TRANSCRIPT: The Incredible Egg

This is a transcript of the Gastropod episode The Incredible Egg, first released on October 23, 2018. It is provided as a courtesy and may contain errors.

MARY CASWELL STODDARD: Well, I guess, to many, an egg is a favorite breakfast food. But when I think about vertebrate animals, what stands out to me is that an egg really represents an amazing evolutionary invention.

CYNTHIA GRABER: Such an amazing evolutionary invention— and such a delicious breakfast food—that many of you have asked us to do an episode about them. And so that’s just what we’re doing!

NICOLA TWILLEY: We, of course, are Gastropod, the podcast that looks at food through the lens of science and history. I’m Nicola Twilley.

GRABER: And I’m Cynthia Graber. And recently Gastropod listeners Jessie Svet, Jeff Stoyanoff, and Benjamin Holloway all wrote in with egg questions. When did we first start eating eggs, and how did archaeologists figure it out? Which animals do humans eat eggs from, today and throughout history?

TWILLEY: What scientific mysteries does the egg still hold—or do we know everything there is to know about this incredible edible evolutionary invention?

GRABER: Is there still such a thing as wild chickens? And, if so, how are wild chicken eggs different from farm eggs?

TWILLEY: In fact, you all had so many questions that we can’t answer them all this episode—like the story of how we came to grow all our vaccines in eggs? We had to save that one for our special supporters email.

GRABER: Which you can get straight to your inbox if you support Gastropod at at least $5 per episode on Patreon or $9 per month on our website,

TWILLEY: But in the meantime, we have more eggs than a Vegas breakfast buffet—plus answers to a bunch of your questions.


TIM BIRKHEAD: Well, one of the common questions that I’m asked is: Which came first, the chicken or the egg? And everybody that asks that thinks they’re asking it for the first time.

TWILLEY: This is Tim Birkhead. He’s a professor of zoology at the University of Sheffield, in the UK. And he wrote a book called The Most Perfect Thing: Inside (and Outside) a Bird’s Egg. So I think we know what his answer is going to be.

BIRKHEAD: Of course it’s eggs that came first because pretty well all organisms produce eggs. Birds evolved from reptiles and birds are in fact dinosaurs, they are a continuation of the dinosaur line.

GRABER: And of course dinosaurs laid eggs. Most reptiles still lay eggs.

TWILLEY: There are crocodile eggs. There are turtle eggs. There are snake eggs. They look a lot like bird eggs, and people eat them, although online reviews are mixed.

RAURI BOWIE: But one of the interesting things about birds is that the egg is very hard.

GRABER: Rauri Bowie is the curator of birds at the UC Berkeley Museum of Vertebrate Zoology.

BOWIE: So if you ever get the chance to feel a reptile egg like a crocodile egg or a lizard egg you’ll see that it’s much softer. It’s sort of leathery rather than having a much harder surface.

TWILLEY: A bird egg shell has an extra layer of calcium that makes it rigid. And that extra strength makes all the difference.

STODDARD: It was a very specialized egg with a shell that allowed vertebrates to colonize habitats away from water. And it’s this type of egg that that birds lay today.

TWILLEY: This is Mary Caswell Stoddard—she goes by Cassie. You heard her voice at the start of the show—she’s an evolutionary biologist at Princeton and her research is focused on eggs.

GRABER: Cassie explained that reptiles have always had to lay their eggs somewhere moist. That way the moisture in the environment around the egg can seep through the leathery flexible shell to provide water for the growing dino. But those first bird eggs that were hard, they formed a protective layer around the water already inside the egg. So this new type of egg could be laid anywhere, not just somewhere wet.

STODDARD: And when I think about that and try to reconcile out with the scrambled eggs on the plate—it does blow my mind still.

TWILLEY: So the egg really did come first—there would be no chicken, or any other birds for that matter, without the invention of the hard-shell egg. But I have a more basic question. Which is, what is an egg?

STODDARD: Eggs have a really specific role, and that’s simply to nourish and protect the growing chick up until it hatches.

GRABER: Cassie’s answer is a simple one, but it turns out that how the egg does that, how it nourishes and protects the growing chick, it’s actually really cool.

TWILLEY: OK, so I started us down this rabbit hole by asking what an egg is, but I just want to take a moment to warn you all that the natural history of eggs turns out to be quite amazing, and we are going to spend a minute explaining why.

GRABER: The shell itself is pretty amazing. It has thousands and thousands of pores, and those pores let oxygen in, which the developing chick needs. And they let carbon dioxide out. So basically the chick breathes through that shell.

TWILLEY: Different species of birds have completely different amounts of pores, and pores of different sizes, and no one knows quite why.

GRABER: Let’s continue traveling inward. Next is the albumen, and it’s watery, and that’s one of its main jobs—the egg white provides water to the chick as it grows.

