EGG ANATOMY & PHYSIOLOGY

The avian egg is the equivalent of a furnished apartment at a vacation resort: it contains all the ingredients and structures required for safely housing the developing embryo, but it needs regular parental servicing and support to function fully. The internal architecture of the egg includes both the genetic machinery and the biochemical structures required to build a viable hatchling.

The already fertilized embryo is enclosed in the amnion, nourished by the yolk and the allantois. The yolk contains high amounts of fat, cholesterol, protein, vitamins, and minerals needed by the developing embryo, while the allantois aids respiration by storing nitrogeneous waste.

These compartments are enclosed within the chorion, and surrounded by the albumen (commonly known as the "egg white"), which supplies hydration for the embryo, and acts as a shock-absorber against sudden movements by the egg. Twisted threads called chalazae (the textured strings sometimes seen in raw or soft-cooked eggs) attach the compartment to the shell for added stability.

Membranes on the inner and outer surface of the shell act as physical and biological barriers to desiccation and bacterial infestation. In addition, both the albumen and the eggshell cuticle contain enzymes and other proteins that have active antimicrobial properties. These enzymes are activated by heat, and so sitting on the eggs at night, even before full incubation begins, may serve to protect the eggs from infections. The whole of this package is contained in the hardened eggshell, which is made up mostly of calcium carbonate. However, this eggshell is semi-permeable, with microscopic pores that penetrate the shell, providing channels for gas exchange necessary for respiration by the developing embryo.

Because of the hard shell, the egg must be fertilized while it is still inside the female's body, before the shell has formed. The chicken's ability to lay eggs whether they are fertilized or not accounts in part for its eggs being such a food staple for humans.

While the egg itself provides much that the embryo needs, the parents must still provide critical services. Typically, one or both parents provide the external heat necessary to jump-start embryonic metabolism, and maintain the necessary microclimate, including high levels of humidity, to keep the eggs from desiccating. They select nest sites and build or usurp nests for the eggs to shield them from predators, sun, dryness, and other threats. They also rotate the eggs in the nest to assure even heating or cooling, and to prevent embryonic malformation.

EGGS & ENVIRONMENTAL TOXINS

The avian egg is a compact and adaptable product of evolutionary engineering. Yet human activities are capable of compromising and even breaking down this highly functional reproductive system. Toxic chemicals such as DDT, introduced into the environment during the 1960s, interfered with some birds' ability to produce the calcium needed to harden the eggshells. The result was thin-shelled eggs unable to bear the weight of the incubating adults; the eggs were crushed, ending in reproductive failure. It took 30 years for populations of Peregrine Falcons, Osprey, and Brown Pelicans to recover their numbers after the banning of DDT.

EGG SIZE & SHAPE

Bird eggs come in extreme sizes and shapes. The ostrich's egg is the largest and heaviest of any living bird; weighing in at over 4 lb (2 kg), it represents over 30 chicken-egg equivalents. At the other end of the scale, the smallest eggs are laid by hummingbirds; they are 1/5,000th the size of an ostrich egg and may weigh less than a paper clip. Some species of the extinct elephantbirds laid eggs that were well over twice the size of an ostrich egg.

SIZE MATTERS

In general, larger birds lay larger eggs; they also have thicker shells, which makes mechanical sense, with each egg having to withstand some of the weight of the incubating adult sitting on top of it. But when considered in relation to adult body weight, egg sizes tell a different story. In this context, ostrich eggs are rather small for the adult's weight and hummingbird eggs are rather large. Some of largest eggs of any bird relative to the female's body weight are laid by the several kiwi species of New Zealand.

Thus, relative egg size, compared to adult body weight, is far from constant and varies extensively with ecology and evolutionary history. For example, birds that lay larger clutches also tend to lay smaller eggs, and species whose chicks hatch fully feathered and ready to follow the parents, tend to produce larger eggs than species whose chicks hatch blind and naked and require prolonged parental care. An egg's size often varies with its internal content, including lipids in the yolk, and the concentrations of hormones, vitamins, and maternal antibodies in the yolk and the albumin. Eggs with smaller yolks typically hatch earlier, with the young soon having to obtain additional nutrition for growth on their own, or by begging for provisions actively from the parents.

