I was frying eggs for my breakfast one morning recently when it occurred to me that it was a pretty neat trick for birds to protect their embryos with a hard shell. I thought that this is a subject worth looking into and before I knew it I was reading all sorts of interesting things about birds’ eggs. That led, naturally I suppose, to reading about bird sex, how eggs are made and that led to this essay.
Let’s start at the beginning, with bird sex. We have all seen a male bird hop on the back of a female and understood that it is the way birds mate, but I have never understood exactly what is happening. For most birds species, both males and females have a posterior opening called a cloaca (pronounced kloh-ey-kuh) from which they urinate, pass feces, and, depending on their sex, lay eggs or transfer sperm. When a male mounts a female she bends forward to provide a flat surface and moves her tail feathers aside to expose her cloaca. He then exposes his cloaca and touches it against hers in what is called a “cloaca kiss.” It can take less than a second, but that is how he deposits sperm that will work its way via her oviduct—the tube that connects her cloaca to her ovary—and fertilize her egg. Usually they “kiss” several times in one event.
Waterfowl, like ducks, geese and swans often have sex at least partly under water, and since that could cause the sperm to be washed away, the males in these species have developed a phallus-like extension of their cloacae that they insert into a female cloaca.
After the bird’s egg is fertilized it becomes a protein-packed yolk. As the mass passes through the oviduct it is like a car on a manufacturing assembly line. The first addition to the yolk is albumen, which contains the protein necessary for the embryo’s development, as well as anti-bacterial enzymes, and a liquid medium in which the embryo develops. Further along the oviduct a little water is added to the mass and further still it is wrapped in a stretchy membrane. At one of the last stops in its progress the mass enters a section where a calcium carbonate shell is developed around the egg. The final stage is when color—if there is to be color—is added to the shell.
In order to produce a hard shell that will protect her eggs, female birds have to eat things that contain calcium. Some vegetation that they eat is high in calcium, but birds also peck at old bones, at shed deer antlers, and they find mineral deposits of calcium in soil. Further, some birds eat snails or other small mollusks and convert the calcium in mollusk shells into their own eggshells. Birds of prey like hawks and owls, as well as crows, blue jays or grackles eat small mammals, other bird babies and in some cases adult birds, and they get calcium from their preys’ blood and bones. If necessary, female birds also draw on their own bones to get calcium for their eggs. A female bird can loose ten percent of the calcium in her bones when she is producing her eggs.
The shape of birds’ eggs varies from almost round to an elongated oval, with a chicken egg being in the middle of this spectrum. Cavity-nesting birds tend to have nearly round eggs, but in a miraculous example of evolutionary adaptation, sea birds that nest on cliff ledges tend to have elongated oval eggs with one end larger than the other. Nests on cliff ledges are often quite insubstantial and the elongated oval eggs, if loose, will roll around in circles instead of rolling right off a ledge.
An egg will usually lie in a nest with its large end pointed up. There are tiny pores everywhere for air to enter but there is a higher concentration of them at the larger end. The large end is where the embryo’s head is located and the brains and eyes of the baby bird require more air than the rest of the body to develop.
The default color of birds’ eggs is the white of calcium carbonate but their eggs also come in different colors, and with spots or lines of many colors. No one is quite sure why an American robin’s eggs are blue, “robin’s egg blue” to be exact, but birds that build nests in trees generally have blue or greenish eggs. Ground-nesting birds’ eggs are wonderfully camouflaged. In species that nest in large colonies, like guillemots or gannets, each female bird’s egg is distinctly marked with color, spots or lines, which helps her find her own egg when she leaves it and returns.
Birds require differing lengths of time for their eggs to hatch and for the babies to fledge. Most songbirds require about two weeks to hatch and about the same to fledge. Larger birds lay bigger eggs that require a longer time to hatch and it takes a longer time for the babies to fledge. It may take as much as six weeks for many raptors’ eggs to hatch and up to ten weeks for the babies to fledge. Eagle babies may need several months to fledge.
Birds’ eggs have played an important role in mythology and religion for millennia. Ancient Egyptian religion proposed that the earth was hatched from an egg. The Greeks and Romans left eggs or nests with eggs in them in tombs, signifying renewed life after death. Most Christians today probably do not pay much attention to the symbolic role of eggs at Easter, the time of Christ’s resurrection, but in earlier times the connection was more consciously recognized. Eggs enclose nascent life and their very shape, without a beginning or an end, can easily be seen as life everlasting.