What makes a boo 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 reach and color of shells to the self-sanitizing world 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 argument on which conclusion of the egg comes first, before carrying off into a rich description of the chick ‘s emergence from its brood labratory, out into the big and beautiful populace .
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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 adult goal, 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 ascent to no fewer than six rebellions. Swift ’ south endian haggle 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 salute in the Host at communion .
There has been a similar endian dispute over which room an egg emerges from the shuttlecock ’ sulfur cloaca : big end or fiddling end first ? Although there are some dissenters, most people—thanks largely to Aristotle—think that the deaden end emerges first. In turning this has led to an erroneous explanation for how the egg is propelled along the fallopian tube. 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 testis travels down the fallopian tube. He suggested that it is pushed along by peristaltic forces, much like a food bolus in the catgut, with the fallopian tube ’ s round muscles contracting behind the egg while those at the front are relaxed. It was thought that the trail, pointed end—that is the longer part of the egg—gave the fallopian tube wall greater leverage to squeeze the testis on its room .
One of the proponents of this estimate, the nineteenth-century anatomist Heinrich Meckel von Hemsbach, was so confident about this explanation he called it a “ numerical necessity. ” To be honest, the mind does have an intuitive attract, 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 cerebral 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 biota can persist for so long in the face of at odds evidence. How could D ’ Arcy Thompson, J. Arthur Thomson and others, ignore the evidence that flew in the face of their testis motion theme ? adenine 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 normally emerges numb end first, it passes down the fallopian tube pointed end first. Others confirmed that this was besides genuine for pigeons, hawks and canaries, thus why did Thompson and Thomson persist in their contrary opinion ? Did they not believe their celebrated predecessors ? possibly they didn ’ thyroxine learn German ( for which they can be forgiven ). The one newspaper 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 lay. ingeniously, he was able to use a pencil to mark that snatch of the egg he could see inside the hen ’ south fallopian tube, through its cloaca anterior to laying. ( I can barely imagine his wife popping into his study with a cup of coffee and seeing Wickmann with his pencil up a hen ’ sulfur buttocks : “ What are you doing, dear ? ” she asks. .. ). This allowed him to establish that, in the hour or indeed before it is laid, the egg is orientated with its pointed egg directed towards the shuttlecock ’ randomness raise even though all eggs were all laid deaden end first. Wickmann deduced that the egg must turn immediately before it is laid .
When I first hear of eggs turning in this way I imagined them doing therefore vertically, along their farseeing axis—that is by “ pitching ” — but they actually do therefore 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 late separate of their travel through the fallopian tube. The brooding hens sit immediately in front of the x-ray sieve, and a succession of images was taken, starting at around noon fair as the egg—covered only by the shell membrane—entered the plate gland. Images were taken, Bradfield says, at intervals until 9 post meridiem and then restarted at 8 ante meridiem the adjacent day. Had Bradfield been my PhD scholar I ’ d have suggested that he stay up all night at least once, although as it turned out it credibly didn ’ thymine matter. He wrote : “ That part of blast secretion which goes on during the night is inescapably missed, but by following an egg which is ovulated early in the day it is potential to trace the first one-half of the march ( which proved to be the most concern ), together with the concluding few hours. ”
When Bradfield examined his x-ray images what he saw was noteworthy. An hour or then before lay, the shell gland with its fully formed shelled egg dropped a few centimetres out of the pelvic girdle, and over a menstruation of fair one or two minutes, during which the hen stood astir, the egg rotated 180° horizontally. The drop of the shell gland is possible because, unlike the mammalian pelvis which forms a r-2 of bone and through which the point of the fetus must pass at birth, a boo ’ mho pelvis is, as Bradfied says, shaped like an overturned boat, allowing the drop and rotation to occur, ampere well as facilitating the lay of boastfully, hard-shelled eggs .
In each boo that Bradfield observed, the pattern was the lapp : the testis entered the shell gland pointed end first, turned and was laid dull end first. Why turning should be necessity is ill-defined, specially for eggs like those of the domestic domestic fowl and most passerines that don ’ deoxythymidine monophosphate actually differ all that much between ends. The fact that it isn ’ thyroxine wholly reproducible within species implies that the way the egg emerges can not be that crucial. It is a pity that those researchers who observed the few chicken eggs laid sharpen end first did not record whether those eggs were a different determine from those laid blunt end first. possibly for most eggs it is better—for some strange reason—to undertake most of their travel down the fallopian tube 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 fertilization and incubation and the third great landmark in the life of an egg. How does the dame break out from the claustrophobic confines of the husk ? Our mental effigy of the process has been corrupted by cartoons, where attempts to romanticise and sanitise the serve often show a hen ’ s egg with its top neatly popping off to reveal a strong, yellow fluffy dame. The reality is not like that. It is distillery pretty remarkable, but it isn ’ deoxythymidine monophosphate as agile, as clean, or equally simple as we have sometimes been led to believe .