BIRKHEAD: But it’s also a barrier so that if any microbes come down those pores, the microbes have got to cross this kind of albumin barrier. And the really striking thing about that albumen is that it contains almost nothing that a microbe could utilize. So I liken it to somebody trying to walk across the Atacama desert. It would be a very tough call for a microbe.

TWILLEY: It’s not just that the egg white doesn’t contain food for the microbe. It also contains active anti-microbials. The first one is called lysozyme. It was discovered by Alexander Fleming, better known for coming across penicillin. And you can also find lysozyme in tears!

BIRKHEAD: So initially it was thought that there might be just a handful of anti-microbials in the albumen. but we now know that there might be hundreds. There might even be thousands. And again that’s the product of natural selection working over thousands and thousands of generations. Because if the embryo becomes infected and dies then the genes for just don’t make it into subsequent generations.

GRABER: There are four different kinds of albumen. Some are more watery, some are more viscous. The closest denser kind of albumen is the type that forms those stringy bits you might see when you crack an egg.

BIRKHEAD: And their job is to suspend the yolk or the ovum in the central part of the egg. So each of those are attached, both to yolk at one end and to the shell membrane at the other end.

TWILLEY: I do not like these stringy bits when I find them in my scrambled eggs because they have a weird texture. But they are quite nifty because they make sure that whenever the parent birds turn the egg, the yolk also flips so that the baby bird is facing upwards. If it faces down, it dies.

GRABER: And finally, of course, perhaps the most business part of the egg of all: the yolk. It has all the nutrients that baby chick needs to develop into a full-grown bird.

TWILLEY: The eggs at the supermarket, they don’t have a baby chick in them, or even the beginnings of a baby chick—they’re unfertilized. But what you do see sometimes is a double yolk, which always feels a bit like winning the breakfast lottery.

GRABER: The double yolk is basically a biological mistake. The chicken releases two ova, those are the yolks, instead of one, and those two yolks get trapped inside one egg.

BIRKHEAD: In the wild, those eggs hardly ever hatch, because although they might have two ova, the amount of albumen doesn’t increase proportionately. And so basically those twins or triplets simply run out of water during the course of development and the embryos die.

TWILLEY: It’s also possible to get a no-yolk egg, although I never have.

GRABER: That sounds really sad.

TWILLEY: I have to imagine I would be devastated. I think egg-white only omelettes are the devil’s work. But fortunately, Tim says these yolk-free eggs are really, really rare. It’s another biological mistake—the albumen and shell form around a stray bit of tissue rather than a yolk.

GRABER: We’ve been exploring the anatomy of an egg. And there are also some pretty amazing things going on as the egg forms inside the bird, too. As we’ve started to describe, there’s an ovum, a yolk, and it travels down a tube and it gets surrounded by the egg white, the albumen, and then that gets surrounded by the membrane. And then that gets surrounded by a shell.

TWILLEY: The whole process of building an egg inside a bird takes about 24 hours from start to finish. The shell alone takes 12 to 14 hours to lay down using calcium nozzles, a.k.a. shell glands.

GRABER: And then come the spray paint. The spray guns inside the bird come in two colors, one’s greenish and one’s reddish.

BIRKHEAD: And by mixing those in different proportions and different concentrations the whole suite of all egg colors that we know can be created.

TWILLEY: Okay, I already was thinking eggs were pretty magical at this point—the antimicrobial in our tears, the little strings holding the yolk, all those clever pores—but this is the moment where they become drop-dead gorgeous, too.

BOWIE: One of the beautiful things about eggs is that no egg is exactly identical to another egg. And the reason for that is, the last phase of when a bird lays an egg is it goes in the shell gland, and that’s where the pigments are squirted on, and that egg’s rotated and it’s never quite rotated in the same way. So every egg is unique.

GRABER: You wouldn’t know this if you only looked at white supermarket chicken eggs, but eggs come in all sorts of different colors, and those colors are sprayed with all sorts of glorious ink-blot squiggly patterns. And then there’s one final step before the egg sees the light of day.

BOWIE: And then the final thing is to cover the egg with a kind of layer that’s equivalent to putting a layer of wax onto a car as it leaves the showroom.

TWILLEY: That final layer, that’s another defense mechanism to stop all those pesky microbes from getting in and feasting on the delicious, delicious yolk. Tim says it’s kind of like breathable Gore-Tex. And in America, in our infinite wisdom, we wash that layer off.

GRABER: Yay, we’re cleaning them! But this isn’t as great as it might sound.

BIRKHEAD: The unfortunate consequence of that is that when the eggs are washed, they are then dried, and the drying process actually literally draws in the water from the eggshell surface with the muck down the pores.

TWILLEY: And that can result in eggs getting infected with bacteria like salmonella.

GRABER: And this is why we in America store our eggs in the fridge. They’re not protected by that extra layer of Gore-Tex-like wax. And so they’re more vulnerable to infection.

