The Art of Hatching an Egg, Explained

Excerpted from THE MOST PERFECT THING: INSIDE (AND OUTSIDE) A BIRD'S EGG. Used with the permission of the publisher, Bloomsbury.​ Published April 2016. Copyright © 2016 by Tim Birkhead. All rights reserved.

What makes a bird egg so spectacular? That's the driving question behind Tim Birkhead's latest non-fiction masterpiece, The Most Perfect Thing, released this past spring. From the making and coloring of shells to the self-sanitizing power of a parent's touch, Birkhead lays bare the entire history of the egg and its survival. In the following excerpt of a chapter, the author digs into the debate on which end of the egg comes first, before carrying off into a rich description of the chick's emergence from its incubated labratory, out into the big and beautiful world.    

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In Gulliver’s Travels, Jonathan Swift describes a conflict between different factions within the kingdom of Lilliput over which end a boiled egg should be broken.

By tradition the Lilliputians had always broken their eggs at the large end, but after the Emperor cut himself while opening the big end, he decreed that the little end should be the end for opening. This was not universally accepted and the quarrels over which end was opened gave rise to no fewer than six rebellions. Swift’s endian wrangle satirises the ongoing eighteenth-century conflict between the Catholics (big-endians) and the Protestants (little-endians) over whether the body of Christ is actually or only symbolically present in the Host at communion.

There has been a similar endian dispute over which way an egg emerges from the bird’s cloaca: big end or little end first? Although there are some dissenters, most people—thanks largely to Aristotle—think that the blunt end emerges first. In turn this has led to an erroneous explanation for how the egg is propelled along the oviduct. Several early authors, including Friedrich Christian Günther, who wrote one of the first books on birds’ eggs in the 1770s, assumed that because the blunt end emerged first that is how the egg travels down the oviduct. He suggested that it is pushed along by peristaltic forces, much like a food bolus in the gut, with the oviduct’s circular muscles contracting behind the egg while those at the front are relaxed. It was thought that the trailing, pointed end—that is the longer portion of the egg—gave the oviduct wall greater purchase to squeeze the egg on its way.

One of the proponents of this idea, the nineteenth-century anatomist Heinrich Meckel von Hemsbach, was so confident about this explanation he called it a "mathematical necessity." To be fair, the idea does have an intuitive appeal, which probably explains why it was picked up and perpetuated by the great Scottish biologist D’Arcy Wentworth Thompson in his book On Growth and Form (1917). Thompson’s intellectual stature was such that others assumed he must be right, including J. Arthur Thomson who repeated the error in his Biology of Birds published in 1923.

I am intrigued by the way certain ideas in biology can persist for so long in the face of contradictory evidence. How could D’Arcy Thompson, J. Arthur Thomson and others, ignore the evidence that flew in the face of their egg movement idea? As early as the 1820s two monumetal figures in biology, the Czech biologist Jan Purkinje and the German Karl Ernst von Baer, both reported that even though the hen’s egg usually emerges blunt end first, it passes down the oviduct pointed end first. Others confirmed that this was also true for pigeons, hawks and canaries, so why did Thompson and Thomson persist in their contrary view? Did they not believe their illustrious predecessors? Perhaps they didn’t read German (for which they can be forgiven). The one paper that surely should have convinced them was by Heinrich Wickmann.

Using eight very tame chickens that would lay their eggs on his desk, Wickmann recorded the events in the hours immediately before and during egg laying. Ingeniously, he was able to use a pencil to mark that bit of the egg he could see inside the hen’s oviduct, through its cloaca prior to laying. (I can just imagine his wife popping into his study with a cup of coffee and seeing Wickmann with his pencil up a hen’s bottom: "What are you doing, dear?" she asks . . . ). This allowed him to establish that, in the hour or so before it is laid, the egg is orientated with its pointed egg directed towards the bird’s rear even though all eggs were all laid blunt end first. Wickmann deduced that the egg must turn immediately before it is laid.

When I first heard of eggs turning in this way I imagined them doing so vertically, along their long axis—that is by "pitching"— but they actually do so by rotating through 180° in the horizontal plane (i.e. yawing). This was discovered in the 1940s by John Bradfield, who used X-rays to observe hens’ eggs on the later part of their journey through the oviduct. The broody hens sat immediately in front of the X-ray screen, and a succession of images was taken, starting at around midday just as the egg—covered only by the shell membrane—entered the shell gland. Images were taken, Bradfield says, at intervals until 9 p.m. and then restarted at 8 a.m. the next day. Had Bradfield been my PhD student I’d have suggested that he stay up all night at least once, although as it turned out it probably didn’t matter. He wrote: "That part of shell secretion which goes on during the night is unavoidably missed, but by following an egg which is ovulated early in the day it is possible to trace the first half of the process (which proved to be the most interesting), together with the last few hours."

