Overview of a Birds Nesting Cycle from NestWatch/Cornell Lab of Ornithology

More than 700 bird species breed in North America, and the variations in their behaviors are fascinating and complex. Different species find mates, build nests, lay eggs, and raise their young in different ways…

1. Finding a place to breed

Throughout the year, day length is the cue that tells most birds what season it is, triggering physiological changes, such as the readiness to migrate or to breed. Most birds, especially those in temperate regions, must time theirbreeding activities so they will be feeding their nestlings when food is most abundant.

2.Choosing a Mate

While territories are being claimed, males and females also try to attract mates.Males often try to advertise their ability and their vigor by exhibiting bright breeding plumage and by singing, drumming, or calling.

Well before nestlings arrive, however, birds need to select a breeding territory. Most bird species form a pair bond with the male and female staying together throughout the breeding season. It was once thought that nearly 90 percent of all birds were monogamous, but new evidence using DNA profiling has revealed that strict monogamy is not so common. Many birds may have a single social mate who helps raise the young, but they may actually have mated with additional partners. DNA analysis shows that even birds that presumably “mate for life,” such as bluebirds and Bald Eagles, may not always be faithful. The young in one nest actually may have been fathered by different males!

Some birds are polygamous, forming social bonds with more than one bird of the opposite sex. Sometimes males have more than one mate at the same time (polygyny). Less commonly, females may have social bonds with more than one male (polyandry).

In most bird species, the female chooses her mate. This is one reason whymales are often the “showier” or more colorful sex. Besides physical appearance, females also may assess factors such as nest site quality and the male’s ability to provide food resources, vocal behavior, age, and courtship displays.

Well before nestlings arrive, however, birds need to select a breeding territory.Resident species may keep a territory throughout the winter or look for a new one in spring. Migratory birds begin looking for and defending a territory or nest site as soon as they arrive in spring. Good territories provide nest sites, reliable food sources, and protection from predators.

3.Nest building

Nest provide a safe place for the eggs and young. Bird nests are extremely diverse, although each species typically has a characteristic nest style. Some birds do not make a nest and simply lay eggs in a scrape in the ground or other substrate. Other birds may make nests from natural materials such as grasses, leaves, mud, lichen, fur, and dung, or manmade materials such as paper, plastic, trash, yarn, and even barbed wire! Nests can be found almost anywhere—on the ground, in trees, in burrows, on the sides of cliffs, in and or on manmade structures, etc. Females typically build the nest, but sometimes both parents or just the male will build it.

4.Copulation and Egg Formation

During the breeding season, hormonal changes cause the internal testes of males to swell to more than 1000 times their normal size, and the ovaries and oviduct of females to increase in size in preparation for egg fertilization and egg development. During copulation, the male’s cloaca contacts and ejects sperm into the cloaca of the female. This is sometimes referred to as the “cloacal kiss.” The sperm travel to the oviduct where they can be stored for long periods in sperm storage tubules. If all goes well, the sperm penetrate through the wall of the ovum (egg) and fertilization takes place. During the first stage of embryonic development, known as the blastocyst stage, the egg shell develops. Ovulation and laying take about 24 hours, so females produce at most a single egg per day.

5.Egg Laying and Incubation

The number of eggs a female lays in one nesting attempt (also known as clutch size) varies widely depending on the species. For example, many tropical birds have clutches of only two or three eggs. Waterfowl such as Wood Ducks can lay up to 15 eggs in one nesting attempt. Clutch size can also vary widely within a species depending on food and calcium availability, latitude, age of female,seasonal timing, weather, and time of year. The size, shape, color, and texture of bird eggs are also extremely variable.

Birds incubate their eggs to keep them at the proper temperature to ensurenormal development. Female songbirds usually begin incubation after they have finished laying all the eggs so that they will hatch at approximately the same time. Other birds, such as herons, cranes, cormorants, and raptors(eagles, hawks, and owls) begin incubating as soon as the first egg is laid, and the eggs may hatch on different days. In some species both the male and female may incubate eggs. Incubation time varies depending on the species, buttypically, the larger the bird, the longer the incubation period.

6.Hatching

Songbirds and most sea-birds have altricial young—the newly hatched birds are blind, naked, and helpless. They remain in the nest where the parent can feed them and brood them, sitting on them to keep them warm. Immediately after hatching, altricial birds can do little more than open their mouths to beg for food. Their mouths are often very bright and patterned. For a long time, scientists thought this helped guide parents to place food; it may also provide information on chick identity, health, and quality.

Unlike altricial birds, precocial birds such as ducks and many shorebirds are born fully feathered, mobile, and with eyes open. Incubation periods arelonger for precocial birds than altricial birds, allowing for increased embryonic development in the egg, and therefore they have enhanced motor and sensory functions at hatching. Precocial young are much more independent after hatching, but in most cases need several more weeks before they can fly.

7.Feeding and Raising Young

For the first week of life, most altricial birds cannot control their own body temperature—similar to reptiles and other cold-blooded animals—and must be constantly brooded (kept warm) by the parents. Usually, by the end of the first week, their eyes are open and their feathers are beginning to emerge. During these first 10 days, nestlings experience remarkable growth—some are able todouble their body weight several times!

To keep up with demand, parents must supply food for themselves and their young. This is an extremely dangerous time for the parents who are constantlyforaging for food, and for the young, whose noisy cries for food may attract predators. After two to three weeks, most songbirds are usually ready to leavethe nest. Other birds, such as birds of prey, may stay in the nest for as long as8 to 10 weeks.

In contrast, precocial birds such as ducks and many shorebirds spend hardly any time in the nest and are often seen wandering in search of food alongside their parents hours after hatching. Most precocial birds gain the ability to flywithin a few weeks to a few months after hatching.

8.Leaving the Nest

After leaving the nest, or fledging, the young birds typically remain close to parents for a short period of time. During this time, young birds must learn to survive on their own and are very vulnerable to predators and starvation. Most birds nest only once per year, but some species can nest up to four or even five times in one breeding season.