Color of Eggs
  Bird eggs show an enormous diversity of colors. Some bird groups that are considered relatively "primitive," such as cormorants and pelicans, are thought to have retained the pale, uniform white or bluish color typical of their reptilian ancestors. In more "advanced" groups, unmarked white eggs are found mostly among some cavity-nesting species where there is no need for the eggs to be camouflaged. Other cavity nesters, such as certain titmice, have spotted eggs -- presumably an indication that they once nested in the open. Pale eggs are also common among some duck species that cover them with bits of nesting materials when they take a break from brooding, or among those species such as doves, owls, and herons that start incubating as soon as the first egg is laid and never leave them exposed.

Seabird species that nest in gigantic colonies tend to have eggs that are extremely variable in both color and markings. Their colors, like all egg colors, are from pigments produced by glands in the female's oviduct. As the egg moves down that tube the colors are squeezed out onto the shell. As ecologist Bernd Heinrich put it: ". . . the motion of the egg affects the color patterns. It is as if innumerable brushes hold still while the canvas moves. If the egg remains still there are spots, and if it moves while the glands continue secreting, then lines and scrawls result."

Chester Reed, an early egg collector, assumed that murres didn't know whose egg they were attending when they returned to their colony, but actually the variability of designs produced by the oviduct "brushes" permits individuals to recognize their own "painting." Experiments have shown that murres learn the pattern of their own egg and will reject others. If its egg becomes discolored gradually with guano, a murre will continually adjust the image of the "proper" egg and will reject an unstained egg of its own basic pattern. Thus the birds are not genetically programmed to recognize their own egg pattern, but rather learn the pattern of the egg they've laid and then continually update its image.

In most birds, however, the colors of the eggs in one way or another help with their concealment, as anyone who has sought Arctic Tern or Killdeer eggs against a pebbly background can testify. How, though, can one explain exceptions where one might expect camouflage, as in the case of the unmarked blue eggs of robins and catbirds? Their uniform colors may now permit easy identification of the eggs of nest-parasitic cowbirds, which may pose a greater threat to the survival of the brood than do nest robbers. It is doubtful, however, if robins and catbirds were in wide contact with cowbirds before deforestation and the importation of cattle opened the East to those parasites.

SEE: Incubation: Heating Eggs; Variation in Clutch Sizes; DDT and Birds; Empty Shells.
Copyright ® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.