Egg Coloration and Pattern
Only birds lay colored eggs. Shell pigments are secreted by the uterine glands and are chemically related to the bird's blood and bile pigments. Background colors are deposited first during shell formation and secondary markings are deposited on the outer cuticle. These pigments result in only two basic colors, blue and buff-brown. Blue, when present, permeates the entire shell as in the American Robin and other thrushes. Sometimes brown pigments are present on or just under shells outer surface. On a white egg this produces the yellow-buff-brown range of colors. When brown is present on a blue shell, colors are in the green-olive range.
Distinctive markings are made by another chemical substance which may appear black or red-brown. These marking can be incorporated at any level as the layers of the shell build up just prior to laying. When seen through white or blue shells, the spots will appear gray, blue or mauve. Spots added on the final layer often do not dry immediately after the egg is laid. This results in blotches, scrawls, streaks and marbled patterns produced because the spots smear as the eggs are laid. In some species the eggs may have all the markings on the large end, producing a capped or wreathed pattern. Despite the endless array of possible patterns, eggs of most species are readily identified by shape, color and pattern. Individual variation is usually slight. Only certain colonial nesting species have a great deal of variation. This may aid birds in locating their own eggs in dense colonies. A good example of this is the marine species the Common Murre. Its cliff colonies are especially crowded and egg colors and patterns vary widely from nest to nest.
Color and pattern variation in Commn Murre eggs.
Eggs Color and markings of eggs probably provide camouflage from enemies. Most cavity nesting birds such as woodpeckers, kingfishers, martins, small owls, etc.., lay white, unmarked eggs, consistent with the fact that they are well hidden. Cavity nesting ducks further camouflage their eggs with white down. Ground nesting ducks cover their eggs with brown and gray down so they better match the surrounding nest materials. Some large birds such as Bald Eagles and Great Horned Owls lay white eggs in open nests, as do a few smaller species like the Mourning Dove. Most of these birds begin incubation as soon as the first egg is laid and there is constant incubation by an adult, so the eggs are seldom visible to predators. Some species lay eggs with only a few small spots, while close relatives have unmarked eggs. For example the eggs of the Barn Swallow are slightly marked while eggs of the Bank, Cliff, Northern Rough-winged and Tree Swallows unmarked white. The latter all have enclosed nests while Barn Swallows have a more cup-like nest. In the thrush family, the eggs of the Eastern Bluebird are the palest, and it nests in cavities and bird boxes. In recent years pure white bluebird eggs have been recorded. Blue eggs are often found in species nesting in heavily shaded locations. Buff-brown eggs are usually associated with ground nesters. The amount of marking often corresponds to the location of the nest, open and exposed or well concealed. Collectors have often sought out unusual or atypical egg sets for their collections. The unmarked, blue Herring Gull eggs shown below are an example.
"Blue" Herring Gull eggs have no markings.