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Birds Guide


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In most birds and reptiles, an egg (Latin ovum) is the zygote, resulting from fertilization of the ovum. It nourishes and protects the embryo. Oviparous animals are animals that lay eggs, with little or no other development within the mother. This is the reproductive method of many fish, amphibians and reptiles, all birds, the monotremes, and most insects and arachnids.

Reptile eggs, bird eggs, and monotreme eggs, which are laid out of water, are surrounded by a protective shell, either flexible or inflexible.

The 1.5 kg ostrich egg contains the largest existing single cell currently known, though the extinct Aepyornis and some dinosaurs had larger eggs. The bee hummingbird produces the smallest known bird egg, which weighs half a gram. The eggs laid by some reptiles and most fish are even smaller, and those of insects and other invertebrates are much smaller still.

The study or collecting of eggs, in particular bird eggs, is called oology.


Bird eggs

Usually after fertilization, the bird egg is laid by the female and is incubated for a time that varies according to the species; then a single young hatches from each egg. Average clutch sizes range from one (as in condors) to about 17 (the Grey Partridge). Some birds lay eggs even when not fertilized, and it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs.

Shell structure

Eggs are usually smooth, but there are exceptions. A cormorant's egg, for example, is quite rough and is very chalky. In contrast, tinamous have very shiny eggs, and ducks have oily and waterproof eggs. Another variation is the very heavily pitted eggs of cassowaries.

There are tiny pores in the shells of eggs to allow the unborn animal to breathe. The domestic hen's egg has around 7500 pores.


Most bird eggs have an oval shape, with one end rounded and the other more pointy. This shape results from the egg being forced through the oviduct. Muscles contract the oviduct behind the egg, pushing it forward. The egg's wall is still shapeable, and the pointy end develops at the back side. Highly conical eggs are often seen in cliff-nesting birds. They are less likely to roll off, tending instead to roll around in a tight circle, and thus are believed to have been selected for by evolution. In contrast many hole nesting birds have nearly spherical eggs.

Brown-headed Cowbird eating another bird's egg
Brown-headed Cowbird eating another bird's egg


There are numerous animals that feed on eggs. Principal predators of the Black Oystercatcher's eggs, for example, include raccoons, skunks, mink, river and sea otters, gulls, crows and foxes.

The Stoat (Mustela erminea) and Long-tailed Weasel (M. frenata) steal ducks' eggs. Other mammals, like humans, also eat bird eggs. The egg-eating snakes (genera Dasypeltis and Elachistodon) specialize in eating eggs.

Brood parasitism also occurs in birds when one species lays its eggs in the nest of another. In some cases, the host's eggs are removed or eaten by the female, or expelled by her chick. Brood parasites include the cowbirds and many Old World cuckoos, most famously the Common Cuckoo.


See also

  • Oology - the study or collecting of eggs.

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