What’s in an egg? In addition to a growing chick, quite a lot. An egg is one of nature’s marvels, a well-constructed container that not only protects a developing embryo but also provides it with almost everything it needs, all in one perfect package. While most of us are familiar with the white and sometimes brown chicken eggs at the grocery store, the variation in these avian vessels covers a wide spectrum of sizes, shapes, and colors, depending on the species. Plus, the structure of an egg is a remarkable puzzle in itself. Ready for a little “eggs-ploration?” [caption id="attachment_120051" align="aligncenter" width="1024"] SAME CONTENTS, DIFFERENT SIZE
Whether it’s a seven-inch ostrich egg, a two-inch chicken egg, or a quarter-inch hummingbird egg, each one contains all the structures and nutrients needed to make a chick.[/caption]

Sizing It Up

It makes sense that a big bird lays a big egg, and the ostrich lays the biggest: seven inches long, five inches wide, and weighing three pounds. At the other end of the scale are hummingbird eggs, which can be about a half-inch long and a quarter-inch wide, weighing less than a pea. And there are eggs of just about every size in between. Some bird species lay clutches of several eggs, while others lay only one or two. Species that have more developed, precocial chicks tend to produce larger eggs than those with less developed, altricial chicks. That’s because precocial chicks typically spend more time incubating inside the egg before they hatch, and they need more yolk and protection to support them. But there is one unusual bird that stands out in the egg department: New Zealand’s kiwi. The female kiwi manages an incredible feat—she produces one egg that is almost one-third the size of her entire body, taking up a huge amount of internal space until it is laid. Apparently, nature decided that after all that, she needed some help, since the male kiwi takes over incubation duty. [caption id="attachment_120052" align="aligncenter" width="1024"] EGG PALETTE
At the Zoo’s off-exhibit Avian Propagation Center, eggs pulled for propagation are carefully labeled. This assortment shows some of the remarkable diversity in bird egg sizes, shapes, and colors.[/caption]

Works of Art

Only bird eggs come in colors other than white. Every egg starts out white, and then a base color is added to the eggshell from cells that secrete pigment as the egg passes through the female bird’s oviduct. Only two major pigments make up the amazing number of colors seen in bird eggs: protoporphyrins, which create colors from yellow to red to brown; and biliverdins, which make blues and greens. Different amounts of these pigments mixed together can also result in colors from violet blue to olive green. Of course, some bird species do not add color to their eggs, especially those that nest in tree cavities or deep nests where the eggs would be hidden from predators and don’t need to blend into their surroundings. In addition to the base color of an egg, some birds deposit additional pigments on the eggshell just before it is laid, marking it with blotches, streaks, speckles, or dots. If the egg is moving at the time, the marks may come out looking like a modern painting or obscure calligraphy. Scientists think that these dots, dashes, and lines may be more than just artistic: the additional pigment can serve to fill in weaker parts of the shell and make it stronger. Plus, these patterns often help camouflage the egg in its environment. Egg color is largely determined by a bird species’ genetics. The females of some species all lay eggs of about the same color, although the strength of the pigment may vary from bird to bird, resulting in lighter or darker eggs. In some species, however, the added markings are unique to individual birds. It is thought that in colony nesters, parent birds can pick their own eggs out of thousands, based on the markings. Now that’s a good eye for pattern and line! [caption id="attachment_120054" align="aligncenter" width="1024"] OVAL, ROUND, OR POINTY?
Left: Flamingos lay eggs that are the classic oval shape. Center: The violet turaco’s egg is almost round rather than elliptical. Right: Wood partridge eggs are pyriform in shape—they have one smaller, more pointed end, which keeps them from rolling like a round egg would.[/caption]

