Birds are the true architects of the natural world. Not only do they design fascinating and complicated structures, but the variety of materials used is absolutely incredible. From tree cavities to cliffs and simple platforms, the birds use any kind of natural substance that they can maneuver with their beak or foot. They’re also not above making use of man-made structures or objects, which is why it’s so easy to enhance your birding pursuits simply by providing a properly designed birdhouse or some dog hair, and then watch them work.

The act of building the nest is triggered by hormones. Other factors such as migration, day length, food availability, and territoriality also contribute, although such behaviors are subject to hormonal influence. Birds’ reproductive organs are significantly reduced in size for most of the year, an adaptive strategy so that they have less weight to carry during flight. Once the hormones kick in, a man’s testicles and a woman’s ovaries increase enormously in size. At post-mortem examination, it is easy to tell the reproductive status of a bird simply by checking the size of these organs. After the breeding season is over, the ovaries and tests are reduced to small structures located ventrally to their kidneys.

nest types

Most people are familiar with the size and shape of a robin’s nest, which is a mud and grass structure often found in our yards or even built over a porch light on our front steps. But there are many other types of nests made with a host of innovative materials. Here are some examples of nests that you can find when you are exploring.

• Cavity Nests – Created in trees by woodpeckers or natural weathering, tree cavities are homes for many species, including swallows, woodpeckers, nuthatches, chickadees, wrens, bluebirds, wood ducks, saw owls and many more. Some birds can even build a nest where they find ‘artificial’ cavities in human structures, such as stove vents, dryers, fireplaces, eaves, soffits and under shingles. The classic birdhouse, made by humans in a variety of shapes and styles, caters to many cavity nesters.
• Cups: Commonly seen on ledges or in the hollow of a branch, a nest cup is generally a circular woven bowl made of grass, leaves, yarn, hair, and mud or saliva to help hold it together. Many species make this type of nest, such as blackbirds, goldfinches, robins, and barn swallows. The latter two species may build their nests on an artificial nest platform secured to the side of your house or garage.
• Hanging Baskets: The colorful Baltimore oriole makes a delicate grass basket that hangs from a branch and sways in the wind, as if it might fall apart in a light storm. Orioles aren’t the only basket weavers, though. Other species such as the bushtit also weave baskets, using moss, hair and plant fibers.
• Mud tubes: Who has not seen the amazing globular nests of cliff swallows? It takes hundreds of trips from the source of mud to the bridge, building or cliff face that houses a cliff swallow’s tubular nest. Each tiny ball of mud is mixed with saliva to create enough adhesive power to hold it all together.
• Platforms: Gathering branches, twigs, grass (even green grass) into a rather disorderly mass, many birds of prey build their nests on a support platform, using natural and artificial foundations. Some are relatively small and inconspicuous, hidden from potential predators. Cooper’s hawks and sharp-shine hawks are likely to build them. Others are enormous in both size and stature, such as those made by ospreys and eagles. Adding materials year after year, both ospreys and eagles can accumulate hundreds of pounds of nest material which increases in size, perhaps reaching 7 feet across and as many feet deep. One nest was known to weigh over a ton when it finally collapsed. Platform nests can be located on top of an old magpie nest, in a strong tree, on the edge of a cliff, or on a man-made structure such as a power pole, bridge, or artificial nest platform. The green plants added each year are known to contain natural repellents that help ward off insects and parasites.
• Tunnels: Kingfishers and puffins, two fish-eating species, tunnel into earthen banks or cliff edges overlooking their favorite hunting grounds. Burrowing owls will use the pre-made tunnels created by rodents such as Richardson’s ground squirrels.
• Ground: Many species hide their nests somewhere in the ground, sometimes protected by a low bush, sometimes protected only by the fantastic camouflage of the substrate itself. Shorebirds, ducks, and many songbirds build this type of nest. Some birds of prey even nest on the ground, such as the long-eared owl, the snowy owl and the royal hawk. These birds of prey have evolved in areas where they may not see a tree for weeks, so they have adapted accordingly.

