Wonder of Eggs

Image Source: Ruben Alexander

With Easter now upon us, I’m sure many of us are about to partake in the bizarre custom of eating large chocolate eggs delivered by a beribboned bunny. But as you’re smashing up your chocolaty treat, take a moment to celebrate the marvel of its form. The humble egg, an item found in millions of kitchens worldwide, is a symbol not only of Easter, but of spring, fertility, and new life – yet it’s something we seldom stop to truly appreciate. As it turns out, even if you’re not a chocoholic, there are plenty of reasons to be excited by eggs. 

How to Make an Egg

In springtime, nature bursts into verdant glory. Animals of all kinds are preparing to bring new life into the world. But perhaps no preparation is quite as intense as that of birds. Precisely what this involves varies from species to species. Coastal birds flock to cliffs in their thousands, squabbling among themselves for the best nesting sites on impossibly narrow rocky ledges. Industrious little songbirds carry what looks to be their own body weights’ worth of moss, feathers and twigs to their chosen tree or hedge. Nests have to be constructed and intruders have to be chased away.

But inside those birds, an invisible preparation is also happening: their bodies are getting ready to produce one of nature’s most delicate structures. A bird’s ovum (i.e. the female sex cell) may spend several days, or even weeks, in the ovary accumulating nutritive material in the form of yolk (from the Old English geolca, ‘the yellow part’), which will be essential for the embryo’s growth. The proportion of an egg that is yolk reflects how much the embryo will develop before hatching. In songbirds, which hatch blind and helpless and do much of their development outside the egg, it is just 20%; in kiwis, which develop extensively before hatching and emerge almost fully formed and independent, it is as much as 70%.

A kiwi
Whereas mammals have a paired system of twin ovaries and oviduct, almost all bird species have just a single ovary and oviduct, which is a weight saving adaptation for flight. Kiwis are a rare exception; they have two functional ovaries, but they don’t need to save weight because they are flightless.
Image Source: Smithsonian’s National Zoo

The sperm from a recently-mated male waits at the top of the oviduct for the ovum to be released. When it is, they immediately swarm all over its surface, seeking out the female’s DNA. Once fertilised, the ovum then begins its passage through the oviduct to emerge some 24 hours later as a fully-formed egg. During its journey, the embryo and yolk are surrounded by a jelly-like layer of albumen (from the Latin albus, ‘white’). Although albumen is normally translucent, it turns white when it is cooked. Water is pumped into the albumen, and a layer – the porous shell membrane, which allows gases and water vapour to pass through – is added to the outside. Next, it passes through a section of oviduct that releases a concentrated solution of calcium into the egg membrane, which forms the main body of the shell. It’s a process that requires a lot of calcium – about 6 grams for a hen’s egg, which accounts for roughly 10% of the egg’s total weight.

In raptors and fish-eating birds, this calcium comes from bones in their normal diet. Other species, however, must find extra calcium, and females about to lay may develop a craving for it. Some seek out snail shells; others find grit; and a few eat fragments of bone from the faeces of carnivores or the pellets of raptors. If a bird cannot find sufficient calcium, its eggs may have very thin shells or no shells at all.

In the final three or four hours of egg development, any colour or markings are deposited on the shell. Despite the large array of egg colours, there are just two pigments: blue-green, which is derived from bile, and reddish-brown, which is blood-based. Altering the concentration and proportion of these two pigments create the entire palette of egg colouration. If an egg twists as the pigment is added, wriggling lines and scribbles will form on its shell.

A man holding a bird and revealing its brood patch, a naked patch of skin on its underside for incubating eggs
Because feathers are very good insulators and prevent efficient incubation, birds during the nesting season usually develop a bald patch on their underside, which will be used to transfer heat to the eggs when sitting on them through direct skin contact.
Image Source: VSPYCC

A Perfect Design

Eggs, as everyone knows, are fragile things – but at the same time, they’re also incredibly strong. Picture for a second an emperor penguin egg, which is robust enough, with the help of a selfless father, to house and protect the growing chick inside, even though it is subjected to the coldest conditions on the planet. Swans, meanwhile, lay eggs in riverside nests that may need to deal with multiple floods during their development, but, so long as they aren’t completely submerged, the cygnets within usually hatch without issue.

