Eggs are laid by females of many different species, including birds

Bird egg

Bird eggs are laid by females and incubated for a time that varies according to the species; a single young hatches from each egg. Average clutch sizes range from one to about 17...

, reptile

Reptile

Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

s, amphibian

Amphibian

Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...

s, and fish

Fish

Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, and have probably been eaten by mankind for millennia. Bird and reptile eggs consist of a protective eggshell

Eggshell

An eggshell is the outer covering of a hard-shelled egg and of some forms of eggs with soft outer coats.- Insect eggs :Insects and other arthropods lay a variety of styles and shapes of eggs. Some have gelatinous or skin-like coverings, others have hard eggshells. Softer shells are mostly protein....

, albumen (egg white

Egg white

Egg white is the common name for the clear liquid contained within an egg. In chickens it is formed from the layers of secretions of the anterior section of the hen's oviduct during the passage of the egg. It forms around either fertilized or unfertilized egg yolks...

), and vitellus (egg yolk

Egg yolk

An egg yolk is a part of an egg which feeds the developing embryo. The egg yolk is suspended in the egg white by one or two spiral bands of tissue called the chalazae...

), contained within various thin membranes. Popular choices for egg consumption are chicken

Chicken

The chicken is a domesticated fowl, a subspecies of the Red Junglefowl. As one of the most common and widespread domestic animals, and with a population of more than 24 billion in 2003, there are more chickens in the world than any other species of bird...

, duck

Duck

Duck is the common name for a large number of species in the Anatidae family of birds, which also includes swans and geese. The ducks are divided among several subfamilies in the Anatidae family; they do not represent a monophyletic group but a form taxon, since swans and geese are not considered...

, roe

Roe

Roe or hard roe is the fully ripe internal egg masses in the ovaries, or the released external egg masses of fish and certain marine animals, such as shrimp, scallop and sea urchins...

, and caviar

Caviar

Caviar, sometimes called black caviar, is a luxury delicacy, consisting of processed, salted, non-fertilized sturgeon roe. The roe can be "fresh" or pasteurized, the latter having much less culinary and economic value....

, but the egg most often consumed by humans is the chicken egg, by a wide margin.

Egg yolks and whole eggs store significant amounts of protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 and choline

Choline

Choline is a water-soluble essential nutrient. It is usually grouped within the B-complex vitamins. Choline generally refers to the various quaternary ammonium salts containing the N,N,N-trimethylethanolammonium cation....

, and are widely used in cookery.