Wild Birds - Nests and Nest Building

Birds are the true architects of the natural world. Not only do they design fascinating and complicated structures, the variety of materials used is absolutely amazing. From tree cavities to cliff banks to simple platforms birds utilize any sort of natural substance they can maneuver with beak or foot. They're not above making use of man-made structures or objects either, which is why it is so easy to enhance your bird watching activities simply by providing a properly designed bird house or some dog hair, then watching them go to work.

The act of nest building is triggered by hormones. Other factors such as migration, day length, food availability, and territoriality also contribute, though such behaviors are themselves subject to hormonal influence. The reproductive organs of birds are significantly reduced in size for most of the year, an adaptive strategy so they have less weight to carry in flight. Once hormones kick in, a male's testes and a female's ovaries increase in size enormously. On post-mortem examination, it is simple to tell the reproductive status of a bird simply by checking the size of these organs. Once the breeding season is over, ovaries and testes shrink back to tiny little structures located ventral 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 the porch light on the front steps! But there are many other types of nests made with a host of innovative materials. Here are a few examples of nests you may find when you're out exploring.

• Cavity nests: Created in trees by woodpeckers or natural weathering, tree cavities make homes for many species, including tree swallows, woodpeckers, nuthatches, chickadees, house wrens, bluebirds, wood ducks, saw whet owls and many more. Some birds may even build a nest where they find the 'artificial' cavities in human structures, such as stove vents, dryer vents, chimneys, eaves, soffit, and under shingles. The classic bird house, manufactured by humans in a variety of shapes and styles, caters to many cavity nesters.


• Cups: Commonly seen on ledges or in the crook of a branch, a nest cup is usually a woven circular bowl made from grass, leaves, string, 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 two latter species may build their nest cups on an artificial nest platform secured to the side of your house or garage.


• Hanging baskets: The colourful Baltimore oriole makes a delicate grass basket that hangs from a branch and sways in the wind, looking for all the world as if it will fall apart in a mild storm. However, orioles aren't the only ones who weave baskets. Other species like the bushtit also weave baskets, using moss, hair, and plant fibers.


• Mud tubes: Who hasn't seen the amazing globular nests of cliff swallows? It takes hundreds of trips from the mud source to the bridge, building, or cliff wall that is host to a cliff swallow's tubular nest. Each little mud ball is mixed with saliva to create enough sticking power to hold it all together.


• Platforms: Gathering branches, twigs, grass, (even green herbs), into a rather untidy mass, many raptors build their nests on top of a supportive platform, using both natural and man-made foundations. Some are relatively small and inconspicuous, hidden from potential predators. Cooper's hawks and sharp shinned hawks are likely to build these. Others are huge in both size and stature, like those made by osprey and eagles. Adding materials year after year, both ospreys and eagles can accumulate hundreds of pounds worth of nest material that increase in size, perhaps reaching 7 feet across and as many feet deep. One nest was known to weigh well over a ton when it finally collapsed. Platform nests may be located atop an old magpie nest, in a sturdy tree, on a cliff edge, or on a man-made structure such as a power pole, a bridge, or an 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 species that feed on fish, gouge out tunnels in dirt banks or on cliff edges overlooking their favourite hunting zones. Burrowing owls will use the pre-made tunnels created by rodents such as Richardson's ground squirrels.


• Ground: Many species hide their nests somewhere on the ground, sometimes protected by a low-growing bush, sometimes protected only by the fantastic camouflage of the substrate itself. Shorebirds, ducks, and a lot of songbirds build this type of nest. Even some birds of prey will nest on the ground, such as short eared owls, snowy owls, and ferruginous hawks. These raptors have evolved in areas where they may not see a tree for weeks on end, so they have adapted accordingly.

The main purpose of a nest is to hold eggs while they are incubating. This is a very vulnerable time, not only for the eggs but also for the adults who are sitting on them. It usually takes several weeks for the eggs to hatch so it's critical they remain hidden from predators and protected from the elements. Nest structure often provides insulating properties, either by the materials used - such as feathers - or the nest location 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 above nest types. Indeed, all of these are common 'fabrics' in nest architecture. We could add to that list what may seem like strange items: guano, spider web, snake skin, shells, saliva, fur, even human artifacts such as string, plastic, buttons, shoelaces, and jewelry! Each of these has a purpose, believe it or not. For example, manure helps disguise the scent of nestlings, thereby fooling predators. (Look up references on burrowing owls!) Saliva acts as a super-glue. Insect and spider silk add tensile strength and can also repel rain. Nuthatches smear pitch around the entry hole of their nest and then rub ants in the pitch; the combination is both a both a chemical and a mechanical deterrent to ward off marauders.

If that weren't enough, it is not uncommon for a bird to deliberately build near a colony of bees or wasps, a truly ingenious way to keep parasites like botflies and warbles away.

The Construction Process

So how complicated is it to be a bird 'contractor'? Some species are so lazy, they prefer not to build at all and simply take over a previously built nest masterminded 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 shorebird use little more than a slight depression in natural substrate, camouflaged by rocks, earth, and vegetation. Other species have such complicated nests they literally need to make over a thousand trips back and forth, carrying needed materials.

In most cases, a new nest is built each year though a few species will return to the same nest year after year, adding materials and making repairs to any damage that may have occurred over the winter.

A newly constructed nest is beneficial for several reasons. Firstly, nests need only last a short time as the entire time-line from egg-laying to fledging may be as little as six weeks so it is counterproductive to spend a lot of time and energy on something that is required temporarily. Secondly, avian parasites can accumulate in the nest materials, then persist over winter so when the nesting pair returns in the Spring, there may be enough parasites to pose a serious health risk to nestlings, perhaps even contributing to their mortality.

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

To maximize efficiency, birds tend to build relatively small nests. 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-speedy rate at which nestlings grow into fully-flighted juveniles. Imagine developing from a new-born baby to an Olympic caliber athlete in just six to ten weeks! The nutrition required to accomplish this is astounding but even the best nutrition is not enough without adequate leg support for maturing birds. As a nestling grows, it must keep its legs firmly tucked beneath its body or the legs grow sideways, in a splayed-leg fashion that is ultimately fatal. Therefore, the close proximity attained in a crowded nest ensures that won't happen.

Bird 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 is extended to their nests and all of its occupants. It is against the law to harm, move, or harass nesting birds. If a bird builds its nest in a location that may be dangerous or inconvenient to a homeowner, they must obtain permission from their local and/or federal wildlife authorities in order to remove it. The wisest choice is generally to leave it where it is; it is not difficult to co-exist peacefully with most species. Keep in mind how short the nesting season is and how enjoyable it can be to watch the labour-intensive process of rearing baby birds until that momentous day when the fledglings 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! Watch for a future article on this subject, as well as a fistful of other cool topics on wild birds and mammals.

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