Small Miracles: The Wonder of Birds' Nests
There is evidence that some dinosaurs built primitive nests on the ground and even cared for their hatchlings there. Today many creatures besides birds—from wasps to mice to alligators—shelter their young or their eggs in nests that they construct themselves. But for variety of placement and material, and for sheer complexity of design, nothing can compare with birds’ nests. Especially among smaller birds, nests are often remarkable for their inventive use of local materials to provide support, shelter, and camouflage. The nests are tiny marvels of disposable architecture.
The skill to create them comes almost entirely from instinct (although there is evidence that young adult birds, making their first nests, do improve with practice). Studies have shown that at least some birds, hand-raised in captivity, can build a nest typical of their own species without ever having seen one. The instinct to do this must be flexible, because the locations and materials available for nests in the wild vary, but it must be based on a considerable amount of precision as well.
Even a small bird’s relatively simple nest may be composed of several kinds of material used for different purposes. For example, a white-crowned sparrow’s may have coarse twigs at the base, finer twigs and weeds intertwined with rootlets and bark strips to form the open cup, dry leaves in the outer edge, and fine grasses and other soft materials molded into an inner lining. And that’s just a simple number. A more complex nest, such as the long hanging pouch of an American oriole, may involve actual weaving or sophisticated knots tied in long plant fibers, and it may take days of intense effort to build.
Most birds are opportunistic when it comes to building materials, and will readily incorporate manmade items into their nests if they fit basic requirements of size and texture. Paper, string, nails, pieces of wire, and bits of fabric regularly show up in the handiwork of suburban birds. In some areas house finches have become a minor nuisance by dismantling nylon window screens to use the strands for their nests.
In many cases, though, the materials chosen must have specific properties. Studies of nesting European starlings have found that the birds were selecting certain plants, such as wild carrot and yarrow, containing chemicals that would inhibit the growth of mites and other parasites. In eastern North America the great crested flycatcher often adds a piece of shed snakeskin, and the power of suggestion imparted may help deter predators or other intruders. Chipping sparrows often use animal hair (gathered in farmyards, or even plucked from startled pets) for their nest lining. Feathers are also ideal for soft, insulating lining material. Big birds like quail or ducks use their own down feathers for this purpose, but swallows and other small birds prefer feathers dropped by larger birds. A truly extreme example of material gathering is practiced by certain tropical swifts, fast-flying small birds that will actually strike much larger birds in midair to knock feathers loose.
As a very general rule, females are the skilled builders. For many species they do all the construction, including some (such as hummingbirds) for which males abdicate any responsibility for helping with the nest or young. In other cases, the male provides the basic foundation and the female adds the detailed lining. A male marsh wren may build 20 or more “dummy nests” around his territory; the female chooses one, adds lining, and uses it as the actual site for the eggs. The male’s building spree is not wasted effort: The presence of all those decoys may provide some protection for the real one, as predators tire of raiding nests that turn out to be empty.
Some of the most impressive nests are also among the smallest. A hummingbird nest is a wondrous creation of tiny plant fibers, mosses, and spiderwebs, so small that a 50-cent piece would completely cover it. It is as soft as felt but strong, with the spiderwebs making it pliable enough to stretch and expand as the rambunctious young hummers grow and exercise in it. Many hummingbirds will camouflage the outside with bits of lichen. At a distance such an object looks, for all the world, like a natural bump or knob on the branch, thereby deceiving potential predators in the mother bird’s absence.
At the opposite extreme are eagles. A pair of bald eagles may use the same nest for years, adding material to it annually until it becomes huge (an extreme example can reach a depth of 20 feet and a weight exceeding two tons). Such a nest is merely a ramshackle heap of sticks, hardly an admirable piece of avian architecture, but it does have its admirers: House sparrows and other small birds sometimes tuck their own nests into the lower crevices, and great horned owls may commandeer the entire nest, perhaps even driving the eagles from their aerie.
The finest nests are crafted by smaller birds, however, and the majority are never reused, not even by their original builders. It seems all the more remarkable that birds should create these intricate structures for such ephemeral use. It seems ironic, too, that most of us are not legally allowed to possess these nests, even after they have been abandoned. But when we are lucky enough to find them in the wild, or see them revealed in works of photographic art like the ones reproduced here, we cannot avoid holding them and the birds that made them in absolute awe.