Among terrestrial vertebrates, birds are a highly successful group due in large part to their superior mobility and the fact that they raise their young in nests. Not any old nest, but structures that are often tailor-made to protect eggs and young birds from harsh weather and, if need be, conceal them from predators.
Consider the following facts about nesting birds. First, instructions on how to build a nest is encoded in a birdís DNA, which is why related species within a taxonomic group usually build similar nests. All raptors, for example, build open cup nests by piling on sticks, while all woodpeckers excavate hollows. The fact that nest building is not a skill that birds need to learn helps explain why only a few construction techniques are used by the 9700+ species of birds.
Although a handful of speciesósuch as woodpeckersóuse the bill to chisel out a nest, for most birds the beak is used solely to transport and manipulate nesting material. In other words, none of the multi-faceted shapes and sizes of beaks has evolved to facilitate construction of a nest. To the contrary, a billís primary role is obtaining food, an activity that is so critical to a birdís survival that evolution is constrained from making modifications to bills relating to other uses. In essence, all birds build nests without the use of specialized tools..
Given the facts, naturalists are left with an intriguing question: How do birds manage to construct nests that function well in a wide variety habitats ranging from deserts to tropical rainforests to the Antarctic? Part of the answer hinges upon the materials used to construct a nest as well as a birdís skill at selecting a safe location to build, presumably a spot where chicks are protected from predators and harsh weather.
Indeed, the location of a nest site can have profound consequences. Historically, Cliff Swallows in western North America nested exclusively under ledges of cliffs. But now, most Cliff Swallows prefer to nest under bridges; as a result, this small species has significantly expanded its range across the NA continent. Likewise, ancestors of Eastern Bluebirds nested in tree hollows. But today many seek out nest boxes that enthusiasts provide in the thousands. Bear in mind that the success Cliff Swallows and bluebirds, is due to a change in where the birds nest and not by any modification to the type of nest that they instinctively know how to build.
There is one group of birds, owls, that seems to break all of the rules. Whereas most large owls raise their family in an open nest usually in trees, small Screech Owls seek out tree hollows and Burrowing Owls nest underground. What makes this group of birds so adaptable? Is it because owls are intelligent and wise? Probably not. Most owls typically do not build their own nests because they are content to use abandoned nests or hollows built by other animals. In essence, with no nest building instinct to control their options, they are free to be opportunistic in their choices..
Among species that build nests, the complexity of architectural designs exhibited vary from densely packed condos to simple scrapes in the ground as is the case for skimmers that nest on beaches. The depression made in the sand is deep and wide enough to hold a clutch of four eggs. To add material to such scrapes would only make a skimmerís eggs conspicuous and an easy target for predators. Furthermore, once hatched, the prococial chicks have hatched they spend little time in the nest.
For the majority of ground nesting birds a simple scrape is not sufficient. Marsh dwelling stilts, for example, often need to build a platform to raise the nest above the water and build a shallow wall to keep the eggs from rolling into the drink. Furthermore, the texture and color of the building materials chosen by stilts help the nest blend into the background.
Elevating the nest is also essential for Gentoo Penguins that breed in South Shetland Islands. In such an open barren landscape, stones are the only building material available to the birds. As a rule, the more stones a pair can collect, the higher they raise their single egg above the cold melt water flowing down the adjacent slopes
Understandably, pairs that nest early have the best chance for gathering enough stones to build a sturdy nest. In contrast, late nesters have to search far and wide to locate stones remaining on the barren patches of ground not covered by snow. Presumably, a scarcity of stones can prevent penguins from successfully breeding, which explains why some birds attempt to steal stones from a neighborís nest.
For the vast majority of birds, the ability to fly gives them the option to nest on cliffs and in trees where their eggs and offspring are safe from most terrestrial predators. If you think, however, that a home in the tree requires sophisticated construction, you would be mistaken. Many species simply add sticks to a pile on which they lay their eggs. The greatest challenge is getting the sticks to stay in place when first starting to build. But once a firm foundation emerges, adding the finishing touches is relatively easy. By thrusting sticks of varying sizes and shapes into the wall of the nest, a tangled mass is created that holds together remarkably well. Whether an inner lining of insulation is eventually added depends upon the need to keep the eggs and chicks warm.
