The Science of Eggs

To supply you for Easter morning conversations, we looked up a whole basketful of facts about this fascinating, ubiquitious culinary staple. (We’ve kept the focus on bird eggs, since it’s pretty rare to find an alligator, platypus or insect egg in your Easter basket.) Enjoy.

How do eggs get their color in nature?

While we dye eggs once a year, birds have found natural ways to make colored eggs all year round. Bird eggs can vary greatly in hue, from the bright blue found on the eggs laid by the tinamou or the robin to the speckly eggs of the killdeer.

Egg color comes primarily from two kinds of pigments: biliverdin, which is blue-green (also found in bruises), and protoporphyrin, which is reddish-brown. These are laid on top of a shell that gets its base white color from its chief ingredient, calcium carbonate.

Colors on eggs serve a variety of purposes: Bright eggs may help parents find their young, and are common to birds that nest in trees. Birds that nest on the ground like the nightjar or the piping plover typically lay “cryptic” eggs that are camouflaged to blend in with the vegetation and ground around them. Pigments may also act as a kind of sunscreen for the baby bird by reflecting UV light, and may also impart extra strength to the eggshell. Some eggs may bear unique patterns that let a mother bird tell her eggs apart from her neighbors’; this most notably occurs in the Common Murre, which “nests” in densely packed colonies on bare rock ledges.

But we humans are only seeing part of the picture in egg coloration. Bird eyes have four kinds of color receptors, compared to the three kinds found in human eyes. This extends a bird’s range of vision into the ultraviolet end of the spectrum. So there may be some cues painted on bird eggs that we just can’t perceive.

Experiments suggest the UV component of egg coloration may be important. When British researchers measured the reflected light from more than 2,100 eggs laid by 251 different kinds of birds, they found that the UV spectrum proved most important in distinguishing between species. In a separate study, when Czech researchers painted Blackcap warbler eggs with a UV blocker, it caused the birds to reject eggs of their own species as they would the eggs of a brood parasite like a cuckoo or cow bird (which tries to dupe other birds into raising its young).

How is an egg made?

A female bird’s mature ovary looks like a cluster of grapes, and usually only one of the pair of ovaries fully develops (probably to cut down on weight so the bird can keep flying). After ovulation, a tiny (unshelled) egg travels down a tube called the oviduct, analogous to what’s usually called a fallopian tube in mammals. The yolk develops from part of the initial egg cell, while the egg white and eggshell are made from layers of compounds secreted in the upper and lower parts of the oviduct. In order to manufacture the eggshell, a female bird has to maintain high levels of calcium in her blood – she may use up to 10 percent of her total body calcium in a single day to lay eggs.

Fertilization isn’t required for laying eggs, which is why there aren’t bones in your omelette. But when fertilized, the egg serves as a baby bird’s life support system. As long as there’s enough warmth, and oxygen passing into the egg through the microscopic pores in the eggshell, the embryo will typically develop normally without any additional help.

Though the reptile invented the egg, the bird improved its design in one key aspect: environmental adaptability. Reptiles typically have to find very moist areas to lay their eggs in, to provide the developing young with water. But a bird egg is designed to survive in drier environments: When the embryo starts consuming the nutritious yolk in the egg, one of the byproducts is water. Once laid, an egg may incubate for anywhere from 11 days, as in small songbirds, to 85 days, as in the Wandering Albatross.

How strong is an egg?

The egg is also a feat of natural engineering: strong enough for a mother bird to sit on, yet delicate enough for a baby bird to peck its way out of. This is due to the egg’s three-dimensional arch structure, a naturally strong shape when force applied is evenly distributed and from the outside. So a chicken egg can support the weight of a mother bird sitting on it, or even a standing human being, and will hold up even if you wrap your hand around it and squeeze it hard. But apply concentrated, uneven pressure—by, say, cracking it on the side of a bowl, or from the inside—say, by pecking at it with your beak—and it cracks easily. And the egg crack’d, as they say, cannot be cured.