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.
Who Builds the Nest?
In general, the female bird builds the nest. She gathers nesting materials and constructs a nest, with an occasional visit from her mate to check on progress. In some species, both parents contribute equally to the construction of a nest. A male bird might forage for precisely the right sticks, grass or mud, but it is often the female that forms or puts together the nest. She uses her body to form the egg chamber. Rarely does the male build a nest by himself.
Fledging
Fledging is the interval between hatching and flight or leaving the nest. Some birds leave the nest within hours of hatching (precocial), but it might be weeks before they are able to fly. This is common with waterfowl and shorebirds. Until they start to fly, they are called fledglings. Birds that are still in the nest are called nestlings. Other baby birds are born naked and blind, and remain in the nest for several weeks (altricial).
Why Birds Migrate
Why do birds migrate? The short answer is simple–food. Birds migrate to areas with high concentrations of food, as it is easier to breed where food is than where it is not. A typical migrating bird–the Purple Martin, for instance–will migrate from the tropics of South America to nest in forests of North America, taking advantage of billions of newly hatched insects to feed its young. This trip is called complete migration.
Some birds of prey return from their complete migration to northern regions that are overflowing with small rodents, such as mice and voles, that have continued to breed in winter.
Complete migrators have a set time and pattern of migration. Each year at nearly the same time, they take off and head for a specific wintering ground. Complete migrators may travel great distances, sometimes as much as 15,000 miles (24,150 km) or more in one year. But complete migration does not necessarily imply flying from the frozen northland to a tropical destination. American Tree Sparrow, for example, is a complete migrator that flies from Canada to spend the winter in New Hampshire and Vermont. This is still called complete migration.
There are many interesting aspects to complete migrators. In the spring, males usually migrate several weeks before the females, arriving early to scope out possibilities for nesting sites and food sources, and to begin to defend territories. The females arrive several weeks later. In the autumn, in many species, the females and their young leave early, often up to four weeks before the adult males.
All migrators are not the same type. Partial migrators, such as American Goldfinches, usually wait until their food supplies dwindle before flying south. Unlike complete migrators, partial migrators move only far enough south, or sometimes east and west, to find abundant food. In some years it might be only a few hundred miles, while in other years it might be nearly a thousand. This kind of migration, dependent on weather and the availability of food, is sometimes called seasonal movement.
Unlike the predictable ebbing and flowing behavior of complete migrators or partial migrators, irruptive migrators can move every third to fifth year or, in some cases, in consecutive years. These migrations are triggered when times are really tough and food is scarce. Pine Grosbeaks are a good example of irruptive migrators, because they leave their normal northern range in search of food or in response to overpopulation.
How Do Birds Migrate?
One of the many secrets of migration is fat. While we humans are fighting the battle of the bulge, birds intentionally gorge themselves to put on as much fat as possible while still being able to fly. Fat provides the greatest amount of energy per unit of weight, and in the same way that your car needs gas, birds are propelled by fat and stalled without it.
During long migratory flights, fat deposits are used up quickly, and birds need to stop to “refuel.” This is when backyard bird feeding stations and undeveloped, natural spaces around our towns and cities are especially important. Some birds require up to 2-3 days of constant feeding to build their fat reserves before continuing their seasonal trip.
Some birds, such as most eagles, hawks, ospreys, falcons and vultures, migrate during the day. Larger birds can hold more body fat, go longer without eating and take longer to migrate. These birds glide along on rising columns of warm air, called thermals, which hold them aloft while they slowly make their way north or south. They generally rest during the night and hunt early in the morning before the sun has a chance to warm the land and create good soaring conditions. Birds migrating during the day use a combination of landforms, rivers, and the rising and setting sun to guide them in the right direction.
Most other birds migrate during the night. Studies show that some birds which migrate at night use the stars to navigate. Others use the setting sun, while still others, such as doves, use the earth’s magnetic fields to guide them north or south. While flying at night might seem like a crazy idea, nocturnal migration is safer for several reasons. First, there are fewer nighttime predators for migrating birds. Second, traveling at night allows time during the day to find food in unfamiliar surroundings. Finally, nighttime wind patterns tend to be flat, or laminar. These flat winds don’t have the turbulence associated with daytime winds and can actually help carry smaller birds by pushing them along.
HOW TO USE THIS GUIDE
To help you quickly and easily identify birds, this field guide is organized by color. Simply note the color of the bird and turn to that section. Refer to the
