Speckled Eggs in Bird's Nest

The colors of bird eggs is varied and  often are done for specific reasons.  As mentioned in an earlier post brightly colored eggs are to “black mail” the male bird of the species to be more aggressive to protect their nest from predators.  Other birds that produce special coloring are most ground-nesting species whose eggs are cryptically colored to prevent predation.   The killdeer is a perfect example which place their clutch of eggs of 4 eggs laid in a slight depression in a gravel area.  The eggs look almost like the pebbles surrounding it,  almost impossible to see.  The adult killdeer also has coloring that blends into its surrounding.  On the other hand some species of birds lay eggs that do not match their background.  Included in this group include the poorwills,  ibises,  doves  and thrushes whose eggs  are often white or a bright blue green placed in a brown mud nest of the flimsy built nest of the several species of doves.

The “black mail” hypothesis has its doubters  who say that five predictions about birds that lay conspicuous  eggs should be studied further.  First the birds should attempt to hide them better.  Second,  shorter incubation periods.   Third,  brightly colored eggs species should be more apparent in birds that are more susceptible to predators that are visually oriented.  Fourth,  incubation should start earlier to keep their nest concealed.  Fifth,  the small bodied birds are usually unable to prevent a brown headed cowbird  from laying eggs in their nest even though they are ever-present attendance at the nest.  However its presence does inhibit  parasite from removing an egg from its nest.  One thing is certain.  Many bird species do lay very conspicuous eggs and the reason has bewildered ornithologist for over a century.

Killdee Eggs in Gravel Nest

House Finch Nest

Bluebirds' "sky-blue" Eggs

Have you ever wondered why some birds and absolutely beautiful and others are plain white or very ordinary?  For instance the female American robin lay beautiful “robin egg” blue color.  The Eastern bluebird lay “sky blue” blue eggs.  Some ornithologist suggest that the female bird may use the color to  “blackmail” the male bird into working harder to combat the risk of nest predation or brood parasitism by having beautiful colored eggs exposed in an open nest.  The  colors of birds’ eggs may be breathtaking.  Collection of bird eggs in the past was done and some of these are still intact.  Fortunately,  today laws exist to prevent the taking of eggs for private collections.

Where do these extravagant colors come from and why do birds lay colored eggs?  Some birds such as the screech owl and doves lay plain ,  all white eggs,  however other birds such as the Cetti’s warbler (an European species) lay deep brick-red eggs.

The pigments in eggs  are laid down in the wall of the oviduct particularly the uterus.  There are two basically two classes of pigment: porphyrins  – deprived from hemoglobin,  which produces browns,  olive and reds; and cyanin,  from bile, creates blue and greens.  Some species of birds have eggs without pigment and are plain white appearance of  calcium carbonates.  Written by Jack Kidd

Eggs in a Robin's Nest

Speckeled Eggs In a Nest