Читать книгу Birds of New Hampshire & Vermont Field Guide - Stan Tekiela - Страница 13

Bird nests are truly an amazing feat of engineering. Imagine building your home strong enough to weather a storm, large enough to hold your entire family, insulated enough to shelter them from cold and heat, and waterproof enough to keep out rain. Now, build it without any blueprints or directions, and without the use of your hands or feet! Birds do!

Before building a nest, an appropriate site must be selected. In some species, such as House Wrens, the male picks out several potential sites and assembles several small twigs in each. This discourages other birds from using nearby nest cavities. These “extra” nests are occasionally called dummy nests. The female is then taken around and shown all the choices. She chooses her favorite and finishes constructing the nest. In some other species of birds–Baltimore Orioles, for example–it is the female who chooses the site and constructs the nest, with the male offering only an occasional suggestion. Each species has its own nest-building routine, which is strictly followed.

As you’ll see in the following illustrations, birds build a wide variety of nest types.

ground nest

platform nest

cup nest

pendulous nest

cavity nest

Nesting material often consists of natural elements found in the immediate area. Most nests consist of plant fibers (such as bark peeled from grapevines), sticks, mud, dried grass, feathers, fur, or soft fuzzy tufts from thistle. Some birds, including Ruby-throated Hummingbirds, use spider webs to glue nest materials together. Nesting material is limited to what a bird can hold or carry. Because of this, a bird must make many trips afield to gather enough materials to complete its nest. Most nests take at least four days or more, and hundreds, if not thousands, of trips to build.

The simple ground nest is scraped out of the earth. A shallow depression that usually contains no nesting material, it is made by birds such as the Killdeer and Horned Lark.

Another kind of nest, the platform nest, represents a more complex type of nest building. Constructed of small twigs and branches, the platform nest is a simple arrangement of sticks which forms a platform and features a small depression to nestle the eggs.

Some platform nests, such as those of the Canada Goose, are constructed on the ground and are made with mud and grass. Platform nests can also be on cliffs, bridges, balconies or even in flowerpots. This kind of nest gives space to adventurous youngsters and functions as a landing platform for the parents. Many waterfowl build platform nests on the ground, usually near water or actually in water. These floating platform nests vary with the water level, thus preventing nests with eggs from being flooded. Platform nests, constructed by such birds as Mourning Doves and herons, are not anchored to the tree and may tumble from the branches during high winds and storms.

The cup nest is a modified platform nest, used by three-quarters of all songbirds. Constructed from the outside in, a supporting platform is constructed first. This platform is attached firmly to a tree, shrub, rock ledge or the ground. Next, the sides are constructed of grasses, small twigs, bark or leaves, which are woven together and often glued with mud for added strength. The inner cup, lined with feathers, animal fur, soft plant material or animal hair, is constructed last. The mother bird uses her chest to cast the final contours of the inner nest.

The pendulous nest is an unusual nest, looking more like a sock hanging from a branch than a nest. Inaccessible to most predators, these nests are attached to the end of the smallest branches of a tree and often wave wildly in the breeze. Woven very tightly of plant fibers, they are strong, watertight and take up to a week to construct. More commonly used by tropical birds, this complicated type of nest has also been mastered by orioles and kinglets. A small opening on the top or side allows the parents access to the grass-lined interior. (It must be one heck of a ride to be inside one of these nests during a windy spring thunderstorm!)

Another type of nest, the cavity nest, is used by many bird species, including woodpeckers and Eastern Bluebirds. The cavity nest is usually excavated in a tree branch or trunk and offers shelter from storms, sun, predators and cold. A relatively small entrance hole in a tree leads to an inner chamber up to 10 inches (25 cm) deep. Usually constructed by woodpeckers, the cavity nest is typically used only once by its builder but subsequently can be used for many years by other birds, such as mergansers, bluebirds and swallows, which do not have the capability of excavating one for themselves. Kingfishers, on the other hand, excavate a tunnel up to 4 feet (1 m) long, which connects the entrance in a riverbank to the nest chamber. These cavity nests are often sparsely lined because they are already well insulated.

One of the most clever of all nest types is known as the no nest or daycare nest. Parasitic birds, such as Brown-headed Cowbirds, build no nests at all! The egg-laden female expertly searches out other birds’ nests and sneaks in to lay one of her own eggs while the host mother is not looking, thereby leaving the host mother to raise an adopted youngster. The mother cowbird wastes no energy building a nest only to have it raided by a predator. By using several nests of other birds, she spreads out her progeny so at least one of her offspring will live to maturity.

Some birds, including some swallows, take nest building one step further. They use a collection of small balls of mud to construct an adobe-style home. Constructed beneath the eaves of houses, under bridges or inside chimneys, some of these nests look like simple cup nests. Others are completely enclosed, with small tunnel-like openings that lead into a safe nesting chamber for the baby birds.