The bird’s egg

What is inside an egg

From the outside in there are only a handful of layers, and each does something different. The shell is the skeleton, the two membranes are the sieve, the albumen is water store, shock absorber and disinfectant in one, and the yolk is the provisions. Only the germinal disc, a spot a few millimetres across on the yolk, is the actual egg cell. Everything else is luggage the female builds around it in a single day.

The albumen is mostly water and defence. Lysozyme breaks down bacterial cell walls, and ovotransferrin locks up the available iron so tightly that bacteria cannot reach it. An egg laid clean and with its cuticle intact therefore stays sterile for a long time — long enough to wait until the clutch is complete and incubation begins.

The chalazae are the most underrated part. They are twisted onto the vitelline membrane and work as a suspension: however the egg comes to lie, the yolk rotates inside it until the light latebra and the germinal disc are on top — right under the parent’s brood patch, at the warmest point of the egg.

The shell close up

The shell is over ninety per cent calcium carbonate, in the crystal form calcite. The rest is a network of proteins that steers the crystals: where and how fast they grow, and how large they get. That network is the difference between a brittle crust of lime and a structure that carries the weight of an incubating bird.

The shell is built from the inside out. The mammillary cones sit on the outer membrane like seeds; from them the columns of the palisade layer shoot up until they meet. Where they fail to meet, a pore stays open — a slanting channel to the outside with a cap of cuticle over it. There are thousands per egg; estimates for a hen’s egg run from 7,000 to 17,000.

Those pores are a compromise. They have to admit enough oxygen for an embryo that is breathing hard by the end of incubation, without letting through so much water vapour that the egg dries out. The higher and drier a bird breeds, the more frugal the shell has to be with gas: eggs of high-mountain species have a lower pore conductance than those of their lowland relatives.

The shell has a second job. During the second half of incubation the embryo, through the blood vessels pressed against the shell, dissolves the mammillary cones and uses that calcium for its own skeleton. The shell becomes noticeably thinner and more brittle — exactly when the chick has to break through it.

How an egg is made

In the ovary the yolks lie in a hierarchy: the largest is due tomorrow, with several more queueing behind it. Such a yolk is not made in one go but over days or weeks, in thin layers that are yellow by day and paler by night. Cut a boiled yolk in half and the rings are still there.

After ovulation the yolk slides into the funnel and is fertilised there, before a single layer of albumen has been added. The sperm need not be fresh: in folds at the junction of oviduct and vagina the female stores sperm that stays usable for days or weeks. One mating can fertilise a whole clutch — and by the same token a clutch can have several fathers.

After that it is a production line. In the magnum the albumen is added over some three hours; because the egg is pushed along while rotating, the chalazae are twisted into cords. In the isthmus the package gets its two membranes in just over an hour, and with them its shape. Then comes the shell gland, where the egg stands for nearly twenty hours while the lime is applied layer by layer. Laying itself takes seconds.

All that calcium has to come from somewhere. Before the breeding season females lay down a reserve in the marrow of their long bones — medullary bone, formed in a few days and broken down again during laying. What the bone does not supply has to come from food; small songbirds actively search for snail shells and grit during the laying period.

Because the shell eats up most of the day, at most one egg fits into twenty-four hours. Many songbirds therefore lay in the early morning, one egg a day until the clutch is full. Where the cycle runs slightly longer than 24 hours the laying time shifts later each day until a day is skipped — and species with a large egg, such as many raptors, lay every two or three days from the outset.

Colour and markings

Every egg colour comes from two pigments. Protoporphyrin IX gives all the red-brown, yellow and black tones; biliverdin gives blue and green. They are applied only in the shell gland: the ground colour throughout the calcification, the spots in the last hours before laying. Markings smeared into streaks or scrawls betray that the egg was still moving at that moment.

The crudest rule in the field is the rule of light. Birds that breed in a dark cavity — woodpeckers, Kingfisher, Sand Martin, Bee-eater, most owls — lay white, unmarked eggs: camouflage is pointless there, and a pale egg is easier to find in the dark. Birds that nest in the open on the ground — plovers, terns, waders, larks — lay the most heavily marked eggs of all.

Why an egg has the colour it has beyond that is an open question with four serious answers. Camouflage explains the ground-nesters. Gosler’s strength hypothesis holds that protoporphyrin is deposited precisely where the shell is thin, like a kind of plasterwork. Moreno and Osorno read the blue-green of biliverdin as a signal from female to male — an idea that has since been strongly criticised, not least because eggshell colour often turns out to be set by the environment rather than by heredity. And Lahti and Ardia pointed to light and heat: dark pigment blocks harmful UV but overheats in the sun, so every species settles somewhere between the two. Worldwide, eggs do indeed get darker towards colder, sunnier breeding areas.

None of the four explains everything, and they are not mutually exclusive. What is certain is that the pattern is recognisably consistent within a female. That is no side issue: it is exactly what allows a Reed Warbler or a Whitethroat to spot a cuckoo egg among her own.

Shape

Egg shape has long been explained from the nest: pointed eggs supposedly pack better into a clutch of four, or fail to roll off a ledge. When Stoddard and her colleagues compared nearly fifty thousand eggs from fourteen hundred species in 2017, none of those explanations held. What did hold was flight: the better a bird flies, the more elongated and asymmetric its egg. A streamlined body has a narrow pelvis, and a long, tapered egg keeps its volume even through a narrow passage.

