Avian Architects Part 2


Some species of birds are opportunistic nesters, using other birds’ nests, never building their own. Birds such as the American Kestrel (Falco sparverius) uses cavities such as an old woodpecker nest hole. Aplomado Falcon (Falco femoralis) and the Merlin (Falco columbarius) take over abandoned nests in trees or shrubs.

Great Horned Owl [Bubo virginianus] adult female with offspring in former hawk’s nest, protecting nestling; Monte Vista NWR., Colorado

Most owls (Strigiformes) do not build their own nests. They may add sticks or lining materials, such as bark, feathers, or leaves to an abandoned nest. Great-horned owls (Bubo virginianus) reuse nests built by other species and don’t make any improvements before moving in. It’s not unusual for their nests to collapse or break up in high winds. They may also use platforms, cavities, or birdhouses depending upon the size available, preferring locations that are protected from potential predators or human intervention.

Great Horned Owl [Bubo virginianus] nestlings, nest in crotch of tree, reused over the course of many years; Monte Vista NWR, Colorado

Some birds use homes abandoned by other animals such as Burrowing owls which often are found in the midst of prairie dog towns using vacated prairie dog burrows to raise their young.

Burrowing Owl [Athene cunicularia] adult at nest burrow entrance in prairie dog colony; Fremont County, Colorado

Mud or earth, of course, is not the only place to make a hole and many birds nest in holes found or excavated in trees, cacti and even termite nests.

The Brown-headed cowbird (Molothrus ater) is considered a brood parasite. They lay their eggs in other birds’ nests and then abandon the eggs for the host bird to raise.

Most simple nests are platform nests which are large and bulky structures formed on top of larger branches with a flat surface and are often constructed primarily with larger twigs and sticks or of plant material, sticks or stones. They are the most common type of bird’s nest and like nestless birds, all the birds that use them are non-passerines. It may have a slight depression in the middle but does not form a cup and can be quite extensive. Many birds reuse platform nests for many years, with a minimal amount of maintenance but often adding new material to the nest each year.

Red-tailed Hawk [Buteo jamaicensis] adult tending new chicks in platform nest; Yellowstone NP.,Wyoming

A common example of a simple platform nest are those constructed by many doves and pigeons. Those, like that of the White-winged Dove (Zenaida asiatica), are often a simple lattice with just enough stiff twigs or plant fibers to support one or two eggs. In these cases, the eggs can often be seen through the nest from underneath. Sometimes a flimsy lining of grass and rootlets may be added and in some cases, the lining may be more elaborate.

Yet another well-known example of a platform nest and one which is more substantial than a dove’s is that of the European White Stork (Ciconia ciconia), which has for centuries nested on chimneys and specially erected platforms across Europe. European white stork nest on a specially raised platform. More complicated than the other nests, it consists of sticks interwoven then plastered with mud. A depression in the middle is often lined with grass or paper.

The most spectacular nests in this category are those built by various eagles such as the bald eagles (Haliaeetus leucocephalus), osprey (Pandion haliaetus), great blue herons (Ardea herodias), many other hawks, eagles, falcons, owls and large wading birds.

Osprey [Pandion haliaetus] adult bringing an offering of fish to feed mate and nestlings in nest, built on towering rock outcrop; Grand Canyon of the Yellowstone, Yellowstone NP., WY
Ferruginous Hawk [Buteo regalis] adult guarding nest with 2 chicks; San Luis Valley, CO
Double-crested Cormorant [Phalacrocorax auritus] nest colony with adults sitting on eggs; Pueblo City Park, CO
Double-crested Cormorant [Phalacrocorax auritus] adult providing water to thirsty nestlings; Holcim Wetland, Florence, Colorado
Great Blue Heron [Ardea herodias] adult tending two offspring in nest in heron and cormorant rookery; Holcim Wetland, Florence, Colorado
Great Blue Heron [Ardea herodias] adult pair feeding juvenile on nest in rookery shared with Double-crested Cormorant; Holcim Wetland, Florence, Colorado

The Hoatzin (Opisthocomidae) from South America is another signature bird of the Amazonian lowlands that builds a simple platform nest.

Hoatzin [Opisthocomus hoazin] perched, carrying twig for nest construction; Napo Wildlife Center, Ecuador
Hoatzin [Opisthocomus hoazin] incubating eggs on nest; Napo Wildlife Center, Ecuador

One of the largest platform nests belongs to the bald eagle (Haliaeetus leucocephalus) who builds bulky sturdy nests that are often used year after year. Like ravens, an eagle’s territory contains an active nest and inactive nests. Those nests that are not used are repaired and maintained. Most bald eagles build their nests in trees and in fact, some of these nests can be so heavy that they can damage the tree supporting them. Eagles’ nests are found in mature or old-growth trees but also on rocky high points or cliff ledges beside a water source, such as rivers, lakes, or coastlines. These areas are great sources of food such as marine invertebrates, waterfowl, fish, or carrion.

Bald Eagle [Haliaeetus leucocephalus] guarding nest with current year’s new offspring; Yellowstone NP., WY

Bald eagles’ long-lasting nests are built out of large sticks and lined with softer plant materials such as weed stalks, grasses, moss, lichens, sod or even seaweed for added insulation, cushioning and to safequard an egg from wedging between the twigs. They build these nests high, ranging 85 to 115 feet up, away from potential disruptions, including human intervention. A pair can take up to three months, several hours a day to build a nest with a typical diameter of 4 to 6 feet with a depth of 3 feet, capable of holding 1,000 pounds. The reason being, building an inferior nest can cost the pair an entire breeding season if it collapses under the weight of a full grown family. After making improvements and adding to the nest over the breeding season and time, it can weigh over a ton! In 1963, the largest bald eagle nest ever discovered was 9 feet and 6 inches wide, 20 feet deep, weighing over 4,400 pounds.

Osprey [Pandion haliaetus] carrying twig to mate in nest; Yellowstone NP., Wyoming
Osprey [Pandion haliaetus]; adult bringing stick to nest during trade off for incubation duties, Turquoise Lake, Colorado
Great Horned Owl [Bubo virginianus] nestlings in nest created in hollowed-out tree strump in an old forest burn area; Yellowstone NP., Wyoming

Stay tuned for Part 3 of this series, Ledge and Minimalist

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