Architects of The Air

Whether a nest lasts for decades or survives only a few weeks, it performs several functions. Nests are incubators for eggs, nurseries for young and hiding places for brooding parents, all vulnerable to predators and foul weather during nesting season.

Nest building is a complex activity and, although the ability is instinctive, birds get more skilled with practice. Some birds must make a thousand trips for materials to complete their nest. Others face the task of gouging out a cavity to use.

The drive to build nests is the result of a cascade of hormonal changes in birds that usually is brought on by lengthening daylight in the spring. Courtship, mating and brooding are all governed by hormones, though external factors also play a part.

The sound of bird song and the availability of food also have been shown to induce nesting. In addition, some birds build nests before they court or use nestbuilding as part of their courtship ritual. The mere sight of eggs induces some female birds to brood, while the site of brooding females can induce males of some species to sit on the clutch.

Nearly everyone can identify a robin's cup-shaped nest, fashioned of mud and grass. The reason they are so common is that robins usually build a new nest for each brood they raise. They've even been known to build several nests at once, sometimes side by side. Whether they do this because they are confused or to confuse predators isn't known, but both parents participate in this flurry of building.

Cardinals, finches, blue jays, mocking birds, crows and many other species also build some variety of the cup-shaped nest. Hummingbirds build the tiniest nest of all, little more than an inch across. It usually straddles a small, downward sloping branch and is easily overlooked. Female hummingbirds construct the nests and use spider webs and bits of lichen to disguise them. Because their nests are so tightly built, hummingbirds can return to them year after year.

The female red-eyed vireo builds a basketlike cup that hangs by its rim from the fork of a thin branch. She often uses grapevine bark in the construction. Male and female Baltimore orioles weave a pouch suspended from a drooping branch. The inside is lined with hair, fine grass and bits of string. This intricate pouch can take up to two weeks to construct.

Barn swallows and chimney swifts actually have benefited from human encroachment. Many of our structures provide them with ideal nesting habitat. Their nests are a variation of the cup nest.

Barn swallows build their nests of mud pellets. Both parents deliver the pellets in their beaks to the nest site, which can be in barns, under bridges, on buildings and on cliffs. They construct a cone of mud and top it with a cup. Females then line the nests with softer materials, including feathers. Swallows commonly nest in colonies.

Chimney swifts construct an even more unusual variation of the cup nest. Swifts favor enclosed vertical spaces, such as chimneys, silos, old wells and attics for their nests. Both males and females will break twigs from the ends of dead branches to use for their nests. They stick the twigs to the wall with their own saliva, which soon hardens into glue. Eventually they accumulate a half-saucer nest for the eggs.

The cup is not the only design birds employ. Many species, including bald eagles, build a simple platform for their eggs. Both eagle partners pile branches, large sticks and even cornstalks in the fork of a tree, which is usually near water. They then line their platform with grasses and moss.

Bald eagles return and add new material to their nests year after year. Sometimes they build two nests, so they can alternate sites annually, which may reduce lice infestation. Some bald eagle nests have been used for up to 30 years, but often by this time they have grown so heavy from their annual accumulation of material that they crash to the earth under their own weight.

Most birds of prey build platform nests. Sometimes they build on cliff ledges, rather than in trees. American kestrels, typically cavity nesters, sometimes build their nests in the underparts of golden eagle nests. Grackles sometimes do the same in osprey nests.

Great blue herons are platform builders and generally nest in colonies. The male brings the sticks to the female and she arranges them on the nest, usually in the highest part of the tree. Mourning doves also build platform nests, sometimes on top of abandoned robin nests. Mourning dove nests are often insubstantial, and the eggs in them may be visible from below.

Woodpeckers meet the nesting challenge by gouging cavities in the trunks of dead trees. Commonly, that's the extent of the nest, though they may soften it with wood chips in the bottom. The entrance to the pileated woodpecker's nest usually faces south or east. For the downy woodpecker, it seems important that the entrance be on the underside of a limb.

Wrens, chickadees, titmice and many owls also are cavity nesters. They line the nest cavity with soft grasses, feathers or fur. Tufted titmice and great crested flycatchers add cast-off snakeskin to their nests, which may scare away predators.

Though some of these birds will excavate their own holes, most prefer to use holes abandoned by other birds. In recent years, bluebirds have benefited from the proliferation of bird houses built to suit them. These artificial cavities have helped increase the once declining population of Missouri's state bird.

Purple martins take readily to colony houses set out for them. They add green leaves to their nests. The leaves can help regulate humidity and may emit a natural fumigant to control lice.

Some cavity nesters don't rely on trees or houses to raise their brood. Belted kingfishers dig a cavity into a stream bank. Both partners participate in the excavation, and they often return to the same nest in subsequent years. The first clutch is laid directly on the dirt floor, but subsequent clutches usually rest on fish scales and other debris from past feedings.

Cavity nesters have a higher success rate raising their broods than open nesters, but the open nesters compensate by tending to have more broods in the season.

And then there are birds that don't bother with nest building at all. A cowbird will lay her eggs in the nest of another species, leaving incubation and rearing to the adoptive parents.

Killdeer just use a scrape on the ground for their nest. They may line it with gravel, wood chips or other debris, but they rely on the mottled coloring of the eggs to hide them. Killdeer parents often feign a broken wing to draw predators away from their nest.

Turkeys make minimal nests. A hen scratches out a small depression in leaf litter, often near a fallen log, to lay her eggs. While she is brooding, she sometimes draws leaves over her back. When she gets up, the leaves fall on the eggs and conceal them until her return.

Birds that make virtually no nest tend to produce young that are mobile within hours of hatching. The young of some species, such as bobwhite quail, may fly within a week.

Not all ground nesters produce mobile young. Turkey vultures lay their eggs on the bare ground and must tend to their young for as long as three months before they are able to fly. Nor do all ground nesters forsake a nest. The females of both Canada geese and mallards build nests on the ground, usually in dense vegetation, though geese readily build in washtubs set out for them. Both birds line their nests with grass, cattails and reeds, eventually adding down from their own breasts.

The female ovenbird builds her nest among the leaves of grasses, moss and even root fibers. Its arched top made of leaves and surrounding plants completely hides it from predators above. Female meadowlarks build similar nests.

The spring frenzy of nest building is fascinating. Birds are especially active, and their comings and going for nest materials can lead you to their building sites. Yet nests should be left undisturbed during nesting season. Poking around too close may keep the birds from their nests or cause them to abandon them. Eggs left unattended for even a few minutes can die.

One of the best times to find nests is in the winter, when leaves are gone and you can spot most silhouettes in the trees. An area with several nests suggests that the site is favorable to birds, and they likely will return to it in the spring.