TWILLEY: And in my home country, the UK, when you go into the supermarket looking for eggs, you’ll find them out on the shelf. Because they’ve still got their Gore-Tex on.

GRABER: Those supermarket eggs are pretty boring to look at, frankly. They come in white and brown, maybe a little blue if you’re lucky. But we wanted to see all those incredible patterns and even shapes and sizes that eggs can come in—in person.

BOWIE: We have about 14,000 eggs.

TWILLEY: Rauri, as we mentioned, is keeper of all the eggs! His official title is Curator of Birds at the Museum of Vertebrate Zoology at UC Berkeley.

BOWIE: And so what I’m going to take you through now is showing you some of the variety of eggs that we have in our collection and sort of some of the things that I think of as being very special, unusual.

TWILLEY: We started by looking at a drawer full of seabird eggs—auks, guillemots, puffins,

BOWIE: These are the same species but some of them have a more blue background. Some of them are very brown. Some of them are very white. Some of them have these really joined kinds of ink dot where the ink looks like it’s sort of run around. Some of them are much more finely speckled.

GRABER: Cassie’s looked at everything in the Berkeley collection and she has her own favorites.

STODDARD: In terms of pattern, I’d say that the jacana lays one of the coolest eggs out there. Jacana eggs are covered with these elaborate squiggles. And then I can’t not mention the tinamou. The tinamou are pretty drab colored birds themselves from Central and South America but they lay the most extraordinary glossy eggs that come in blue and green and even pink sometimes.

GRABER: Eggs come in an infinite variety of patterns and colors. And on top of that, they come in all sorts of sizes.

BOWIE: The largest bird that ever existed, at least that we’re aware of, is the elephant bird from Madagascar and it went extinct somewhere between 700 and 3,000 years ago. And we are very fortunate to have two elephant bird eggs.

TWILLEY: Remember, an ovum, the yolk, that’s a single cell. So the elephant bird yolk is also the largest single cell ever known.

BOWIE: You could fit over 100 chicken eggs in it. It’s probably about two to three times the size of the average human head. And then you know these are some of our smallest eggs. So these are—some of these are Anna’s hummingbird eggs.

TWILLEY: It’s like a cannellini bean.

GRABER: So eggs come in beautiful colors and patterns, and they come in all kinds of sizes. And eggs come in all kinds of shapes, too.

STODDARD: So owls, for example, tend to lay eggs that are almost spherical. They look a lot like golf balls. While other birds like hummingbirds tend to lay eggs that are more elliptical. and the hummingbird egg looks like a little Tic Tac. And some other birds like sandpipers tend to lay eggs that are highly asymmetric and pointy. So we see that variation and shape across the bird world.

TWILLEY: For generations, humans have marveled at all this variety. But Cassie wanted to understand it as well as appreciate it.

GRABER: She started with this question of shape.

STODDARD: Well, there are a lot of classic hypotheses about the evolution of egg shape. One of the popular ones was the idea that cliff-nesting birds might lay pointy eggs so that if the egg is bumped it will spin in a circle and not topple over the cliff edge. So that was one idea. Another idea is that egg shape might be related to clutch size or the number of eggs that a bird lays at a time. So some shapes might be optimal for incubation efficiency. Because eggs of certain shape might be optimally packed into a nest when there is a particular clutch size. And there were still other ideas that have hypothesized links between egg shape and a bird’s diet, or egg shape and egg strength.

GRABER: These are all hypotheses that needed to be tested. And Cassie decided to do that. She actually used the very collection that Rauri is in charge of in Berkeley.

TWILLEY: The Berkeley collection had recently photographed all their eggs and put the images online.

GRABER: Cassie created a computer program called an ‘Eggstractor.’

TWILLEY: Pun fully intended.

GRABER: And it extracts the shape of all those eggs. Cassie’s Eggstractor created a way to evaluate the eggs for two different shapes: how symmetrical or asymmetrical the egg is, and how elongated or round it is.

STODDARD: And so in this way we could actually create a map almost like an astronomer mapping the stars in egg shape space.

TWILLEY: And what that map enabled her to do was to try out all the various theories and see which fit the data best. So for example, Cassie looked to see whether birds that lay similar size clutches all cluster together in the pointy egg part of the map? But they didn’t.

GRABER: She checked whether birds with similar diets could all be found in the same region of the egg shape map. Nope. Or whether birds that lay their eggs on precarious cliffs and ledges had all evolved toward the same sweet spot on that egg shape map. Also no.

STODDARD: And we were surprised to find that egg shape is correlated with flight ability. We found that strong flying birds that migrate long distances like terns or spend their lives largely in the air like swifts, these birds on the whole tend to lay eggs that are also more asymmetric or more elliptical.

GRABER: Cassie has a theory for why the egg shape is related to the bird’s flight ability.

TWILLEY: As birds become more powerful fliers, over evolutionary time, they also become more streamlined.

