All around us, birds are back in the business of producing the next generation, their eggs hidden away in nest cups and tree holes. Bird eggs are marvels of biological engineering. All of the materials needed to build a new individual are packaged as a self-contained unit. And while the contents of the egg—the viscous, liquid egg white, the lipid-rich yolk, the developing chick—are amazing, the package itself—the shell—incorporates a lot of secrets too.
Production of a bird egg is a complex process. The egg cell with yolk is released from the ovary into the upper oviduct. As the egg passes through that tube, fertilization takes place (if sperm are available; some birds can store viable sperm for several weeks), then layers of egg white (albumen) and surrounding membranes are added. The last contribution, of course, is the shell, which is deposited in the terminal region of the oviduct, called the uterus or shell gland. That entire process can occur in less than 24 hours. Birds often lay one egg per day, sometimes for a dozen or more consecutive days, to complete their clutch.
Eggs of our local birds vary in size from a minuscule ruby-throated hummingbird egg (0.5 g; one-fifth of a penny) to that of a Canada goose (160 g, just over one-third of a pound). Egg size must meet the ability of the female to carry a load while flying and to pass the shelled egg through her external reproductive opening (cloaca). In general, smaller birds lay relatively larger eggs—10-15% of body mass in the hummingbird, just one-half to one-third of that in the goose.
The primary structure of the shell is a matrix of calcium carbonate. That calcium is provided by the diet—birds don’t store excess calcium in anticipation of egg laying—and then is extracted from the bloodstream by the shell gland. The shell gland also adds the chemicals that provide color to the shell. Biliverdin, derived from heme (part of hemoglobin), provides blue/green coloration, and protoporphyrin, also related in structure to heme, contributes brownish-red colors. The most obvious function of egg color is camouflage. Eggs that are concealed in dark nest holes, like those of owls and woodpeckers, typically are pure white. In contrast, many cup-nesting species, like cardinals and goldfinches, have pale eggs with brown speckles that blend in against a background of nest materials. For some reason, some birds, like robins and other thrushes (including bluebirds, which nest in cavities), have beautiful bright blue eggs. Ornithologists conjecture that the pigments might absorb potentially harmful solar radiation. The two color schemes are not mutually exclusive. Blue jays, for example, lay pale blue eggs with brown speckles.
One critical requirement of the eggshell is that it must be strong enough to survive the parents’ comings and goings as they incubate, but fragile enough that the chick can break out. (One of the catalysts of the 1960s environmental movement was the thin, too-easily-breakable eggs induced by ingested DDT, which led to declines in populations of fish-eating birds like bald eagles.) The curved shapes of bird eggs, which distribute a parent’s weight across a significant area of shell, contribute to their durability. However, larger birds also have eggs with thicker shells (about 0.05 mm thick in hummingbirds, 20 times that in geese) that are heavier relative to the mass of the egg. Although the eggs survive being sat on by the adults, chicks are able to break through from the inside using a small projection on their beak—the “egg tooth”—that falls off shortly after hatching.
Although the egg does contain all of the nutrients required to build a chick, one outside element is required: oxygen. The metabolism that drives a chick’s growth and development, just like adult bodily function, is based on oxidative processes. Those processes also produce carbon dioxide as a waste product that must be eliminated. The exchange of those respiratory gases—oxygen entering the egg, carbon dioxide leaving—takes place by diffusion through tiny holes (pores) in the eggshell.
The ease with which gases can cross the shell depends on the combination of the number of pores and their individual geometry (area and thickness). Not surprisingly, smaller eggs have fewer pores: several hundred for a sparrow-sized bird, ten thousand or more for a bird the size of a duck or goose. The pores of larger, thicker eggshells are longer (which impedes gas movement), but they have larger diameters (which facilitates gas movement). The net result is that individual pores from different size eggs provide similar ease of exchanging gases. And when that is combined with variation in pore numbers and with the rates of respiration of different size chicks, bird eggs of all sizes end up with similar availability of oxygen and carbon dioxide inside. Pretty clever!
The pores that allow passage of oxygen and carbon dioxide also provide a pathway for water vapor, and so eggs lose water to evaporation during incubation. That water loss ends up providing a benefit to the egg, though, as it creates an air space inside the blunt end of the egg. Just before hatching, the chick breaks its beak into that space as it transitions to its first breaths of air. (Until then, the chick fueled its metabolism using oxygen dissolved into blood vessels of membranes underlying the eggshell, without any role for the lungs or breathing.) Still, though, too much water loss would be detrimental to the egg. It turns out that the pore geometry that suffices for oxygen and carbon dioxide exchange also results in a relatively constant fraction of water loss regardless of egg size. Successful eggs of all sizes evaporate away about 18% of their initial mass by the time of hatching.
A robin about to lay an egg probably is not doing a lot of mental geometry to plan its eggshell pore dimensions or consulting designers about what shade of blue to color its eggs. But evolution has done that work over the millennia. The result is that we now see a range of egg sizes, shapes, and colors, all constrained by the physics and physiological needs of this self-contained system. Like any good shell game, it’s more complicated than meets the eye.
Article and photo contributed by Dr. David L. Goldstein, Emeritus Professor, Department of Biological Sciences, Wright State University.