The Science of Birds - Eggs: Incredible and Commendable

Episode Date: June 24, 2022

This episode—which is Number 54—is all about bird eggs. This is an important topic. Eggs are a fundamental aspect of bird biology. Recently, in Episode 49 of the podcast, I covered the topic of nests. So it seems like a logical next step for us to get the lowdown on eggs.Oology is the science of studying bird eggs. So today, we are all honorary oologists. We’re egg-heads on a mission to better understand how baby birds come into the world.~~ Leave me a review using Podchaser ~~Book RecommendationsThe Most Perfect Thing: Inside (and Outside) a Bird’s Egg [affiliate link]Links of InterestHow are Eggs Made (BBC/Attenborough clip) [VIDEO]Chicken embryo development animation [VIDEO]Bird Eggs Warn Each Other About Danger [VIDEO]Link to this episode on the Science of Birds websiteSupport the show

390 million years ago, during the sultry days of the Devonian period, some adventurous fish started crawling out of swamps to explore the wide open frontier of dry land. These fish were the ancestors of the earliest amphibians, which evolved some millions of years later. Amphibians were much better at getting around out of the water, what with the legs and lungs and all that. But most amphibians, even to this day, can't venture too far from watery habitats, if for no other reason than because their eggs won't survive unless they're immersed in water. Some amphibians eventually evolved into reptiles, about 320 million years ago. One of the defining evolutionary inventions of reptiles was the ability to lay enclosed

eggs that can survive on dry land. Your standard-issue reptile egg has a leathery shell that protects the growing embryo inside, protects it from things like harmful microbes, solar radiation, and drying out. As early reptiles evolved into the first dinosaurs, somewhere along the way, these animals started laying eggs not with leathery, flexible shells, but with hard shells. dinosaur eggs with their hard shells were an upgrade from early reptile eggs. One lineage of dinosaurs became the birds, of course. Birds with their warm, feathered bodies incubate their hard-shelled eggs at an optimal temperature. This brooding behavior in birds, combined with the structure of their eggs, opened even more opportunities for them.

They could raise their young just about anywhere. So, even though the fossil record of this story is very incomplete, we can look back and see the evolution of vertebrate eggs. From the delicate, slimy eggs of fish and amphibians to the self-contained eggs of reptiles, dinosaurs, and birds. Mammals are another story. They went down a different evolutionary path. Most mammals stopped laying eggs over 100 million years ago. Although I'm not sure what's going on with the Easter bunny. It's a giant rabbit, okay, so that means it's a mammal, but it lays these crazy colorful eggs and carries them around in a wicker basket. Oh, wait, wait, what if? Maybe the Easter bunny isn't actually a rabbit, but instead it's a monotream,

like the duck-billed platypus, because monotrems are egg-laying mammals. So maybe we should call it the Easter platypus. Well, whatever we call it, Easter bunny, Easter platypus, this enormous, furry, dead-eyed beast is just going to keep doing what it does best. Terrify small children. But we're talking about birds, right? Bird eggs are wonders of nature. They work like little self-contained survival pods. Like feathers and like nests, the avian egg is a special feature that has helped these animals spread to every corner of the globe, and to diversify into thousands of species. Bird eggs are a lot more complex and fascinating than you might think. Hello and welcome. This is the science of birds.

I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted, guided exploration of bird biology for lifelong learners. This episode, which is number 54, is all about bird eggs. This is an important topic. Eggs are a fundamental aspect of bird biology. Recently, in episode 49 of the podcast, I covered the topic of nests. So it seems like a logical next step for us to get the lowdown on eggs. Back in the 19th and early 20th century, many well-to-do British gentlemen made a hobby

of collecting bird eggs and displaying them. For many of these guys, collecting became an obsession. They called their hobby oology. This is a strange-looking word, oology. It's like zoology without the Z, like the legend of Elda, or the ambi apocalypse, or that giant city-destroying lizard, God Illa. Oology as a hobby mostly disappeared a hundred years ago, when egg collecting for fun and profit became illegal in many countries. But oology is also the science of studying bird eggs. So today we are all honorary oologists, the sciencey kind. We're Eggheads on a mission to better understand how baby birds come into the world. Along the way, let's keep in mind that the structure of eggs, as well as things like

clutch sizes and the behavior of chicks and parents, are all evolutionary adaptations related to reproductive success. These things have been under a lot of pressure from natural selection for hundreds of millions of years. Eggs and chicks have had to deal with challenges like the elderly. Elements, competition among siblings, disease, and, of course, predators. All right, let's get into it, shall we? We begin with the external structure of eggs. We all know the basics, right? Most of us have seen a chicken egg or two.