ďPiling onĒ material can also be used to add a dome over a clutch of eggs as Cactus Wrens are known to do. In the western United States, American Dippers build exceptionally large domed nests near water often under ledges overhanging streams, behind waterfalls, and under bridges. Presumably, such structures help keep nestlings warm and dry. But apparently domes are expensive to construct, which may explain why some dippers - specifically those living under bridges - have dispensed with adding a dome all together.
Does having access to specialized tools require birds to exhibit extraordinary skills and inventiveness when constructing large or elaborate nests? Probably not. Instead, many birds rely on building supplies that literally assemble themselves. Essentially, even naÔve, young birds can build a functional nest if they choose the right materials. This is particularly true when silk or mud are used to hold a nest together.
Hummingbirds collect silk by wrapping spiderwebs around their beaks; the silk is later removed by rubbing the beak on the outer walls of the nest. Plumbeous Vireos (image) collect egg cases of spiders to adhere bits of leaves and twigs to their nests, while gnatcatchers and Vermilion Flycatchers use silk to attach bits of lichen to the outer surfaces, presumably as a means of camouflage. Because silk is lightweight, pliable, and strong, it is an ideal building material for small birds.
On the other hand, mud is the material of choice for many birds. Swallows possess a short beak that widens toward the base, a feature that is useful for catching flying insects. In fact, Barn Swallows build their nests almost entirely out of mud reinforced with small amounts of fibrous material. After depositing a load of mud and grass on the rim of the nest, swallows liquefy the mixture by rapidly vibrating the bill, which leads to the mud flowing into existing crevices in the nestís wall.
By collecting mud pellets, Cliff Swallows can attach their nests to vertical surfaces. In South Dakota, we filmed swallows transporting mud from the top of an embankment to the support beams of a nearby bridge. Being colonial, dwellings of Cliff Swallows are tightly clumped together so much so that most nests share walls with their neighbors. Without support underneath, however, it is remarkable that such structures do not collapse or become distorted under their own weight. To minimize nest sagging, the birds build in stages during which fresh mud is allowed to dry before more pellets are added to the nest.
A handful of birds in Australia, including the Magpie Lark, also use mud to construct the walls of their open cup nests. Unlike Cliff Swallows the nest is started on top of a branch in the fork of a tree. In addition, they mix plenty of fiber with the mud that they collect. Watch closely and you will see that Magpie Larks use both their breast and feet to help shape the nest while the mud is wet.
When constructing a hanging nest, piling on sticks or plastering with mud does not work. What is required is the ability to rivet or weave fibrous materials together. In North America, Baltimore Orioles lay their eggs in a pouch-shaped nest that hangs from the outer branches of trees. Such breeding sanctuaries are slowly stitched together out of stringy plant material and discarded fishing line. Surprisingly, the process of weaving is relatively straightforward. While holding a strand in her beak, a female pokes her bill through the wall of the nest or around a branch and then releases her hold; alternatively, she pokes an empty beak through the nestís wall and then grabs hold of material which she then pulls back through. Inadvertently, knots form, but not because the builder intentionally ties strands together.
When there is a critical mass of fibers in place, a poke and pull style of weaving works remarkably well. It is getting a nest started that is the most difficult phase of construction. Initially it takes trial and error to attach multiple fibers at both ends -- strands that will support the weight of the nest. The amount of effort required to start a nest depends on the weather and on how many anchor points a builder can reach when installing the first dozen or so fibers.
To summarize our conclusions, birds can breed in wide variety of habitats in part because of the varied types of nests that they are capable of building. And this is all accomplished without the aid of specialized tools. Success, apparently, depends on: selecting building materials that have inherent adhesive qualities and to a lesser degree, adopting a building technique that is appropriate for the materials available.
Undoubtedly, behaviors required to build a nest have been honed by natural selection. In fact, such skills are hardwired in the brain, which helps explain why there are only a limited number of designs and building techniques used by birds (e.g., open cup nest, dome nests, nest hollows, vase-shaped nests, and hanging nests). Yet, when birds have choices regarding where to build, they select locations that enhance their breeding success regardless of the type of nest they construct. Such flexibility can help birds adapt to rapid changes in their environment