A amply train embryo lies scrunched up inside the egg with its ankles at the bespeak end and its question towards the blunt end ; its neck is flex so that the fountainhead lies adjacent to the front with the beak poking out from under the right wing up against the testis membrane. This pre-hatching military capability seems to be typical of all birds, except for megapodes .
Before starting to break out of the egg the dame has three things it must accomplish. It must first switch from being dependent on the oxygen diffusing through the pores in the shell into the network of rake vessels that line the inside surface of the shell and begin to use its own lungs to breathe. The chick takes its first proper breath and fills its lungs the here and now it punctures the air cell inside the top of the egg. This pace is all-important because by this stage of development there is not adequate oxygen diffusing through the pores in the plate to support the dame ’ second respiratory requirements. Taking a breath from the air cell provides the oxygen and the energy necessary to break through the shell .
Before it takes that first breath, the chick has to start shutting off the rake provide to the network of lineage vessels that line the inside open of the shell, and withdraw that blood into its consistency. The blood vessels are programmed to close off at the point where they emerge from the bird ’ randomness navel, and good before the dame starts cutting round the plate .
Third, the chick has to take what is left of the egg yolk and draw it into its abdomen. It does this by sucking up the remaining egg yolk through the haunt that connects the egg yolk to the chick ’ s humble intestines. This “ yolk theca ” is a food modesty for the first few hours or days after hatching.
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basically, the dame has to do what a human baby does as it switches from addiction on the placenta for both oxygen and food to independent breathe with its lungs and the consumption of food through its mouth. think of it like that, it is a pretty major conversion .
The chick is now ready to break out of the husk and starts by thrusting its beak against the inside wall of the shell. To help puncture it the dame employs a bantam structure of particularly hardened material at the peak of the charge. Known as the egg tooth, its function 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 break up of shell, he could see the abrupt fiddling 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 ) besides have an egg tooth, as do the egg-laying mammals, the duckbill platypus and echidna : it is the cardinal for getting out of a plate. In birds, the egg tooth is made of calcium and is normally confined to the tip of the upper lower jaw, although some species such as avocets, stilts, and woodcock have an egg tooth on the gratuity 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 beak. 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 tune : its first breath of air outside the shell. Energised by this pulsate of excess oxygen, the dame continues to peck away at the inwardly of the shell and simultaneously starts to press its shoulders and legs against the inside of the husk. It besides begins to rotate its consistency inside the shell in an anti-clockwise commission ( if you are looking down on the blunt end of the egg ). The egg tooth then makes a trap in the shell, a process known as “ shoot. ” I suspect it was in the first place called “ peep ” 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 finally results in the top of the shell, above the widest point of the testis, falling barren and allowing the dame 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 territory or fermenting vegetation, they can afford to have a relatively thin shell because their eggs don ’ t have to bear the weight of an brood parent. besides, because each egg lies in brilliant 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 ’ sulfur slender shell facilitates gas exchange, but it besides means that breaking out is relatively easy. Megapode chicks don ’ thyroxine have an testis tooth, although one does appear—like an evolutionary ghost—early in development entirely to disappear by the time of hatching. rather, megapode chicks hatch feet first, kicking their way out of the shell. To avoid injuring themselves as they hatch, the chicks ’ crisp claws are covered by jelly-like caps that fall off soon after they emerge above ground. A promote difference is that megapode starting signal to breathe air angstrom soon as they break through the shell because the business of digging themselves out of the dirty or vegetation, which takes around two days, is energetically demanding and requires a good provide of oxygen. It was once thought, presumably because they were besides buried, that dinosaur eggs hatched in a like way to megapode chicks, but the discovery of an egg tooth on one of the extremely rare fossils of near-hatching dinosaur embryo suggests that this is not true .
In a wide range of birds from owl to budgerigars, the parents sometimes help their chicks out of the egg by breaking off bits of shell at the period where it is penetrated by the chick ’ sulfur beak. In other species, the parents aid by tipping the chick out of the shell once the ceiling is removed .
Among those birds that cut the peak off the shell, 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 flinch cut properly round the circus tent of the shell, neatly removing the entire crown before emerging. The bobwhite quail, which besides removes a dispatch cap, even goes circle more than once .
Researchers have speculated about why there should be such variation in the way different boo species emerge from the shell. One mind 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 mind seemed improbable as species with precocial chicks include those that cut both the smallest ( ostrich ) and the largest ( flinch ) pipping perforation tracks before emerging. much more plausible is that eggs whose shell membrane and shell are baffling and flexible command more cut before the dame can escape than eggs that are intemperate and brittle. The eggs of ducks and chickens are hard and require only a few pips to destabilise the husk ’ mho integrity, and chicks can emerge after a relatively few pips. Quail, pigeons and the guillemot, on the other hand, have less brittle, relatively street fighter eggs and membranes and require more perforations to release the dame .
