Most bird species build their own nest, which is a necessary container for their eggs. Exceptions include some species of cuckoos and cowbirds, that lay their eggs in other species’ nests. Nest-building is often done by the female alone, but in some species the male may help or even build it himself. Eggs are incubated either by females only, or by males and females, depending on the species. Eggs, consisting of 60 percent water, contain a fatty yolk for nourishment of the embryo,as well as sugars and proteins. 

Eggshells are hard enough to sustain the weight of incubating parents, yet soft enough for a chick to break its way out. Hatching is an energy-draining process, and can last for several hours.

In addition to the four types shown below, nests range from a simple scrape in the ground with a few added pebbles to an elaborate woven basket-like structure. Plant matter forms basic nest material. This includes twigs, grass stems, bark, lichens, mosses, plant down, and rootlets. Some birds add mud to their nest for strength. Others incorporate animal hair or feathers to improve its softness and insulation.

 Female eider ducks line their nest with down feathers plucked from their belly. Some birds include bits of plastic or threads in their nests. Several species of flycatchers add shed snakeskins to their nests. Many birds make their nest or lay their eggs deep inside the empty burrows of other animals.
Burrowing Owls nest in prairie dog burrows, where they coexist with the rodents.

There are six basic egg shapes among birds, as illustrated to the right. The most common egg shapes are longitudinal or elliptical. Murres lay pear-shaped eggs, an adaptation for nesting on
the narrow ledges of sea cliffs; if an egg rolls, it does so in a tight circle and remains on the ledge. 

Spherical eggs with irregular red blotches are characteristic of birds of prey. Pigeons and doves
lay white oval eggs, usually two per clutch. The eggs of many songbirds, including sparrows and buntings, are conical and have
a variety of dark markings on a pale background.

After a period of incubation, which varies from species to species, chicks break the eggshell, some of them using an egg tooth, a special bill feature that falls off after hatching. After a long and exhausting struggle, the chick eventually tumbles out of the shell

 The transition from the watery medium inside the egg to the air outside is a tremendous physiological switch. Once free of their shell, the hatchlings recover from the exertion and either beg food from their parents or feed on their own.