When Bradfield examined his X-ray images what he saw was remarkable. An hour or so before laying, the shell gland with its fully formed shelled egg dropped a few centimetres out of the pelvic girdle, and over a period of just one or two minutes, during which the hen stood up, the egg rotated 180° horizontally. The dropping of the shell gland is possible because, unlike the mammalian pelvis which forms a circle of bone and through which the head of the fetus must pass at birth, a bird’s pelvis is, as Bradfied says, shaped like an upturned boat, allowing the drop and rotation to occur, as well as facilitating the laying of large, hard-shelled eggs.

In each bird that Bradfield observed, the pattern was the same: the egg entered the shell gland pointed end first, turned and was laid blunt end first. Why turning should be necessary is unclear, especially for eggs like those of the domestic fowl and most passerines that don’t actually differ all that much between ends. The fact that it isn’t entirely consistent within species implies that the way the egg emerges cannot be that important. It is a pity that those researchers who observed the few chicken eggs laid pointed end first did not record whether those eggs were a different shape from those laid blunt end first. Perhaps for most eggs it is better—for some unknown reason—to undertake most of their journey down the oviduct pointed end first, but for the finale to emerge blunt end first is better.

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Hatching is the climax of incubation; indeed, it is the climax of both fertilisation and incubation and the third great landmark in the life of an egg. How does the chick break out from the claustrophobic confines of the shell? Our mental image of the process has been corrupted by cartoons, where attempts to romanticise and sanitise the process often show a hen’s egg with its top neatly popping off to reveal a warm, yellow fluffy chick. The reality is not like that. It is still pretty remarkable, but it isn’t as quick, as clean, or as simple as we have sometimes been led to believe.

A fully developed embryo lies scrunched up inside the egg with its ankles at the pointed end and its head towards the blunt end; its neck is bent so that the head lies adjacent to the breast with the beak poking out from under the right wing up against the egg membrane. This pre-hatching posture seems to be typical of all birds, except for megapodes.

Before starting to break out of the egg the chick has three things it must accomplish. It must first switch from being dependent on the oxygen diffusing through the pores in the eggshell into the network of blood vessels that line the inner surface of the shell and start to use its own lungs to breathe. The chick takes its first proper breath and fills its lungs the moment it punctures the air cell inside the top of the egg. This step is essential because by this stage of development there is not enough oxygen diffusing through the pores in the shell to support the chick’s respiratory requirements. Taking a breath from the air cell provides the oxygen and the energy necessary to break through the eggshell.

Before it takes that first breath, the chick has to start shutting off the blood supply to the network of blood vessels that line the inner surface of the shell, and withdraw that blood into its body. The blood vessels are programmed to close off at the point where they emerge from the bird’s umbilicus, and just before the chick starts cutting round the shell.

Third, the chick has to take what is left of the yolk and draw it into its abdomen. It does this by sucking up the remaining yolk through the stalk that connects the yolk to the chick’s small intestines. This "yolk sac" is a food reserve for the first few hours or days after hatching.

Essentially, the chick has to do what a human baby does as it switches from dependence on the placenta for both oxygen and food to independent breathing with its lungs and the ingestion of food through its mouth. Thinking of it like that, it is a pretty major transition.

The chick is now ready to break out of the shell and starts by thrusting its beak against the inside wall of the shell. To help puncture it the chick employs a tiny structure of especially hardened material at the tip of the bill. Known as the egg tooth, its role in hatching was discovered by the ornithologist William Yarrell in 1826. Watching domestic ducks and hens hatching in an early incubator, and by removing a fragment of shell, he could see the sharp little egg tooth pressing against the inside of the egg, ultimately enabling the chick "by its own efforts to break the walls of its prison." Reptiles (including at least one dinosaur) also have an egg tooth, as do the egg-laying mammals, the duck-billed platypus and echidna: it is the key for getting out of a shell. In birds, the egg tooth is made of calcium and is usually confined to the tip of the upper mandible, although some species such as avocets, stilts, and woodcock have an egg tooth on the tip of the upper and lower beak. In most birds the egg tooth falls off a few days after hatching, but in passerine birds (such as finches and sparrows) it is absorbed back into the bill. In petrels, the egg tooth remains visible for up to three weeks after hatching.