And we know the answer to Bilbo Baggins's riddle. A box without hinges, key, or lid, yet golden treasure inside is hid. Indeed, if you're hungry, the inside of a bird egg is a treasure. No wonder so many animals are trying to get their grubby paws on bird eggs. The list of egg thieves includes rats, raccoons, squirrels, monkeys, mongooses, gollum from the Lord of the Rings, skunks, foxes, birds like ravens, and other corvids, and snakes. Snakes are especially crafty egg predators. There are even some snakes, like the 17 African species in the genus Dacepeltis, that are egg-eating specialists.

They only eat bird eggs. We humans, too, love us some eggs. We came up with catchy tunes like this. Yes, eggs are incredible, without question. But if you're like me and you tend more towards a vegan diet, you might prefer another word instead of edible. How about the incredible commendable egg? Or the incredible three-dimensional egg?

Hmm, not as catchy. There aren't that many words that rhyme with incredible. Oh, well. Let's go ahead and look at the sizes of eggs. their three-dimensional shapes and their colors. Bird eggs vary in size most dramatically across species or across different taxonomic groups, like raptors compared to songbirds, for example.

But egg size can also vary among individuals, within clutches at different times of year, and so on. Among all birds, there's an enormous size range for eggs. Naturally, hummingbirds lay the smallest. The world record for the smallest egg belongs to the Vervein hummingbird, Melasuga Minima, native to the islands of Jamaica and Hispaniola. Its egg is only 10 millimeters long, the size of a small jelly bean. Then we have the common ostrich, whose gigantic egg is about 160 millimeters long, which is 6.3 inches. If you think that's big, remember the elephant bird that I talked about in episode 52? That extinct avian monster laid the biggest bird eggs ever

known. At 13 inches, an elephant bird egg was twice the length of an ostrich egg and seven times the volume. And it had 150 times the volume of a chicken egg. It was bigger than a basketball. Not only did the elephant bird lay the biggest bird egg ever, it was the biggest egg of any animal, period, bigger than the eggs of even the largest dinosaurs. Besides these absolute sizes, we should consider the relative sizes of bird eggs. That is, how big is an egg relative to the body size of the female who made it? Bigger birds tend to lay bigger eggs in the absolute sense, sure. And small birds lay small eggs, duh. But big birds lay small eggs relative to their body size. For example, the weight of an ostrich egg is only about one to four percent of

the adult bird's weight. That's relatively small. A tiny bird, like a warbler on the other hand, might lay an egg that's 10 to 15 percent of its body weight. In case you're wondering, human babies typically weigh about 5% of what their moms weigh. Kiwis in New Zealand are famously unusual for many reasons. Besides being flightless, nocturnal, and mammal-like, Kiwis lay ginormous eggs. Well, relatively ginormous eggs. Relative to their body size, proportionally,

Kiwis lay the largest eggs of any living species. A female kiwi can lay an egg weighing up to 25% of her body weight. If a human mother gave birth to a baby that size, it would weigh about 37 pounds, which is 18 kilos. That would be like mom giving birth to a four-year-old preschooler. The 37-pound newborn would come out, look around, and be like, what's up, lady? You got any cookies? I want to watch Finding Nemo. Moving on to the shapes. Moving on to the shapes. of bird eggs. The shape of the domestic chicken egg is universally recognized as the egg shape. It's curvy and beautiful. I wouldn't be surprised to learn that humans are genetically

imprinted to instantly recognize that shape as an egg. We use it as icons, emojis, logos, and on and on. But the classic chicken egg shape, while it is common across many species in the avian world is just one form of bird egg. Some lineages of birds make rather differently shaped eggs. For example, many owls, trogons, and kingfishers lay eggs that are spherical, or nearly so. These eggs can look like little ping pong balls. Other types of birds make eggs that are both oval-shaped and symmetrical. Some lay eggs that are more elongated. Other birds lay eggs that are what biologists call p-y-riform. That's P-Y-R-I-F-O-R-M.

