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.
A baby tortoise emerges from a reptile egg.
Insect eggs, in this case those of the Emperor Gum Moth, are often laid on the underside of leaves.
Fish eggs, such as these herring eggs are often transparent and are fertilized after laying
An average Whooping Crane egg is 102 mm long, and weighs 208 grams
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.
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.
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.
- Marine Biology notes from School of Life Sciences, Napier University.
- Speckles Make Bird Eggs Stronger, Study Finds John Pickrell, National Geographic News, 11 Oct 2005.
- Andrew Gosler, Yet even more ways to dress eggs in British Birds, vol 99 no 7, July 2006
- Oology - the study or collecting of eggs.