As it breaks through the shell the chick takes its first breath of atmospheric air: its first breath of air outside the shell. Energised by this pulse of extra oxygen, the chick continues to peck away at the inside of the shell and simultaneously starts to press its shoulders and legs against the inside of the shell. It also begins to rotate its body inside the shell in an anti-clockwise direction (if you are looking down on the blunt end of the egg). The egg tooth then makes a hole in the eggshell, a process known as "pipping." I suspect it was originally called "peeping" after the noise the chick makes at this point, since there’s a note in Fabricius’s account of the development of the chick from entitled ‘Peeping is a sign that the chick wishes to leave the egg’. As pipping continues, it eventually results in the top of the eggshell, above the widest point of the egg, falling free and allowing the chick to emerge. This is the commonest way that chicks get out of eggs. In a few species the chick splits the side of the egg and emerges through an untidy hole—a method of hatching that seems to be most common in birds with longish beaks, like waders.

Megapodes are different. Incubated in warm soil or fermenting vegetation, they can afford to have a relatively thin shell because their eggs don’t have to bear the weight of an incubating parent. Also, because each egg lies in glorious isolation in its incubator, there’s no risk of their being damaged by colliding with others or being kicked or pecked by the parent bird. The megapode’s thin shell facilitates gas exchange, but it also means that breaking out is relatively easy. Megapode chicks don’t have an egg tooth, although one does appear—like an evolutionary ghost—early in development only to disappear by the time of hatching. Instead, megapode chicks hatch feet first, kicking their way out of the shell. To avoid injuring themselves as they hatch, the chicks’ sharp claws are covered by jelly-like caps that fall off soon after they emerge above ground. A further difference is that megapodes start to breathe air as soon as they break through the eggshell because the business of digging themselves out of the soil or vegetation, which takes around two days, is energetically demanding and requires a good supply of oxygen. It was once thought, presumably because they were also buried, that dinosaur eggs hatched in a similar way to megapode chicks, but the discovery of an egg tooth on one of the extremely rare fossils of near-hatching dinosaur embryos suggests that this is not true.

In a wide range of birds from owls to budgerigars, the parents sometimes help their chicks out of the egg by breaking off bits of shell at the point where it is penetrated by the chick’s beak. In other species, the parents help by tipping the chick out of the shell once the cap is removed.

Among those birds that cut the top off the eggshell, some, like the ostrich, cut through no more than a quarter of the egg’s circumference before shattering the shell and breaking out. At the other extreme, Barn Owls, pigeons, and quail cut right round the top of the shell, neatly removing the entire cap before emerging. The bobwhite quail, which also removes a complete cap, even goes round more than once.

Researchers have speculated about why there should be such variation in the way different bird species emerge from the shell. One idea is that hatching might be influenced by the degree of development, with precocial species, like chickens, being stronger and more able to break out of the shell than altricial species, like blackbirds and robins. But this idea seemed unlikely as species with precocial chicks include those that cut both the smallest (ostrich) and the largest (quail) pipping perforation tracks before emerging. Much more plausible is that eggs whose shell membrane and eggshell are tough and flexible require more cutting before the chick can escape than eggs that are hard and brittle. The eggs of ducks and chickens are hard and require only a few pips to destabilise the shell’s integrity, and chicks can emerge after a relatively few pips. Quail, pigeons and the guillemot, on the other hand, have less brittle, relatively tough eggs and membranes and require more perforations to release the chick.

The final, climax phase of hatching, in which the chick emerges from the shell, varies from a few minutes in small songbirds to a day or more. In the chicken, the chick punctures the air cell about thirty hours before hatching, makes its first pip of the eggshell at twelve hours before hatching, and starts to rotate within the shell just fifty minutes before it emerges. In the guillemot, the air cell is punctured thirty-five hours before hatching; the first pip appears at twenty-two hours, and rotation starts about five hours before the chick emerges. As well as the effort required to cut through the relatively thick shell membrane and shell, there is another reason for the more protracted process in the guillemot—the chick and its parents have got to be able to recognise each other’s voices beforethe chick hatches. Remember that guillemots live beak by jowl with their neighbours at incredible densities and with no nest. They can recognise their own egg, but they also need to be able to recognise their chick and it may take a couple of days to complete that process. Soon after the guillemot chick breaks through into the air cell, it starts to peep, and there is something magical about hearing a guillemot chick inside a still intact egg and its parents calling in response. Their individually distinct calls create a bond between the parents and the chick that ensures they can recognise each other the moment the chick breaks free from the shell. In the closely related razorbill, which breeds in solitary sites away from other razorbills, such immediate parent–offspring recognition does not occur because there’s no risk that chicks from different families will become mixed up.