STODDARD: So we think that as birds’ bodies over time became sleeker, this presented a challenge for birds because birds couldn’t lay eggs that were too wide across. And so one way to pack a large volume into an egg without increasing its egg width would be by increasing that egg’s ellipticity or asymmetry.

GRABER: So in Cassie’s new hypothesis, narrow, sleek strong fliers needed to lay skinnier, longer, or asymmetric eggs through their narrower, sleeker pelvis.

TWILLEY: Whereas owls, which are kind of chunky—they have round eggs like ping pong balls.

GRABER: This strong flyer-shape match seems to map onto the data in the broadest sense. But Cassie says it probably won’t explain all the variety in egg shapes.

STODDARD: When you zoom in on particular bird families or bird species, we expect that there will be different factors responsible for egg shape variation.

TWILLEY: Like, for example, Tim Birkhead’s favorite bird, the guillemot. Guillemot nests are covered in greeny-white poo—these birds are notoriously drippy shitters. Based on Tim’s research, the most likely explanation for the guillemot’s pear-shaped egg is that it helps keeps the blunt end of the egg out of the poo. This is still a theory, but it makes sense: the blunt end of the egg is where the baby chick’s head is—and where it comes out when it hatches—so keeping it clean matters

GRABER: Cassie’s theory is also still a theory, it’s just a correlation so far.

STODDARD: We need to do a lot more work to pin down whether there is a mechanism here that could explain why birds’ bodies and adaptations potentially for flight are correlated with egg shapes.

TWILLEY: But even though the egg shape puzzle continues, Cassie’s also looking at egg color. This is another area where there were lots of theories.

GRABER: Scientists have hypothesized that maybe the color has to do with how well the egg absorbs or reflects sunlight and heat. Or maybe how easy it is to find that egg in dark environments for cave-dwelling birds. Or maybe the colors are meant as camouflage. That’s one of the most popular theories.

TWILLEY: Cuckoos are famous for tricking other birds into raising their chicks. They fool the other birds by disguising their eggs to look like the eggs of the bird they’re trying to trick—they can make their eggs look like a robin’s egg, or a warbler, or whatever.

STODDARD: And so we have been trying to take a bird’s eye view in approaching this. And that’s important because birds see the world very differently than humans do. Birds have a fourth color cone in their retinas that’s sensitive to ultraviolet light.

STODDARD: So birds are tetrachromatic and we humans are mere trichromats, we have just three color cones.

TWILLEY: This means that birds can see up to a hundred times more colors than us. Unless you are one of the very, very, very rare humans who is a tetrachromat, in which case, I am so jealous!

STODDARD: And so we use a combination of spectrophotometery and UV photography, both tools that allow us to quantify colors in a way that is relevant to birds, to look at egg color and egg pattern in new ways.

TWILLEY: When Cassie looks at eggs the way birds would see them, she can pick out patterns that seem to be anti-cuckoo security devices—signatures that parent birds put onto their eggs to try to make sure they’re only putting their energy into raising their own chicks.

GRABER: But then on to another type of camouflage—are birds trying to hide their eggs from hungry animals that might want to eat them? Cassie has also created models for the ways that those predators might be looking at the eggs, too.

STODDARD: And it looks like some of our assumptions might not always hold up. It doesn’t look like egg camouflage is as straightforward as we thought.

TWILLEY: The defense of eggs is a complex thing. And it’s to do with structure as well as color. Because egg shells also have to be strong enough to be sat on by parent birds.

BOWIE: And I’ve done this—if you take an ostrich egg and it’s full, a 200 pound man can stand or woman can stand on the egg and it won’t break. If you jump on it, it’ll break but you can stand on the egg and it won’t break.

GRABER: But those egg shells also have to be fragile enough for a tiny baby chick to break out of. Cassie’s trying to figure out this mystery by studying the structure of the eggshell at the nanoscale. It’s just another one of the many questions that scientists are still trying to tease out when it comes to eggs.

STODDARD: So the egg presents a very good puzzle for researchers.

TWILLEY: But the egg is also delicious and makes a very nice breakfast. Or dinner.

GRABER: And we promise that we are now going to get back to the topic of food! Like—how long have we been eating eggs? But first we have a couple of sponsors to tell you about.


GRABER: Humans have been eating eggs for probably hundreds of thousands of years.

JULIA BEST: So we think other species such as Neanderthals, they were using birds definitely because we find the cuts of flint tools upon the bird bones. So we think they were probably using wild bird eggs. And even other species such as Homo heidelbergensis—there’s been egg shells found at sites that are 300,000 years old and associated with that species.

TWILLEY: This is Julia Best. She’s a zooarchaeologist at Bournemouth University in the UK.

GRABER: And we’re not just talking chicken eggs here.

ADELE WESSELL: So we have a really long history with eggs of all different kinds.

GRABER: Adele Wessell is a food historian at Southern Cross University in Australia.