THE ADVANTAGES OF BEING "EGG-SHAPED"

Most eggs have a clearly identifiable blunt end (or pole), typically formed where the shell is physically closer to the cloaca in the oviduct; the sharp end forms nearer the ovary. The shell is typically thinner near the blunt pole and thicker near the equator and sharp pole. But as the embryo grows, the calcium from these thicker shell regions is recruited as building material for the developing bones in the skeleton, so that at the time of hatching, most regions of the shell are equally thin. Chicken embryos typically face the blunt pole with their beaks, and start breaking the shell in this region in preparation for hatching.

"Egg-shaped," or ovoid, has many advantages: despite the fragile shell, an ovoid can withstand surprising compression (for example, from the incubating adult's weight) before it breaks. An ovoid is also easier for the female to push out of her body. The symmetrical shape and smooth texture of the shell is achieved by muscles rotating the entire egg in the oviduct during shell formation; when this process is interrupted, for example due to trauma or aggression by predators or competitors, an asymmetrical and rough-textured egg may be formed, causing difficulties, or even death, to the female during laying.

EGG COLORATION & PATTERNING

The photographs in this book reveal the astonishing colors and patterns of avian eggs. Eggs are made primarily of calcium carbonate, which is white to the human eye. While many bird eggs are also white, all the additional variation in eggshells is the result of the interaction of physical and chemical properties of the shell and just two major pigments: biliverdins, which are responsible for the blue-green hues of eggs, and protoporphyrins, which make the rusty colors, from yellow to red to brown. Spotted, lined, blotched, or scrawled eggs have higher concentrations of protoporphyrins. When the two pigments combine in different proportions they can create hues from violet to green.

PIGMENT POWER

How is it possible that just two pigments, interacting with the crystalline structure of the eggshell, can generate the diversity of shell coloration and patterning seen in nature? The surprising answer is that we simply do not know yet. All the studies that have attempted to extract pigment-like compounds from avian eggshells have produced chemicals consistent with the structure of biliverdin and protoporphyrin, or chemicals whose structure could not be identified even with the latest analytical instruments. Biologists and chemists now must combine forces to solve this conundrum. Despite this outstanding question, some reasons for egg colors are relatively straightforward to explain: for example, the typically whitish eggs of the Horned Grebe quickly become stained red from the wet plant matter used to cover the eggs when the incubating parent is off the nest.

The shell gland compartment of the oviduct has to take resources away from the laying female and divert them into producing colorful pigment molecules, which then combine to generate the background color and the spotting, streaking, and blotching on the eggshell. Scientists therefore argue that white eggs are "cheaper" than colorful ones and more likely to be produced by birds whose eggs are hidden in a deep nest (or nest hole) or under the cryptic plumage of a dedicated incubating parent. This is the case for the white eggs of woodpeckers, hummingbirds, ducks, and owls. Sometimes, patterning on eggs can be critical to identifying individual eggs laid in a colony, such as the fantastic variation among individual eggs of Common Murres. In this species, variation has evolved to allow parents to pick out their own egg from a thousand on a crowded cliff face.

A critical piece in the conversation about egg colors is that we have only recently begun to learn about what the birds themselves see. All birds have four photo receptor proteins, compared to three in humans, which provides them with instantly more accurate and detailed color perception, relative to humans, including seeing color in the ultraviolet (UV) region, not visible to people. Researchers are now surveying and analyzing eggshells with physical instruments, such as UV-filter camera-lenses and reflectance spectrometers to reveal unexpected variation between and within eggshells, that birds can see, but, until recently, scientists had not.

NESTS & EGGS

Birds have evolved a fantastic package—the egg—to nourish and support the development of their embryos after they are ejected from the mother's body when the egg is laid. As part of the same set of successful reproductive strategies, birds have also developed an extraordinary array of structures to shelter, protect, and help warm the eggs and chicks: the nest.

The simplest definition of the nest is any structure or space that surrounds and houses the egg(s). Constructed nests are a form of tool use by animals: birds take materials found in the environment (including human castoffs), manipulate and modify them, and generate a novel use. This basic definition of tool use covers most bird nests, from the loose stick nest of a Rock Dove breeding on a window ledge, to the hundred-unit nesting aggregations of colonially breeding weavers.

