Which came first, the chicken or the egg? That’s a standard puzzler often offered when people are faced with a problem in causation. But the answer is very simple, as noted by a famous biologist (S. J. Gould): the egg came before the chicken, because birds evolved from reptiles (specifically, certain dinosaurs) and reptiles produce eggs. So, of course, do all the predecessors of reptiles—eggs are just gametes (sex cells) that contain some nutrition for an embryo, in contrast to sperm, which normally contain just genetic material and the internal machinery needed for swimming to meet an egg.
Bird eggs, however, differ from those of most reptiles in having a hard shell (a few turtles and crocodilians produce rather hard shells, and so did some dinosaurs, but bird shells are even harder). A hard shell was one invention that greatly facilitated the evolution of living on land and the subsequent radiation of many kinds of birds. The hard shell protects the embryo from predation by invertebrates and microbes, but the hard shell also limits the uptake of water that is needed by the embryo. In compensation for that limitation, the avian egg contains albumen (the egg white), which is composed of water and a little protein. The albumen also serves as a shock absorber if the egg is jostled and an insulator from sudden changes of temperature.
Inside the hard, calcified external shell and the layer of albumen lies a membrane that encloses the embryo and its attached yolk sac. The yolk is composed of fats, proteins, and water. It provides all the food needed for growth of the embryo. As the embryo grows, the yolk gets used up and gradually absorbed into the body cavity.
The growing embryo produces waste products, of course, as it metabolizes the yolk proteins. Another membrane develops, forming a sac to contain wastes. Vertebrates have three basic options for disposing of nitrogen-containing waste products of metabolism: ammonia, urea, and uric acid. Ammonia is not a good option for terrestrial animals, especially enclosed systems such as eggs, because it would rapidly accumulate to toxic levels. Space is limited inside that hard shell, so there isn’t room to produce water-soluble urea (as mammals do), which can’t be flushed out of the egg and would take up a lot of space. Urea also would upset the embryo’s osmotic balance (of water and larger molecules inside and outside the embryo). Instead, birds (and reptiles) produce uric acid (actually a salt of uric acid), which is not soluble and is deposited by the avian embryo as tiny crystals inside the waste-disposal sac. Production of uric acid costs more energy than producing urea, but it solves the waste-storage problem.
The hard shell of a bird egg limits water uptake, requires a self-contained food supply, and prevents external waste disposal, but the embryo is not entirely isolated from the outside environment. Like all vertebrates, the avian embryo needs to breathe: to take in oxygen from the air and get rid of carbon dioxide and water vapor produced by cellular metabolism. To allow this necessary exchange of gases (respiration), the egg shell has pores that allow the gases to move in or out. Tiny blood vessels in the various embryonic membranes link the pathway of gas exchange to the embryo. As the embryo grows, calcium is removed from the inside of the shell and used for growth of the skeleton. So the shell gets thinner and gas exchange increases as the embryo grows and its respiration increases.
It ain’t easy, being an egg! Avian eggs are amazingly complex, solving many problems that accompany early development. They come in a wide range of sizes, too, depending on the size of the female bird, how many eggs are laid in each clutch, on the stage of development of the chicks when they hatch, among other factors. Despite a wealth of data on egg size and adult body size, there remains much to be explained about the factors that determine the size of eggs, both in absolute terms and relative to female body size. In the meantime, here are some curious factoids.
The biggest bird eggs known to science were as much as thirteen inches long and over three feet in circumference; they housed the developing chicks of the now-extinct elephant bird of Madagascar. The smallest eggs, in absolute terms, may be those of hummingbirds, which are roughly the size of my little fingernail.
Everyone probably knows that ostriches lay large eggs; but the bird is very big too, so the size of the egg relative to the female is fairly small. If egg size is assessed relative to size of the mother, it is often said that the record is held by New Zealand brown kiwis. A female brown kiwi produces one egg that can be as much as one quarter of her body weight. (Imagine a female human who weighs a hundred and twenty pounds carrying a fetus weighing thirty pounds!) Female kiwis have to eat enormous amounts of food when that huge egg is developing. But kiwi eggs are so big that, for several days before the egg is laid, there is no room inside the mother for food, so she has to fast for those last few days.
However, kiwis are not alone in vying for the record of relative egg size. A wading bird called the crab plover, which lives on the shores of the Arabian Sea and nearby areas, is also reported to lay eggs weighing about a quarter of the body weight. Similarly, certain tropical terns also lay relatively huge eggs.
At the other end of the range of egg sizes, some birds, including ostriches, lay relatively very small eggs—as little as one or two percent of their body weight. The biggest penguins are in this category, as are some songbirds.
Perhaps the next problem that needed to be solved, as hard-shelled eggs evolved, was where to put them once you had them. All birds are tied to land (or ice) for reproduction, and indeed some only come ashore for that purpose. But how they deal with their eggs varies enormously, and that may be a topic for a future essay.