WESSELL: In Australia, for example, Aboriginal people would have eaten emu eggs. In South Africa and Zimbabwe and places, people would have eaten ostrich eggs, duck eggs, quail eggs—basically, any kind of egg can be eaten.

TWILLEY: But actually tracing egg consumption in the past is tricky.

BEST: So our evidence generally comes from finding the very fragmentary crumbs of egg shell preserved in archaeological sites. And to do that we need to have a fun game called sieving lots of soil.

GRABER: And then they find tiny bits of eggshells. But you can’t look at those eggshell bits and just figure out what birds they came from, or what people were doing with them. So Julia has a few other tools up her sleeve.

BEST: So you can study egg shell under a microscope and, particularly if you put it under a scanning electron microscope, which is very powerful, you can look at lots of the tiny features of the eggshell.

TWILLEY: Remember we told you different eggs have different number of pores? Yeah you do! Well, Julia uses that information to identify her little egg shell crumbs.

BEST: We can for example measure the thickness of the eggshell. We can count how many pores are in it. And the internal surface of an egg shell has very distinctive little—they look like volcanoes, they’re the bit that the internal membrane clings onto. And these look different for different species. And so using a high powered microscope you can compare that to modern eggs and start to identify them to species. Which is great.

GRABER: But was that egg served for dinner, or did a little birdie hatch out of it? Julia can figure that out, too.

BEST: So as a chick grows within an egg it doesn’t just use up the yolk but it also extracts some of the calcium back out of the egg shell to help form its bones. And this leaves little pockmarks over the inside of the egg shell that I can then identify with the microscope.

TWILLEY: Julia uses all these tricks, and a couple more, to figure out when and where the domesticated egg-laying chicken entered the scene. Her question is: When did people start eating chicken eggs regularly, rather than the whole variety of wild bird eggs they might have enjoyed before?

BEST: So it really depends where you are. So, for example, in places like Iceland even when the chicken finally does become established there, you’ll still surrounded by a glut of wild birds so people still made use of the wild bird eggs.

GRABER: In England, when the chicken arrived, its eggs took over pretty quickly. And chicken eggs came to England with the Romans. Julia studied a 2000-year-old Roman amphitheater in the west of England for clues.

BEST: And at Chester amphitheater we found lots and lots of egg shell underneath the seating banks where people would have sat to watch the events going on within it.

TWILLEY: Julia analyzed them and she found they were chicken eggs, freshly laid.

BEST: So this shows that these were eggs being used as eggs and that people in that amphitheater 2,000 years ago were sat there probably having a snack equivalent having a pie at a football match today and then dropping their waste down under the seats. I like to think that potentially there was somebody sat outside the amphitheater flogging you an egg as you went in.

TWILLEY: It’s evidence like this that Julia can use to pinpoint when chicken eggs took over as the egg of choice.

GRABER: But we have not yet answered one major question that many of you have asked: Why the chicken? Why do we nearly exclusively eat chicken eggs today?

TWILLEY: The chicken’s wild ancestors came from the jungles of Asia. And that’s also where the chicken was first domesticated—we’ve talked about this before on Gastropod. Those wild junglefowl lay between a half dozen and dozen eggs once a year in spring. If you take those eggs away, they might lay another batch, but that’s about it. Domestic chickens, on the other hand—they lay three hundred eggs or more, all year round.

GRABER: It took a few different steps to get to those mega egg-layers. First, it looks like people in what’s now China domesticated chickens from those junglefowl as long as 4,000 years ago. And in doing so, they serendipitously disabled a gene that matched breeding to the length of the day. So originally birds timed their breeding to a specific time of the year. But with that gene disabled, the chickens were more likely to breed all year round.

TWILLEY: Even so, that genetic change wasn’t quite enough to meet our egg needs. Humans weren’t done messing with chickens yet.

WESSELL: Yeah. Well, one of the first things was the introduction of incubators so that you could breed chickens in separate sort of hatcheries. And the hatcheries then themselves would choose the strongest, healthiest kind of birds and different sorts of strains of birds for more efficient production—ones that laid more eggs, ones that laid eggs for a longer period of time, and so on.

GRABER: The Chinese and Egyptians figured out quite early on how to keep eggs warm and toasty and incubate those eggs more effectively. That’s great in terms of getting more chickens and then choosing the best chickens, but it doesn’t make those chickens pump out eggs all the time.

BEST: Where we see a big change potentially is later on when we get artificial lighting. Once we’ve got artificial lighting we can make hens lay more frequently because we can trick them into thinking that it is the time for laying.

TWILLEY: The first successful use of artificial lighting to increase egg production was in 1895, by one Dr. Waldorf. He built a chicken shed lit by kerosene and then later by gas, behind his house in Buffalo, New York.

WESSELL: There’s also, you know, adding vitamin D to their feed so that they don’t necessarily have to go outside as long.