It means pear-shaped. These asymmetrical eggs have a blunt end and a relatively pointy end. Many birds belonging to the order Karadriiformes lay pyroform eggs. These include the plovers and ox. The classic example is the common Mur, or Common Gilamont, Uria Alga. The common mur is a seabird in the Ock family. These birds nest on rocky cliffs and islands in northern seas. The female common myrrhers lays a single, large egg that's distinctly, dramatically,

pyriform. I was going to say pyriform shaped, but that would have been really. redundant, you know, like saying ATM machine, or unexpected surprise, or crazy birder. Across the 10,000-plus species of birds in the world, eggs come in every other shape between the extremes of spherical, paraform, and elliptical. Elliptical is fancy talk for oval-shaped. But why are there these different shapes? Does each have a special function?

Some adaptive purpose in the context of natural selection? Ornithologists have been trying to figure this out for a long time. They've come up with a handful of reasonable hypotheses over the last couple of centuries. Maybe certain shapes are stronger under pressure. Maybe some shapes allow more eggs to be packed into a small space. Other shapes might keep an egg from rolling off a cliff and so on. We still don't know exactly why there are these different egg shapes, but a recent study published in the well-respected journal Science might point to the answer. The researchers in this

2017 study, led by Dr. Mary Stoddard of Princeton University, measured the shapes of almost 50,000 eggs from 1,400 bird species. These species represent 37 orders. Each egg was given a score, for how asymmetrical it was and another score for how elliptical it was. After lots of complicated math and computer analyses, Stoddard and her colleagues discovered that an egg's shape most strongly corresponds or correlates with the flying ability of the bird species that laid it. It seems strong flying species tend to lay eggs that are both more asymmetric and more elliptical. The researchers explain that the muscular, streamlined bodies of birds well-adapted for flight

might have, through the process of evolution, caused these birds to make asymmetric oval-shaped eggs. Those shapes are, for some reason, the best fit. So, at least at the broad scale, looking across all these 1,400 avian species, flight ability might explain a lot about egg shape. But there's still room for other complementary explanations, especially when we look at the smaller taxonomic scale of bird families or species. Nest conditions or other factors during incubation may have also influenced egg shape. For example, many shorebirds with precocial chicks lay asymmetric eggs.

eggs with a blunt end and a pointy end. Precocial chicks are the fuzzy ones born with open eyes and sparky attitudes. Their little brains and bodies need extra oxygen as they grow inside the egg. The blunt end of an asymmetric egg allows more oxygen to seep through the shell. So egg asymmetry, in this case of precocial shorebird chicks, might be related to their oxygen needs. possibly. We've done size and shape, so now it's time for us to do color. Bird eggs may not be as wildly kaleidoscopic with color as the ones laid by the

Easter platypus, but many of them are still incredibly beautiful. Bird eggs come in shades of blue, green, pink, purple, brown, and cream. Some can have an almost metallic sheen. But the default egg color is, of course, white. That's because the main component of the egg shell is calcium carbonate, a white substance. Many bird species lay plain old, nondescript white eggs. But other species jazz things up with fun colors and patterns. There are two types of pigments produced by female birds to add color to their eggs.

The first is Billa Verdon. This can give eggs a blue or green color. Then we have protoporferin, which gives us reds and browns. On top of these background or base colors, many eggs are decorated with dark spots or streaks. The streaks are often more like squiggly lines, really. Some of them look like they were drawn on with a calligraphy pen. For example, you know, I keep wanting to say things like, for example, wink, wink, but I've restrained myself every time, until now.

Sorry. For example, the African jacana, actophilornis Africanus, has some gorgeously patterned eggs. This species breeds in wetlands across sub-Saharan Africa. The African jacana's net, is a flimsy mass of floating vegetation. The female lays two to five pyriform eggs. Each has a creamy, tawny background color overlaid by dark, scribbly brown lines.

The sinuous patterns look to me like the writing of some lost civilization. They look so cool. When I see eggs like this and like those of the common myrrer I mentioned earlier, I can kind of see why some guys back in the 1800s went bananas about egg collecting. And, hey, fun fact about the African jacana, as well as other jacanas.

After the female lays her beautiful eggs, she's pretty much done working. It's the male jacana that does most of the nest building, and he does all the incubating and subsequent parental care. Anyway, I'll put a photo or two of African jaccona eggs, as well as some other examples, on this episode's show notes on the Science of Birds website. Egg shells not only come in a variety of colors and patterns, there's also variation in their texture. Some are glossy, some are matte.

