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Eggs of Birds

Eggs of birds show a great variety in shape, surface texture, colour, and size. Almost spherical eggs are laid by kingfishers, rounded eggs by parrots, and elliptical eggs by emus. Many ground-nesting birds such as waders lay pear-shaped eggs.

Surface texture varies from the rather corrugated eggs of emus to those of birds like grebes which have a slimy coating, the glossy and greasy eggs of ducks which have water-repelling qualities, and the smooth and polished appearance of most bird eggs.

The inorganic material of an egg shell is calcium carbonate, usually known as lime. The basic colour of this is usually changed by means of two pigments, reddish brown and bluish green. All the varieties of colour found in birds' eggs are produced by combinations of these two pigments.

Although there is a similarity in the shape and colour of the eggs of each species of birds there are individual variations. The question of the colour of birds' eggs is a complex one and a number of theories have been put forward to explain particular cases. It is most likely that the main reason for eggs to be coloured in patterns instead of being white is that camouflaged eggs have more chance of survival. Birds such as parrots which nest in holes or dark places, where camouflage is of no value, have white eggs. An added advantage of white eggs in dark places is that in the dim light the eggs are more easily seen and none of the clutch is likely to be overlooked. Birds such as grebes and ducks, which lay their eggs in open nests but cover them should they be forced to leave, also lay whitish eggs. There is more need for camouflage with nests in shrubs or trees and there is a tendency that the more open the nest the more coloured the eggs.

In some species such as the banded plover and the stone-curlew the colour of the eggs can vary according to the kind of ground on which they are laid. It has been observed that when the eggs of the banded plover were laid in a paddock green with new grass they were olive green and that they were lighter in colour when laid in a bare paddock with lighter coloured soil.

The time of egg-laying varies from species to species. The number of eggs in the clutch is an interesting aspect of the story of birds' eggs. Many species always lay the same number of eggs everywhere in their range but some may vary the size of the clutch.

The availability of food can also cause changes in clutch size. Some birds lay clutches of two eggs but rear only one chick. This occurs in a world-wide pattern with herons, eagles, penguins, pelicans, storks, boobies, and owls. Many species can lay extra eggs to replace those destroyed or may nest a number of times in the same season if conditions are suitable.

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