Your Question: Why are bird eggs so variable in their colours and patterns?
The colour and colour pattern of bird eggs vary enormously from species to species (and often between individuals of the same species, and sometimes between the eggs of the same mother).
A tray of eggs from Museum Victoria's H.L White egg collection, showing the diversity of patterns and colours for a single species, the Australian Magpie Gymnorhina tibicen.
Image: Michelle McFarlane
Source: Museum Victoria
Eggs are made of calcium carbonate, which is white. White is therefore the default colour for bird eggs, but many birds lay coloured or colourfully-patterned eggs. Why?
The colouration of bird eggs can often be explained by the animal's biology and behaviour. The eggs of ground-nesting birds, for example, need to be well-camouflaged to avoid discovery by predators. They are usually coloured and patterned to match the substrate they are laid upon.
The highly-camouflaged eggs of the American Golden Plover Pluvialis dominica, which nests on the ground.
Image: MeegsC
Source: Wikimedia Commons
Tree-nesters, on the other hand, usually have blue or green eggs.
The American Robin, Turdus migratorius, which nests in trees, lays bright blue eggs.
Image: Laslovarga
Source: Wikimedia Commons
Birds whose eggs are hidden from view (in hollows, burrows or deep nests), or who sit on their eggs continuously throughout incubation, tend to have white eggs.
The now extinct Paradise Parrot Psephotus pulcherrimus, which laid its eggs in termite mounds, had white, unpatterned eggs.
Image: Rodney Start
Source: Museum Victoria
The patterns on eggs have developed over eons via natural selection – the better the camouflage, the more likely the eggs are to survive and pass on the genes for well-camouflaged eggs to the next generation. Ornithologists have classified egg patterns and given each "style" a name in order to distinguish them: splashed, blotched, spotted, dotted, marbled, streaked, scrawled, overlaid, capped, and wreathed.
Eggs from Museum Victoria's Ornithology Collection
Image: John Broomfield
Source: Museum Victoria
Colour also provides another form of protection: it is thought to act as a sunscreen, protecting the developing foetus from UV light. The addition of colour also strengthens the eggshell. Birds that are calcium-deficient lay thin-shelled eggs, which are more likely to break. Scientists have found that birds that have multiple clutches in a single season have more highly-coloured eggs in the second and subsequent clutches (when the mother's calcium supplies are reduced). Patterned colouration is also more common in areas with calcium-deficient soils.
The specific colours are incorporated into the shell in the final stage of egg development. Blue and green colour comes from a pigment called biliverdin (which is the same pigment that causes green bruises in humans). In egg colouration, biliverdin comes from bile; the red and brown colour on eggs comes from protoporphyrins, which comes from blood.
The Red-vented Bulbul Pycnonotus cafer lays red eggs.
Image: J. M. Garg
Source: Wikimedia Commons
Australia's native birds are protected. It is illegal to collect eggs or to interfere with birds' nests without a permit. Details of regulations and permits can be obtained from the Department of Sustainability and Environment.
Links:
Museum Victoria's Ornithology Collection
H.L. White Collection of Australian Birds’ Eggs
The evolution of egg colour and patterning in birds
Australian Magpie Eggs