In “Egg-cellent” Shape

Saying that something is “egg shaped” is a familiar description. But if you really look at a variety of eggs, the description is not so clear after all. Bird eggs can be oval, round, or elliptical. The shape is a result of the internal structure of the female bird, but it is also influenced by factors such as the nest environment. Some birds lay eggs with one pointed end; the pointed end of each egg faces toward the center of the nest—an efficient arrangement that minimizes the space needed. Some birds lay eggs in a sandy spot on a ledge or cliff, and their eggs are more teardrop or pear shaped—referred to as pyriform—so that if bumped, the egg spins in a circle rather than rolling right off the edge like a round egg would. Round has advantages, though: a round egg can hold more contents with the same amount of shell as an oval egg, and a round shape tends to make the shell a bit stronger. [caption id="attachment_120056" align="aligncenter" width="1024"] SIZE MATTERS
The impressive size of a kiwi egg (left) contrasts with a delicate hummingbird egg (right). The Zoo's Avian Propagation Center handles eggs of many sizes—big or small, they raise them all![/caption]

Oology? Oh!

The branch of zoology that studies eggs is called oology, but the human fascination with these highly variable yet entirely effective containers started long before it had a name. Beyond being a protein-packed and easily obtained food source, people have sought, collected, shared, and admired eggs for centuries. Poems and stories have been written about them, and they symbolize new beginnings, potential, and the Earth in some cultures. They’ve even been a topic for philosophers: who among us hasn’t wondered, at some point, which came first, the chicken or the egg? So be a good egg and don’t take the mighty oval object for granted: consider the wonder and beauty of this marvel of nature before you crack into that shell. [caption id="attachment_120062" align="aligncenter" width="1024"] STRUCTURAL "INT-EGG_RITY"
The architecture of an egg is truly impressive. It contains all the structures, genetic material, and biochemical processes required to build a bird, all in a streamlined, efficient, and quite beautiful form.

1. CHALAZAE
If you’ve ever noticed those white, threadlike pieces when you crack open an egg, you’re seeing the chalazae, which are twisted strands of protein fibers that attach the main compartments of the egg to the shell to aid in stability.

2. CHORION
This membrane encloses the yolk and embryo, but it is permeable to let water in and allow for the exchange of oxygen and carbon dioxide.

3. YOLK
This is the nutrient-rich food source for a developing embryo. It contains fat, cholesterol, protein, vitamins, and minerals that the chick absorbs, and the yolk reduces in size as the chick grows. All bird egg yolks are yellow or red (yes, red!). Why? Because they contain high levels of carotenoids, which provide nutrients and are thought to protect the embryo from free radicals that might damage the DNA of the developing chick.

4. BLASTODISC OR GERMINAL DISC
In a fertilized egg, this is the combined genetic material from the female and male birds that will develop into a chick. In an unfertilized egg, this spot is only genetic material from the female and is unable to become an embryo.

5a/5b.THICK AND THIN ALBUMEN
These layers make up the albumen, known as the egg white, which accounts for about 65 percent of an egg’s weight. The albumen provides hydration and acts as a shock absorber when the egg is moved, such as during incubation when a parent bird turns the egg to keep heat distribution even. The albumen also contains proteins and enzymes that have antimicrobial properties. These enzymes are activated by heat, so incubation helps an embryo develop and protects it from infection.

6. AIR SPACE
A cushion of air between the shell membranes provides extra shock absorption.

7. CUTICLE
Just before an egg is laid, a thin coating called the cuticle or bloom is applied. In some birds, like the tinamou, this layer is almost lacquer-like and partially fills in pores, creating a very glossy, shiny egg.

8. SHELL
The outer shell is made of layers of mostly calcium carbonate, which are deposited within the female bird’s body as the egg travels down the oviduct. It takes about 20 hours to complete the shell. While the shell is rigid and looks smooth, it is actually semi-permeable, with tens of thousands of microscopic pores that allow for gas exchange and the release of water vapor. In other words, eggs breathe! Some shells have more and bigger pores or a looser construction, and they can appear rough, bumpy, or chalky. Tighter construction and smaller pores make a smoother egg.

9a/9b.INNER AND OUTER SHELL MEMBRANES
These two membranes sandwich a layer of air, which provides additional cushioning. Like the chorion, these membranes are permeable to allow gas exchange. They also act as barriers to prevent the egg from drying up and to prevent bacteria from getting in.[/caption]