Nests have evolved to help protect the eggs and the nestlings from competitors, predators, and also parasites. Enclosed nests, whether constructed domes or a hole in a tree, keep the eggs and chicks out of the sight of predators and away from sun and rain. Building a nest in a hidden place, among dense tufts of grasses or foliage, ensures discovery by competitors, and many nest predators, is minimized. A tight or difficult-to-access nest can also provide protection from parasites: Orchard Orioles, for example, protect their clutch by sitting tightly on the nest, so that the contents are not accessible to parasitic Bronzed Cowbirds attempting to lay their eggs into the nest cup.

Finally, nesting sites and nests themselves can serve as an aphrodisiac; in many bird species, males attract females to their territory by building a nest or defending a nest site. Females choose a male based on his ability to build a good nest in a safe location; these traits of the nest architecture and location work together to assure greater reproductive success for both the talented male and the choosy female.

HATCHING EGGS WITHOUT NESTS

Not all birds use a nest; some simply deposit the eggs on bare ground, a cliff ledge, in leaf litter, at the bottom of a tree cavity, or in a shallow ground scraping. The heat for embryogenesis is still provided by the incubating adult. Many seabirds, including Gannets and Emperor Penguins, cannot afford to lose feathers from the chest to develop a barren skin, called the brood patch, because they need to keep diving deep into the cold seawater for fish throughout the nesting period; in these cases the heat is transferred to the eggs in other ways, particularly through the warm blood carried in the veins of the webbed feet.

Most unusually, brush turkeys and other birds in the family Megapodidae lay their eggs into heaping mounds of rotting vegetation or warm sandy soils near sunbathed beaches or volcanic slopes. There, the biochemical, solar, or geothermal energy provides the heat for the eggs to maintain embryonic development.

BREEDING STATEGIES: CLUTCH SIZE

Birds lay eggs in sets, or clutches. Clutch size is the number of eggs laid in one nesting attempt; it varies across species, from a single egg to as many as 20. In part, this is determined by evolutionary history; all albatrosses lay just one egg, hummingbirds consistently lay two, while partridges can lay 10–15 or more. But this is not always the case. Clutch size also can vary within species, within populations, and even for an individual from year to year, based on such variables as habitat, latitude, altitude, nest type, size of a nesting colony, food availability, and body size and health of the mother. Such trade-offs and the choices involved are a critical part of a bird's breeding strategies at all levels.

CLUTCHES LARGE OR SMALL

Broadly speaking, natural selection leads birds to lay as many eggs as they can successfully rear. For example, birds that breed in northern latitudes tend to have larger clutches while related birds from the tropics lay fewer eggs; although temperate regions have a shorter breeding season, there is plenty of food during that time to feed more chicks. Birds that lay multiple clutches each year may have fewer eggs in their final clutches; this may be due to reduced resources in the health of the mother but also could be related to the availability of food late in the breeding season. Likewise, birds with precocial young (those that leave the nest soon after hatching and can feed themselves thereafter) lay more eggs per clutch than do birds with altricial young (those that require prolonged brooding and care). Parents with altricial young may find more success with a smaller brood.

Most small birds lay one egg each day until their clutch is complete; others, typically large birds, lay only one egg or lay consecutive eggs two to three days apart. Female kiwis, whose eggs weigh some 25 percent of their body weight, may go weeks between laying the first and second egg.

SURVIVAL OF THE FITTEST

Parent birds can control to some extent the rate and timing of egg development to influence the success of their breeding attempt. Some start to incubate each egg on the day when it is laid, resulting in asynchronous hatching; the chicks hatch over a series of days, and the first-hatched chicks are larger and more dominant. In Cattle Egrets, asynchronous hatching, and the resulting size and dominance hierarchy of the chicks, is further enhanced by the mother depositing different amounts of testosterone into the egg yolk: the first two eggs receive nearly twice as much as the last, third-laid egg. The result is that in seasons with poor food supplies, the hungry, aggressive, and large first chicks attack, peck, and drive off the nest the small, timid last chick, assuring that the resulting brood of just two chicks receives sufficient food from the parents to fledge successfully.