Eggs of all ColoursProbably the most striking and variable feature of eggs is their coloration. Although many eggs are white, eggs representing almost every colour of the rainbow are known.
Distinctive MarkingsAs well as background colour, eggshells may be intricately marked. Markings can be blotches, scrawls, streaks or speckles. These markings are often concentrated in a ring around the large end of the egg.
Pigments result in Colours
Only three pigments are responsible for the tremendous diversity in egg colour and markings. These pigments are related to the pigment in haemoglobin that makes blood red. One pigment, protoporphyrin, produces colours ranging from yellow and pink to reddish buffs or browns. The other two pigments, biliverdin and the zinc chelate of biliverdin, result in blue and green colours. When these three pigments are applied in different proportions, colours ranging from violet-blue to olive-green may result.
Pigment DepositionColour is added to the eggshell from pigments secreted by cells in the oviduct wall.
The timing of pigment deposition affects colour. Pigment deposited as the egg enters the oviduct results in the base colour of the egg. This background colour may be modified by subsequent addition of eggshell. Pigments added right before the egg is laid form the patterns or markings on its surface. If the egg remains still while pigments are applied, spots appear. If the egg is moving, lines or scrawls appear.
Control of ColouringEgg colouring is controlled to a large extent by genetics. Usually, egg colouring is typical of a species. All robins, for example, lay blue eggs without markings. However, egg colouring or marking may be extremely variable among females of a species. Common Murres lay only one egg during one nesting and there is much variability in markings among eggs of different females.
MimicrySome birds like the Brown-headed Cowbird are called brood parasites. They lay their eggs in the nests of other birds (host species), leaving the host to take care of the parasite's eggs. The European Cuckoo, a brood parasite, is incredibly host-specific producing eggs that mimic its preferred host. The mimicry is essential as hosts have learned to reject Cuckoo eggs that do not match their own. Here a cuckoo has laid one egg in a clutch of four Sedge Warbler eggs. Which egg belongs to the Cuckoo?
The Ecology of Egg ColoursEgg colours and markings have strong adaptive values. Originally, birds' eggs were probably all white, as reptile eggs are. Eggs that are laid on the ground or in open nests in trees, rather than in cavities, often exhibit cryptic colouration. The eggs blend in with their surroundings and are much less visible to potential predators. The nest here belongs to a Killdeer, a shorebird. Most shorebirds lay their eggs in a scrape on a beach. These eggs are camouflaged by their environment.
Sometimes eggs that are laid in open nests are white at first. They then
become stained by the mud and rotting vegetation in the nest. Grebes lay white
eggs that become stained and cryptically coloured over time, like the ones on the right.
In some species, such as the Common Murre, where different females lay
eggs with very different markings, the uniqueness may have a purpose.
Distinctive patterns help females identify their own egg in a colony where
thousands of eggs may dot a cliff face.
Eggs of kingfishers and other cavity nesting birds, such as woodpeckers,
are often white. The brightness of the eggs may help the parents to more
easily locate them in the cavity. These are Downy Woodpecker eggs.
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