hydrocyanic acid, which may inhibit infestation by insect parasites. Such preventive efforts are not limited to birds of prey. Users of old nest sites, such as starlings, can discriminate between helpful and ornamental leaves and select those that deter lice and bacteria for inclusion in their nests.
Avian ingenuity is seemingly boundless. For example, the Great Kiskadee is known to add the entire nest of the Vermilion Flycatcher to its own, presumably to increase its cushioning/insulating properties. Lists of nesting materials published by early ornithologists provide a sort of fossil record allowing us to trace changing patterns of use. Comparison of these records with contemporary observations of nest materials helps to document changes in the availability of materials. For instance, the Chipping Sparrow, at the turn of the century, was commonly referred to as the "hairbird" from its practice of lining its nest with horse hair. With the advent of mechanized travel and the decline of horses, both the trait and the name disappeared. Similarly, a number of contemporary inventions such as plastic insulation and cellophane may substitute for snake skin in nests of some flycatchers and titmice or replace other once common materials. Thus, nests used perennially could serve as storehouses of data. A White Stork nest still in use in 1930 dated to 1549. One 36-year-old nest of a Bald Eagle, which finally collapsed along with its supporting tree in a storm, contained two tons of accumulated material. Dissection of either nest could have proved a fascinating (if messy) enterprise.
SEE: Nest Lining; Masterbuilders; Disease and Parasitism; Eggs and Their Evolution; Incubation: Heating Eggs; Precocial and Altricial Young.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.