TWILLEY: We’re not saying this is entirely a good thing—there are all sorts of problems with industrial egg production. We covered some of them in our episode last year called The Birds and the Bugs. But this is how we got to where we are.

GRABER: So now we have chickens that have been bred to be power layers. And we have artificial lighting that keeps them laying. And we have food supplements that keep them healthy indoors.

TWILLEY: And the end result is a lot of chicken eggs. That’s how chicken eggs became synonymous with eggs. Julia actually has seen this shift play out in the archaeology—she studied some sites in the Scottish islands where traditionally people ate a lot of seabird eggs.

BEST: And we see such a dramatic shift up there between collecting the wild bird eggs and then when the chickens come in, because you can get meat from all of the wild birds at many points of the year. But the chickens—they’re the ones that mean you can have eggs all year round.

GRABER: And now, now that the chicken won, and with all these advances in chicken laying technology, we eat a lot of eggs. In the U.S., today, on average we eat eggs three days out of every four.

TWILLEY: Collectively, we eat an incredible number of eggs every year, and we eat them in an incredible variety of ways. The egg is nothing if not versatile.

BEST: Even from the Roman period we have recipe books that show that eggs were being used in a really wide range of recipes.

GRABER: The Romans made all sorts of tasty-sounding dishes out of eggs, like one that had soft-boiled eggs in a pine nut sauce. They used eggs to make cheesecake. They used eggs to clarify wine. But not all the Roman egg recipes sound so great.

BEST: There’s a horrible recipe—well, it might have been delicious but it doesn’t sound it—for brain sausages, which describes mushing up some chicken brains with four eggs and various spices and forming them into a sausage. So there is a huge range of different applications for eggs even from some of our earliest written records.

TWILLEY: Binding, clarifying, poaching—eggs could do it all. But this is before artificial lighting, and before refrigeration—so eggs had one major Achilles’ heel. They went bad. So humans all over the world devoted a lot of ingenuity to preserving them.

WESSELL: So initially, you know, salting and pickling, those kinds of things would have been treating eggs and that would have made them available throughout the year, because sometimes you get a glut of eggs. The Chinese preserved eggs and I think a lot of people will probably be familiar with those tea eggs. So they were packed in kind of ash and lime and the egg shell just sort of slightly cracked.

GRABER: The famous Chinese century egg isn’t actually preserved for 100 years. And it won’t last for a hundred years either. Although the technique has been around for a few hundred years. The eggs are packed in a type of plaster that contains lime, the same ingredient in mortar and cement. And they cure for anywhere from about two to five months.

TWILLEY: At the end of that time the egg white is a dark, brownish black, translucent jelly, and the yolk is a super creamy greeny-blue color and it tastes really funky and really quite delicious—like a super stinky cheese. I actually much prefer Chinese century eggs to the pickled eggs you get in big jars in British pubs.

WESSELL: Yeah, they are really amazing aren’t they. I mean when you think about an egg, it contains everything that’s required to produce a bird. So there’s a lot of kind of nutrients in it but they also have particular sorts of properties.

TWILLEY: These particular properties—they are what has allowed the egg to embed itself in almost everything we eat.

WESSELL: I think you know cooking eggs really is is a lot about chemistry. So whether you apply heat, or whether you beat them or whether you mix them with fats and those sorts of things, they will kind of take up the different sorts of properties.

GRABER: One of the great things you can use egg yolks for is to bind together water and fat, which usually don’t stick together. This is why eggs are the critical ingredient to make oil and vinegar stick together in mayonnaise. The yolk contains special molecules called lecithins that bury themselves in the fat and also stick to water. So when you slowly beat in an egg yolk with oil and vinegar and some other ingredients, you get the white delicious creaminess that is mayonnaise.

TWILLEY: But eggs can also go from a liquid to a solid that you can cut with a knife, like a custard tart or a quiche. This is a cool trick where you use heat to unfold the proteins in the egg and then knit them back together in a different shape.

GRABER: And now onto another fun way to play with egg proteins. When you whip egg whites, you also unfold those proteins and get them to knit back together. And they capture tiny bits of air. You can use this trapped air to help lighten cakes. And obviously you can use it in a meringue—that is 70 percent air!

TWILLEY: We are not being sponsored by the egg board, I promise, but really—all that textural variety from one ingredient, it’s kind of amazing.

GRABER: Eggs are amazing from a chemistry standpoint. They’re nutritional powerhouses, too—after all, they have to support an entire developing chick. And they basically all taste good!

TWILLEY: Which is kind of curious if you think about it. Because in nature, if you taste good, other things want to eat you. And birds obviously would rather people and other predators didn’t eat their precious genetic material. So why don’t birds just put some kind of nasty chemical in their eggs to put the rest of us off?

BIRKHEAD: Well, the accepted answer to that is that if a female bird was to put distasteful substances into the yolk to deter predators, that might compromise the rate at which the embryo develops. And so there’s a kind of trade-off. I don’t think anybody’s actually tested that, partly because there are no distasteful eggs that we know of. So it seems not to have evolved.