The final, climax phase of think up, in which the chick emerges from the carapace, varies from a few minutes in belittled songbirds to a day or more. In the chicken, the dame punctures the publicize cell about thirty hours before hatch, makes its first pip of the shell at twelve hours before think up, and starts to rotate within the shell merely fifty minutes before it emerges. In the guillemot, the breeze 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. vitamin a well as the effort required to cut through the relatively midst plate membrane and beat, there is another reason for the more drawn-out process in the guillemot—the dame and its parents have got to be able to recognise each other ’ mho voices beforethe dame hatches. Remember that guillemots live beak by lower jaw with their neighbours at incredible densities and with no nest. They can recognise their own testis, but they besides need to be able to recognise their dame and it may take a couple of days to complete that process. soon after the guillemot dame breaks through into the air cell, it starts to peep, and there is something charming about hearing a guillemot dame inside a inactive integral testis and its parents calling in reaction. Their individually distinct calls create a attachment between the parents and the dame that ensures they can recognise each other the moment the dame breaks free from the carapace. In the close relate razorbill, which breeds in lone sites aside from early razorbills, such immediate parent–offspring realization does not occur because there ’ s no gamble that chicks from different families will become assorted up .
I am thrilled by the theme 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 hatch of consecutive eggs by starting to incubate only once the entire seize is complete. Nonetheless, some embryo development occurs even with no, or minimal, brooding, 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 seize hatched with extraordinary synchrony—over just a two-hour menstruation. 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 rhea. tied though the interval between laying and hatching in different rhea clutches varied from 27 to 41 days, the chicks placid hatched over just two or three hours. Faust realised that something must be causing this synchronism but he did not know what .
Margaret Vince, a research worker in Cambridge during the 1960s, solved the problem when she discovered that egg spill to each other. She noticed that if she held a japanese flinch egg close to her ear precisely before it hatched, she could hear a peculiar click noise. This legal is uttered by the dame between 10 and 30 hours after it has first pipped the plate and Vince realised that this might be how egg in the lapp nest bespeak to each other and synchronise their activities. To test her hypothesis 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 partially auditory and partially tactile. indeed, when she exposed quail eggs to artificial vibrations or clicks, she could induce synchronous hatch. The dame ’ s clicks could either slow down or speed up the hatch process in adjacent eggs : most remarkable of all, when Vince added an egg to a clutch 24 hours late than the others, it was able to speed up its hatching to such an extent that the chick emerged from the testis at the same time as the others .
The chicks of different bird species hatch in respective states of development. At one extreme point are helpless ‘ altricial ’ chicks of song-birds ; at the other are the completely independent “ super-precocial ” chicks of the megapodes which hatch amply feathered, their eyes overt and capable of flight. In between, there is the familiar baby chicken—eyes open, covered in down and, although capable of feeding itself, inactive dependent upon its mother for protection and care. The guillemot chick is slightly less precocial than this, in that while its eyes are outdoors and it is covered in devour, it can not run around and it can not control its own consistency temperature. And probably merely a well : cliff ledges are no home to be running about, at least not until the dame has some decent coordination and a commodity sense of what an edge is—which it acquires as it grows. Because the guillemot dame is unable to maintain its torso temperature, it requires warming against its parent ’ s sulk piece, which besides helps to keep it safe .
What ’ sulfur left as the dame hatches ? The answer is, not much : equitable the shell, which is slenderly thinner than it was when the testis was laid because the chick has taken some of the calcium to form its skeletal system. But the empty shell is a liability : Its crisp 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 shell from the nest. Most normally the parents carry the two pieces of shell away. Birds like herons, nesting high up in trees, simply flick the plate pieces out of the nest ; grebes, which nest on body of water, push the shell pieces out of spy 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 specify of field experiments on nesting Black-headed Gulls conducted in the 1950s and 1960s Niko Tinbergen demonstrated both the cues that stimulate shell removal and the survival value of shell removal behavior. The cue that triggers removal is the light system of weights of the empty shell ; 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, merely leave the shell wherever it is, because their chicks are relatively safe from predators.
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photograph : courtesy of Bloomsbury
The Most perfect thing : Inside ( and Outside ) a Bird ‘s egg, by Tim Birkhead, Bloomsbury, 288 pages, $ 20.28. Buy it at Amazon .
I am broadly interested in how human activities influence the ability of wildlife to persist in the modified environments that we create.
Specifically, my research investigates how the configuration and composition of landscapes influence the movement and population dynamics of forest birds. Both natural and human-derived fragmenting of habitat can influence where birds settle, how they access the resources they need to survive and reproduce, and these factors in turn affect population demographics. Most recently, I have been studying the ability of individuals to move through and utilize forested areas which have been modified through timber harvest as they seek out resources for the breeding and postfledging phases. As well I am working in collaboration with Parks Canada scientists to examine in the influence of high density moose populations on forest bird communities in Gros Morne National Park. Many of my projects are conducted in collaboration or consultation with representatives of industry and government agencies, seeking to improve the management and sustainability of natural resource extraction.