The main purpose of a nest is to hold the eggs while they hatch. This is a very vulnerable time, not only for the eggs but also for the adults that sit on them. Eggs typically take several weeks to hatch, so it is critical that they remain hidden from predators and protected from the elements. The nest structure often provides insulating properties, either by the materials used, such as feathers, or by the location of the nest with respect to prevailing winds and sun exposure.

nest materials

When it comes to nest building, we think of grass, mud, feathers, and twigs, as mentioned in several of the nest types above. In fact, these are all common ‘fabrics’ in nest architecture. We could add to that list what may seem like foreign items: guano, spider web, snakeskin, shells, saliva, fur, even human artifacts like rope, plastic, buttons, shoelaces, and jewelry! Each of these has a purpose, believe it or not. For example, dung helps disguise the smell of chicks, thus fooling predators. (Look up references to burrowing owls!) Saliva acts like super glue. Insect and spider silk add tensile strength and can also repel rain. Nuthatches smear tar around the entrance hole of their nest and then rub the ants on the tar; the combination is both a chemical and mechanical deterrent to ward off marauders.

If that wasn’t enough, it’s not uncommon for a bird to deliberately build near a bee or wasp colony, a truly ingenious way to keep pests like flies and chirping away.

The construction process

So how complicated is it to be a bird ‘contractor’? Some species are so lazy that they prefer not to build anything and simply take over a nest previously built by another species in a previous year. The great horned owl is a class example. This species often uses old magpie, crow or hawk nests, adding a new twig or two and being quite satisfied with that. Several species of shorebirds use little more than a slight depression in the natural substrate, camouflaged by rocks, dirt, and vegetation. Other species have nests so complicated that they literally need to make over a thousand trips back and forth, carrying the necessary materials.

In most cases, a new nest is built each year, although some species return to the same nest year after year, adding materials and repairing any damage that may have occurred over the winter.

A newly built nest is beneficial for several reasons. Nests need only last a short time in the first place, as the entire time from egg laying to fledging can last as little as six weeks, so it is counterproductive to spend a lot of time and energy on something that is temporarily required. . Second, avian parasites can accumulate in nest materials and then persist through the winter, so when the nesting pair returns in the spring, there may be enough parasites to pose a serious risk to the health of the chicks, perhaps even contributing to their mortality.

Conversely, making simple modifications to an existing nest, rather than starting from scratch, saves time and energy, priceless resources for almost all birds.

To maximize efficiency, birds tend to build relatively small nests. Once again, temporary accommodations do not need to be large. Another advantage of a small nest is the ability to hide it from predators more easily. Perhaps the most important reason for a small nest relates to the super-fast rate at which chicks develop into full-fledged juveniles. Imagine going from a newborn baby to an Olympic-caliber athlete in just six to ten weeks! The nutrition required to achieve this is staggering, but even the best nutrition is not enough without adequate foot support for maturing birds. As a chick grows, it must keep its legs firmly tucked under its body or the legs will grow out to the sides, in a splay-legged fashion that is ultimately fatal. Therefore, the proximity that is achieved in a crowded nest ensures that this will not happen.

Bird’s nests and the law

The vast majority of bird species in North America are protected by an international treaty known as the Migratory Bird Treaty Act. This protection extends to their nests and all their occupants. It is illegal to harm, move, or harass nesting birds. If a bird builds its nest in a location that may be dangerous or inconvenient for a homeowner, they must obtain permission from local and/or federal wildlife authorities to remove it. The wisest choice is generally to leave it where it is; it is not difficult to coexist peacefully with most species. Consider how short the nesting season is, and how enjoyable it can be to watch the laborious process of raising chicks until that momentous day when the chicks venture out of the nest for the first time.

If you think nest building is fascinating, the intricacies of egg laying will blow your mind! Keep an eye out for a future article on this topic, as well as a handful of other interesting topics on wild birds and mammals.

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