Bird Eggs

It is spring in the northern hemisphere, and many birds are claiming territories, finding mates, and establishing nesting sites.  Raising new families will take most of the summer and early autumn.  As we watch birds who have laid their eggs, it may seem that not much happens until the young hatch, but there is lots of activity taking place. 

An ovum, the female reproductive cell, starts its journey to become an egg in the oviduct where it is fertilized by stored sperm from a male and encased in a glob of protein-filled gelatin to form a yolk.  Additional proteins, known as the albumin or egg white, are added to nourish the embryo as it grows inside the egg.  Calcium carbonate, a mineral added by special cells in the bird’s uterus, encloses the gelatin-like mass of protein and embryo, slowly hardening into the egg’s shell.  Egg shells are not completely solid, but are perforated with many minute holes that allow air to reach the developing embryo.  Pigmentation is squirted onto the shell adding color and pattern to the outside.  A coating of protein determines the outer texture of the egg which may be smooth, glossy, dull, rough, or powdery.

The total number of eggs laid at one time is called a clutch and some bird species lay only one a year, but many species produce two, or more, annually.  A consistent number of eggs is found in each clutch, although additional clutches in the same year may contain fewer eggs.  Eggs vary widely in size, shape, and color among various bird species.  Egg shapes may be determined by the location of the nest.  Cavity nesters tend to have rounded eggs, while cliff dweller’s eggs are often oval with a broad end on one side and a pointed end on the other to prevent the egg from rolling very far.  Where a clutch has a greater number of eggs, pointed shapes fit into smaller spaces enabling the mother to easily cover them all when sitting on the nest.

Incubation is the process of keeping the eggs warm.  An adult usually develops a brood patch underneath its belly where feathers and down disappear and blood vessels close to the skin’s surface can warm the area that is in direct contact with the eggs.  Incubation periods are consistent in each species, but may be lengthened by abnormal cold spells.

Color is added into the shell material before it hardens.  Colors may vary in hue and saturation on individual eggs, and the deepest hues are often found at the largest end.  Pigments are chemicals with complex molecular structures that produce color when mixed with water.  Birds have two pigments:  reddish-brown hues and bluish-green hues.  Here are several theories as to why eggs have markings, but exact reasons are not known. 

Heavier pigments are often found in birds that lay their eggs in open, exposed environments, and the markings may serve as camouflage.  Many cavity nesters lay white eggs, which are well hidden from other birds, predators, and the sun.  Some eggs laid in exposed nests may still be brightly colored, but covered by the female most of the time.

Darker colors and markings are often found among species that breed in cooler climates.  Pigments are known to absorb UV radiation from the sun and not allow it to harm a developing embryo.  But, light  absorption by an area of pigmentation generates more heat inside the shell, which may also be harmful.  Pesticides and other chemicals in the environment can cause thinning of the shell casing, making them more susceptible to breakage.  Pigments tend to gather at weaker spots in the shell casing and have been shown to strengthen the shell.

Several aspects of bird egg development are not yet understood, and many theories are under investigation.  An interesting question currently under review is whether a bird, or another animal such as a predator, sees different aspects of the light spectrum than humans.  If so, what do birds see when they look at an egg?  A new bird’s life starts in an egg, a small and wonder-filled package that has been much studied and yet still presents many unanswered questions.