So how can one design be able to adapt to virtually every habitat on Earth, to survive on each and every continent, to be both strong enough to withstand the brutality of nature yet fragile enough to allow baby birds to escape? The answer is in the perfection of its structure. From its size, shape and colour, all the way down to its molecular structure, an egg is superbly designed for its important purpose.

While the eggs we are most familiar with – chicken eggs (not to mention our yummy Easter chocolate ones) – have a traditional, somewhat oval shape, not all eggs look like that. In fact, the shape of eggs is extraordinarily varied across the bird world. Some are nearly entirely spherical; others are sharply pointed; and a few are elongated and rounded at both ends, looking almost like short, fat sausages. But this huge variety of shapes is not for nothing: each one is designed precisely for its particular needs.

Take, for example, the guillemot egg. Guillemots are seabirds that lay their single large, speckled, pear-shaped egg on extremely narrow ledges on cliff faces, where they are safe from almost all egg thieves apart from the occasional gull or skua. Many answers have been proposed by science to explain why guillemots lay eggs that are fat on one side but more pointed on the other. One possible reason is that such a shape is hard to roll. Although it would seem as though even the slightest breeze could knock such a precariously-placed egg off its ledge, the distinctive shape of a guillemot egg, or so it has been theorised, means that if it is knocked, it does not travel far; it merely rolls in a tight arc or swivels on the spot, making it harder to dislodge. Certainly, this would be an incredibly valuable safety feature for an egg on the very edge.

However, ornithologist and writer Professor Tim Birkhead (who has authored one of our favourite books, Bird Sense) has proposed an alternative theory. Guillemots defecate with absolutely no regard for their neighbours and, as a result, their colonies are notoriously dirty. Professor Birkhead speculates that the more pointed end of the egg keeps the blunt, bulbous end (where the developing embryo is) raised up off the guano-encrusted rocks slightly. By keeping at least the most important end relatively clean from the accumulated filth of the closely packed guillemot colony, the egg is rarely completely smothered and the carbon dioxide that builds up inside the shell can continue to escape through its pores.

And even more recently, Professor Birkhead has come up with another theory. He discovered that a guillemot egg on a steeply sloping rock stays in place much better than eggs with a less elongated shape. The long, straight edge of a guillemot egg means that it has more contact with the ledge, and on a sloping surface the extra friction helps to keep it in place.

A guillemot egg
Three times the size of a hen’s egg and highly nutritious, the unusually-shaped eggs of the guillemot were highly sought for centuries by those people with access to the birds’ cliffside colonies.
Image Source: Didier Descouens

The shape of an egg is often related to where it is laid, but it is also important to consider the time it will spend in its mother’s body. For instance, a longer, thinner egg allows a female bird to maintain a more aerodynamic body shape and does not stifle her aerial talents. Size is another important consideration, for a heavy egg would also limit the mobility of the mother. Interestingly, the size of the parent bird does not always determine the size of the egg laid. The aforementioned flightless kiwi lays the largest of all bird eggs in proportion to the size of the adult – up to 20% of the female’s weight. Though the bird itself may only be around the size of a chicken, its eggs are six times bigger than those laid by any normal hen, and around the same size as those laid by its much larger cousin, the emu. In fact, a kiwi egg is so enormous that the mother cannot even eat during the later stages of development because there simply isn’t enough room in her body for stomach expansion, so she must rely on fat stored in previous months to survive.

Other birds opt for a different strategy: they lay multiple smaller eggs, which tend to have a higher mortality rate. Partridges lay very large clutches indeed, with some containing over 20 eggs. Ostrich nests contain even more eggs, but they are not all laid by the same female. Multiple hens contribute to these enormous broods, creating massive nests overflowing with eggs – the record was a staggering 78! Although an ostrich egg is the largest produced by any living bird (between 1.3 and 1.9kg), it is extraordinarily small in relation to the size of the adult, being only 1-4% of the female’s body weight.

