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.