GRABER: So now we know why eggs all basically taste amazing. But—do they all taste the same?

BEST: I mean a duck egg, you can definitely tell that it’s not a chicken egg. And I think quail’s eggs—they taste more creamy in a way. Now that might just be my modern mind saying I paid more for these therefore I’m going to enjoy them more.

TWILLEY: Duck, quail, goose—those are the only non-chicken eggs ones I’ve tried. You can find them in fancy shops. But Tim has tried something a bit more exotic.

BIRKHEAD: I ate usually scrambled gull eggs for lunch every day during the summer for three or four years and I used to swear blind to my friends that I couldn’t tell the difference with chicken eggs. And then a few years ago somebody gave me a clutch of infertile gull eggs and I ate them and there was quite a strong flavor to them so I was a bit taken aback. So gull eggs are a bit more chalky. They’ve got a slightly stronger taste.

GRABER: And then Tim has also tried another kind of seabird eggs—he’s tasted guillemot eggs.

BIRKHEAD: And guillemot eggs, if you boil a fresh egg, the white never sets in quite the same way as it does in a chicken egg. and it stays a kind of icy blue, slightly gelatinous and therefore not terribly appealing consistency but perfectly nutritious. They taste pretty good. but they do they do have a kind of slight, distinctive flavor.

TWILLEY: Ooh. now I’m intrigued. Icy blue and gelatinous? Distinctive-tasting how?

GRABER: Obviously you can’t just harvest wild eggs—it’s actually illegal, as Tim was quick to point out. He did eat them legally. But so what do scientists know about the different flavors of birds’ eggs?

STODDARD: This is an area of research in my mind that is still that is still wide open but of course difficult to test for for many reasons.

TWILLEY: As it turns out, not a lot of solid scholarship has been done on this issue. The Ancient Greek scholar Athenaeus ranked eggs by taste in his treatise on food. Peacock eggs came first, goose second, and chicken eggs were apparently a distant third.

GRABER: Cassie and Tim both told us the only recent research they could think of about the taste of eggs is from the 1950s—they were taste studies conducted by a British zoologist named Hugh Bamford Cott.

TWILLEY: Cott started down this path using egg taste test panels made up of animals—specifically hedgehogs, ferrets, and rats. You might wonder how a hedgehog can offer tasting notes on different eggs. But Cott set up an ingenious system where he put two dishes, each with a stirred-up egg from a different species, the exact same distance in front of each animal, and then recorded which it chose to eat. So, for example Pickles, one of the hedgehog tasters, he consistently chose wood pigeon eggs over blackbird, coot over owl eggs, and buzzard eggs over linnet.

GRABER: For Cott’s human tasting panel, he served three scientists omelettes from 81 different species. The penguin egg was considered gluey and without any flavor except some slight fishiness. The goose omelette apparently left all the tasters gagging, but Dr. Cott said the eggs themselves may not have been particularly fresh.

TWILLEY: Both Tim and Cassie said that Dr. Cott’s studies were not the most scholarly. Which means there is plenty of room for some new egg tasting science…

GRABER: Okay, are you ready.

TIM BUNTEL: Yes I’m ready.

GRABER: So we have three different types of eggs. We have brown eggs from chickens, we have blue eggs from chickens, and we have quail eggs. And what we need you to do is we need you to make three different little bowls of scrambled eggs and Nicky and I are going to see if we can taste any difference among these three different types of eggs.

TWILLEY: Yes, we ate eggs for science. Two kinds of chicken eggs and then quail eggs because that’s all we could find at the gourmet market.

GRABER: Oh, you’re weighing.

TWILLEY: It’s science, Cynthia.

BUNTEL: We have to be as accurate as possible here. Right.

GRABER: That’s my partner Tim, and he’s a great baker, so he loves to weigh ingredients. In this case, he wanted to make sure he had the same weight of eggs for the chickens as the quail eggs. After all, none of us had any idea how many quail eggs you’d have to crack to equal two chicken eggs.

BUNTEL: Okay. So what’s the plan? So you want me to cook all three and present them at the same time?

GRABER: Yes. And we won’t know which is which. And we’re just going to taste and see if they taste the same or different.

TWILLEY: And can I have a piece of toast with it?

GRABER: Nicky wants it to be lunch.

BUNTEL: And hash browns… All right, so this is the heirloom Andean blue egg. Not blue inside.

TWILLEY: Next up, two dark chocolate brown eggs from a French heirloom chicken variety called Marans. Apparently these were James Bond’s favorite eggs. As featured in From Russia with Love.

GRABER: And then we opened the package of teeny tiny very pretty quail eggs, which had brown squiggles on a cream background.

BUNTEL: All right. This is interesting. How am I to crack these?

TWILLEY: Have you ever cracked a quail egg before?