An ostrich egg
Ostrich eggs are the largest eggs laid by any living bird, though they are actually the smallest eggs relative to the size of the adult. The smallest egg – about the size of a coffee bean, and weighing half a gram – is laid by the smallest bird in the world, the bee hummingbird, which you can read about here.
Image Source: Museum of Veterinary Anatomy

Shape is not the only important factor in an egg’s design. They also come in a myriad of colours and patterns. Of course, this colour isn’t to impress humans. Eggs are highly nutritious and avidly eaten by predators, so many need to be camouflaged to protect them from thieves. Plovers, for example, lay eggs that perfectly blend in with the pebble beaches upon which they are laid. Conversely, cavity-nesting birds such as owls have the opposite problem. In their dark, gloomy nests, their eggs are vulnerable to being trodden on, so the females of these species often lay pure white eggs that their sharp eyes are sure to see.

Four plover eggs camouflaged on the ground
Even though they are laid directly on the ground, plover eggs are usually so well camouflaged that most threats come not from predators actively locating the nest, but from large animals (including humans) accidentally blundering upon it and crushing the eggs.
Image Source:  Brett Hondow from Pixabay

Beneath the Shell

Eggs, with their extraordinary range of shapes, sizes and colours, are, by themselves, incredible – but the real beauty is invisible to the naked eye. Each egg acts as an extraordinary, external life-support system. Like virtually all other animals – including developing infants – chicks require oxygen. Being surrounded by the sticky liquids of an egg, however, means they cannot simply breathe. So the egg itself breathes for them. Eggshells are riddled with microscopically tiny pores that allow oxygen to enter the interior of the egg. From there, a membranous bag called the allantois does the job of a placenta: its blood vessels carry oxygen entering the shell to the developing chick and releases carbon dioxide produced by the chick into the outside world.

Infants of nearly all species are vulnerable to some degree, and few more so than baby birds. While in the egg, they are at their weakest and most fragile. And if an egg allows air in, how can it possibly protect the developing chick from parasites and bacteria, particularly if the egg is subjected to the damp conditions of, say, a waterbird nest? Well, fortunately, the egg has many lines of defence behind the shell! The white of the egg, the albumen, is a truly fantastic substance. Composed of water and a little protein, it prevents the embryo from dehydrating, acts as a shock absorber, and is antimicrobial, actively fighting foreign bodies that permeate the calcium fortress of the shell. But even if this fails, the invader has a veritable desert to cross. Despite the yolk being a rich store of nutrients vital to sustaining the developing chick, the albumen is a harsh place for microbes to survive, and certainly a poor medium for bacterial growth.

Three blue eggs from an American robin
No one really knows why some bird eggs are blue. One theory is that blue is a compromise between protecting the embryo from the damaging effects of overheating and ultraviolet radiation. If eggs are too dark, they will get too hot in the sun. Too pale, and they may not adequately shield against UV.
Image Source: Vijayakumar blathur

The warmth from the incubating parent bird, together with the yolk as a food supply, drives the embryo’s development inside its protective shell, until, eventually, it is time to hatch. Just prior to this, the chick develops an egg tooth, a small, sharp but temporary projection on its beak. This allows the chick to pierce an air-filled space between the inner and outer egg membranes, which has increased in size throughout the incubation period, and take its first breath. Then the chick repeatedly forces the egg tooth through the shell, eventually creating a hole that enables the little creature to burst its way into the outside world. Once hatched, the egg tooth soon falls off or is absorbed into the growing chick’s beak. (A few birds, such as the kiwi, do not develop an egg tooth; they are strong enough at the time of hatching to simply kick their way out.)

This article is the first one that Jason and I have properly collaborated on because it’s a subject that we both find fascinating. Beautiful, fragile and mysterious, eggs have always intrigued and inspired humans. So, this Easter, why not take a moment to contemplate the biological wonder that is a bird egg? They are, after all, a truly remarkable miracle of nature.

If you’re still in an Easter mood, make sure you check out our accompanying article on the largest egg ever laid, and our British Wildlife of the Week posts on the European rabbit and the original Easter Bunny, the brown hare.


  • Alex Mockridge

    Alex is a passionate animal lover with a background in zookeeping. Parrots have always been her favourites and although she adores all creatures great and small, and has a keen interest in all areas of animal husbandry, our feathered friends will always hold a special place in Alex's heart.

  • Jason Woodcock

    With a background in conservation and animal behaviour studies, Jason's passion lies in the natural world. He adores all things nature and enjoys nothing more than spotting rare and interesting species out in the wild. He has also worked in a zoo and knows plenty about keeping the animals inside our homes healthy and happy, too.

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