I am thrilled by the idea of a guillemot chick inside its egg communicating with its parents. But in birds producing clutches of eggs that give rise to precocial chicks, something even more remarkable happens. In such species it is important that all the chicks hatch at the same time and can be taken en masse by the mother to safety. Female ducks, for example, minimize delays between the hatching of successive eggs by starting to incubate only once the entire clutch is complete. Nonetheless, some embryo development occurs even with no, or minimal, incubation, suggesting that the spread of hatching times might still be considerable.

One of the many novel observations made by Oskar and Magdalena Heinroth was that Mallard ducklings from the same clutch hatched with extraordinary synchrony—over just a two-hour period. Despite this remarkable observation, no one thought much about synchronous hatching for a further forty years until another German ornithologist, Richard Faust, reported the same phenomenon in captive American rheas. Even though the interval between laying and hatching in different rhea clutches varied from 27 to 41 days, the chicks still hatched over just two or three hours. Faust realised that something must be causing this synchronisation but he did not know what.

Margaret Vince, a researcher in Cambridge during the 1960s, solved the problem when she discovered that eggs talk to each other. She noticed that if she held a Japanese quail egg close to her ear just before it hatched, she could hear a peculiar clicking noise. This sound is uttered by the chick between 10 and 30 hours after it has first pipped the shell and Vince realised that this might be how eggs in the same nest signal to each other and synchronise their activities. To test her theory she reared bobwhite quail under different circumstances and found that the eggs must be touching for synchronous hatching to occur, suggesting that the communication is partly auditory and partly tactile. Indeed, when she exposed quail eggs to artificial vibrations or clicks, she could induce synchronous hatching. The chick’s clicks could either slow down or speed up the hatching process in adjacent eggs: most remarkable of all, when Vince added an egg to a clutch 24 hours later than the others, it was able to speed up its hatching to such an extent that the chick emerged from the egg at the same time as the others.

The chicks of different bird species hatch in various states of development. At one extreme are helpless ‘altricial’ chicks of song-birds; at the other are the completely independent "super-precocial" chicks of the megapodes which hatch fully feathered, their eyes open and capable of flight. In between, there is the familiar baby chicken—eyes open, covered in down and, although capable of feeding itself, still dependent upon its mother for protection and care. The guillemot chick is slightly less precocial than this, in that while its eyes are open and it is covered in down, it cannot run around and it cannot control its own body temperature. And probably just as well: cliff ledges are no place to be running around, at least not until the chick has some decent coordination and a good sense of what an edge is—which it acquires as it grows. Because the guillemot chick is unable to maintain its body temperature, it requires warming against its parent’s brood patch, which also helps to keep it safe.

What’s left as the chick hatches? The answer is, not much: just the shell, which is slightly thinner than it was when the egg was laid because the chick has taken some of the calcium to form its skeleton. But the empty shell is a liability: Its sharp edges could injure the delicate young chick; the chicks could be trapped inside a shell; but worse, the pale-coloured inside of the shell makes an egg that was once cryptic highly conspicuous to predators. The parents deal with these challenges in one of two ways: Either they eat the shell or they remove the eggshell from the nest. Most commonly the parents carry the two pieces of eggshell away. Birds like herons, nesting high up in trees, simply flick the shell pieces out of the nest; grebes, which nest on water, push the shell pieces out of sight beneath the surface; and ground-nesting birds like gulls pick the pieces up in their beak and fly off before dropping them a few tens of metres away.

In an elegant set of field experiments on nesting Black-headed Gulls conducted in the 1950s and 1960s Niko Tinbergen demonstrated both the cues that stimulate eggshell removal and the survival value of eggshell removal behaviour. The cue that triggers removal is the light weight of the empty eggshell; and the survival value is that it removes the white inner shell that predators like crows cue in on to find tasty young chicks. Ducks simply leave the eggshells in the nest but remove their synchronously hatched chicks to places where they are safer from predators. Guillemots and other cliff-nesting birds, like the kittiwake, simply leave the eggshell wherever it is, because their chicks are relatively safe from predators.

The Most Perfect Thing: Inside (and Outside) a Bird's Egg, by Tim Birkhead, Bloomsbury, 288 pages, $20.28. Buy it at Amazon.