BUNTEL: I have never even seen a quail egg before, let alone cooked with them.

BUNTEL: Alright, let’s give it a little crack here. So tiny.

GRABER: Oh my goodness.

BUNTEL: It really looks just like a wee little egg inside.

GRABER: In case you’re wondering, Tim had to crack a full dozen quail eggs to equal just two chicken eggs.

BUNTEL: Okay, we’re going to go with one tablespoon of whole milk per batch here. I don’t want to cross contaminate the fork from scrambling. Now, you guys go away.

TWILLEY: I’m so hungry! I just wanted some eggs. But, science.

GRABER: I was starving Nicky until the science experiment was ready. Sorry.

TWILLEY: I must say, I was fully expecting there to be no real difference between all the eggs. I’ve eaten quail eggs before and to be honest I thought they tasted like eggs. But when Tim brought out all three bowls of scrambled egg, I spotted the odd one out right away.

TWILLEY: So just on color alone, I’m saying there’s already a difference.

GRABER: Whatever this green bowl is, it has a darker richer yellow color.

TWILLEY: Yeah and this blue bowl is a little lighter.

TWILLEY: Then FINALLY we got to eat.

TWILLEY: Blue bowl is quail for sure.

GRABER: I think blue bowl tastes different than the other two.

TWILLEY: The other two are more eggy and the blue that the eggs in the blue bowl are more creamy and less eggy. Alright, Tim?

BUNTEL: All right. The green bowl contains the brown egg. The blue bowl contains the quail egg. The white bowl contains the blue egg.

GRABER: So, two things: We guessed correctly which bowl had the quail eggs! It was a little lighter in color, and slightly different than the other two. To me it was more of a textural thing than a taste thing, but frankly I had a cold.

TWILLEY: It was pretty subtle. And the brown and blue eggs—they were both from chickens, just different color shells—and to me, they tasted exactly the same.

GRABER: Okay, so did we determine that that quail eggs are fundamentally different tasting from chicken eggs? Turns out that we hadn’t set the experiment up quite well enough to prove that.

STODDARD: Well, there certainly are differences in the amount of yolk that is in different eggs laid by different species.

TWILLEY: Cassie told us that as eggs get bigger, the ratio of white to yolk changes. The bigger the egg, the more white there is, proportionate to the yolk. So, in fact, the quail eggs could have had a creamier texture just because they had more yolk than the chicken eggs.

GRABER: This is something we should have included in the experiment design! Tim?

BUNTEL: Just don’t ask me in the future to actually separate the yolks in these tiny little eggs.

TWILLEY: This is what we need to do! If you portioned it out so we had the same ratios? Tim?

TWILLEY: Science in action, folks. We make the mistakes so you don’t have to. If you have the patience to separate out quail eggs and do this test right, be our guest—and let us know the results.

GRABER: Even if the ratio was the same, there are things that could change the flavor of the eggs. Tim Birkhead—not my partner Tim—he told us that what the birds eat might have an impact on the flavor. And that’s not all.

BIRKHEAD: It could also be that the female is secreting different combinations of chemicals into those eggs to serve different purposes.

TWILLEY: For example, one thing Rauri told us is that what order the egg was laid in makes a difference to what the mother bird puts in the egg.

BOWIE: What’s been shown in some species is that the egg that is laid last, she may put more testosterone into that embryo, which makes that baby more aggressive and assertive and so it catches up by being more more assertive in being fed. So there’s fascinating interactions between the lifestyle of the individual birds, what the female places in the egg, etc.

GRABER: Maybe that testosterone influences the flavor. Or maybe there are other chemicals that the mother bird puts in that we don’t know about yet.

TWILLEY: But also, maybe a chicken that eats the same diet as a guillemot would have an egg that tastes like a guillemot’s egg,

GRABER: But maybe not. The research has not been done.

TWILLEY: Scientists, get on it!


TWILLEY: Uh, Cynthia, I feel like we’re managed to make an episode on eggs and end up with the same number of questions as when we started, if not more!

GRABER: For all you listeners who still have burning questions—I guess we’ll just have to make another egg episode in the future!

TWILLEY: Thanks this episode to Rauri Bowie who showed us around the amazing egg collection at the Museum of Vertebrate Zoology at UC Berkeley. We have links to the gorgeous images of the collection, all digitized thanks to a grant from the National Science Foundation, on our website, so you can ooh and ahh at all the pretty eggs just like we did.

GRABER: Thanks also to Cassie Stoddard, Julia Best, Adele Wessell, and Tim Birkhead, whose book is called The Most Perfect Thing. And yes, that’s how we both feel about the egg, too. You can find links to their research and books on our website,

TWILLEY: As well as a link to find out if you are indeed a tetrachromat, like birds. And finally of course thanks to Tim Buntel, egg cracker and scrambler extraordinaire. We’ll be back in two weeks with a brand new episode. Although frankly, we could keep going with eggs for ever.