That shape is set in the isthmus, by the shell membrane. The lime that follows merely traces what the membrane has already laid down.

The extreme case is the Common Guillemot, with an egg so sharply tapered at one end that it is almost pear-shaped. The textbook explanation — that it rolls back in a circle instead of off the ledge — has never been confirmed; on real ledges the egg does no such thing. Birkhead offered two alternatives: the shape makes the egg less vulnerable to the collisions that are unavoidable in a crowded colony, and because the egg rests on its point, the blunt end with most of the pores stays out of the filth of the ledge.

Size, clutch and incubation

Within Europe, size runs from the Goldcrest’s egg — fourteen by ten millimetres, a little under a gram — to the Mute Swan’s, over eleven centimetres and some 340 grams. A factor of four hundred, from the same machinery.

More telling is size relative to the bird. In an Ostrich the egg is not even one and a half per cent of the female’s weight; in a kiwi it is around twenty per cent, and in the last days the female barely eats because there is no room left. In Europe the waders sit at the heavy end: a Lapwing egg is strikingly large for the size of the bird, and that is no accident — precocial species invest more in the egg and less in the chick.

Clutch size varies as widely. Guillemots, fulmars and most petrels lay one and rarely replace it; Great Tits reach more than ten in a good year, and ducks often eight to thirteen. Some species lay a fixed number however many you remove; others keep topping up as long as the nest is not full.

Incubation length mostly follows size and the state of the chick at hatching. Small songbirds sit for around twelve to fourteen days, most ducks four weeks, and the Bearded Vulture fifty to sixty — the longest in Europe. Over that time an egg loses about fifteen per cent of its weight as water vapour through the pores. It is precisely that loss which grows the air cell into the space where the chick will take its first breath.

Precocial and altricial

The difference between a downy duckling that takes to the water within a day and a naked, blind tit chick is already there in the egg. In precocial species about forty per cent of the egg is yolk; in altricial ones around twenty to twenty-five. More provisions means a longer incubation and a more developed chick — the duckling hatches with open eyes, down, working legs and a remnant of yolk in its belly that feeds it for the first days.

It often shows from the outside: a large egg in a small bird points to a precocial chick. Compare a Lapwing’s clutch with that of a Blackbird of the same weight.

Hatching

Around the seventh day a small horny point grows on the embryo’s upper mandible: the egg tooth. At the same time the hatching muscle swells in the neck, a muscle that exists for this alone. A few days before hatching the chick pushes the egg tooth through the inner membrane into the air cell — the internal pip. From then on it is breathing air and is audible: the first peeps carry through the shell, and parent and chick start answering each other before they have ever met.

Then comes the external pip, the first hole to the outside, and it can still take twelve to forty-eight hours before the chick has worked its way out along a ring of cracks around the blunt pole. The egg tooth drops off of its own accord in the days that follow. In species where the whole clutch must hatch at once, such as quails and many waders, the chicks coordinate by sound: the eggs that are furthest along slow down and the laggards speed up.

What happens to the shell afterwards is itself a classic piece of fieldwork. In Black-headed Gulls, Tinbergen showed that the white inside of an empty shell left beside the nest attracts crows and gulls and measurably raises the risk of predation. The parents carrying it away is therefore not tidiness but camouflage.

The cuckoo’s egg

A female cuckoo has rebuilt everything to do with the egg into burglary equipment. She lays in about ten seconds — an ordinary songbird takes minutes — usually in a brief window while the host is away, and often removes one of the host’s own eggs as she goes.

Her egg is small for her size, so that it does not stand out among those of a Reed Warbler or a Meadow Pipit, and it has a thicker, harder shell than the host’s eggs. It also hatches sooner: eleven to thirteen days, often a day ahead of the rest, so that the chick is first to heave the others over the rim.

The most striking part is the mimicry. Cuckoos are divided into gentes: lines of females each specialised on one host species and matching the colour and pattern of its eggs. The trait is passed down the maternal line, which explains how the lines persist side by side while the males mate across them. Host and cuckoo hold each other in balance: the sharper a host is at spotting a foreign egg, the closer the mimicry has to be to pass inspection.

The egg among related animals

An egg with membranes around it is not a bird speciality: turtles, lizards, crocodiles, dinosaurs and birds all share that construction, and it is what allows an egg to lie out of the water. Dinosaur eggs were long assumed to have a hard calcareous shell. Fossils re-examined in 2020 show something else: in several groups the eggs are soft and leathery, like a turtle’s, while the hard calcareous shell is found in at least three separate groups within the dinosaurs.

The two pigments are not confined to birds either. They have been detected in fossil eggs of oviraptorosaur dinosaurs, which therefore laid blue-green and speckled eggs. How widely the trait is spread is unsettled. The find was presented as belonging to the bird-like dinosaurs alone; the objection is that the same compounds also occur in white bird eggs and in crocodile eggs, which would place the trait far more widely among the archosaurs.

Sources

The figures on this page are rounded and given as orders of magnitude; where sources disagree, this is said in the text.

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