Some are rough and pitted. Others are chalky. Some are even sort of waxy or greasy. As with egg shape, scientists still don't fully understand the purposes of egg colors. It seems obvious that for species that nest out in the open and or on the ground, having camouflaged eggs is an important adaptation. Most shorebirds that nest in open country do indeed have superbly camouflaged eggs. They blend in with the rocks, sand, and vegetation

around the nest. Ornithologists have wondered why some bird eggs lay white eggs, which have no pigments on the surface. It takes energy for a female to make pigments like Billa Verdon in her body, so if special colors or patterns aren't beneficial, then it probably makes no sense to use them. Many cavity nesting birds have white eggs. Since these eggs are hidden from the eyes of predators, maybe they don't need any pigments for camouflage. Or maybe white eggs are easier for the brooding parents to see in the dark cavity. Who knows? Other functions of egg color that have some scientific support have to do with a parent being able to recognize its own eggs. For example, in species that get victimized by brood parasites like cuckus, it's pretty handy if you can tell your own

eggs apart from those of the sneaky cuckoo. Parent birds might simply know what their eggs generally look like when compared to those of a brood parasite or another species. Or, the parents might have the ability to recognize individual eggs. This is where those spots and squiggly lines come in. Once again, let's have a look at the common myrr. These birds breed in dense colonies on rocky coasts. They don't make any nest to speak of. Breeding pairs simply huddle together, each pair defending a small piece of real estate as their own.

Common murs are famous for the seemingly endless diversity of colors and patterns seen in their eggs. They come in shades of blue, tan, and cream. The spotting and streaking on their surfaces is incredibly variable. Each common mer pair has only one egg at a time, and they can tell their own egg apart from all the other eggs. eggs in the colony, which might number in the thousands. The myrrh parents remember the unique combination of color and spotting of their own precious egg. When an egg leaves the female bird's body, it's a self-contained little survival pod, like a tiny spaceship. The embryo inside is packaged with all the resources it needs to

survive in a hostile world. It's got food, water, and a sort of septic tank to contain wastes. And for entertainment, there's a VCR with a collection of family-friendly VHS tapes, a Nintendo, and a basketball hoop. Well, the egg doesn't have all of the resources a growing chick needs. Unlike a spaceship floating through the vacuum of space, an egg doesn't carry its own supply of air, its own oxygen tank. The egg has to exchange gases with the outside world. Oxygen in, carbon dioxide out. So, I guess an egg is less like a spaceship and more like what? A camper van? You like a camper van. I don't know. Maybe after you listen to this episode, you can think of a better analogy. But let's go ahead and look at the internal structure of a

bird egg. Picture a chicken egg you've just cracked open to make an omelet. This is an unfertilized egg. There's no embryo, no baby chicken, just a big yolk and some translucent, gelatinous stuff surrounding it. Now let me describe a fertilized egg, as it might look inside at the halfway point in its development, halfway between being laid and hatching. In the center, floats the embryonic chick. It looks like a cute little dinosaur alien thing. It's wrapped in several layers of membranes. Growing out of the embryo's belly are some tubes with stuff flowing through them, food, waste, or blood. Perhaps the most conspicuous thing attached to our bebe chick is the yoke. This is a big blob of fats, proteins, minerals, and pigments. The yoke

is the food supply for our growing chick. The relative size of the yolk in the egg varies among different types of birds. The yolk is large in species with precocial babies, like chickens and ducks. Those chicks need a lot of nourishment in the egg so they can hatch at a relatively advanced stage. Altricial chicks, on the other hand,

like those of songbirds, are born blind and helpless. The yolks in their eggs are small, since they don't need as much food before hatching. The amount of an egg filled by yolk varies from only 15% in northern Gannets to 70% in the southern brown kiwi. As I mentioned, the bird embryo is enveloped by or connected to several membranes, or sacks. First, the chick is wrapped in the amnion. That's one membrane. The chick is also connected to the yolk sack, and another sack called the Alantuus. The Alantuus is sort of like a septic tank. It receives and contains waste from the chick. Its other important function is to exchange oxygen and carbon dioxide through the shell. A network of tiny blood vessels in the

elantus makes this possible. The embryo, the yoke, and the elantus are all encased in the corian, which is another membrane. The corian and its precious contents form a spherical sort of bubble suspended in the middle of the egg. A clear, liquidy, jelly-like substance called albumin fills the rest of the egg. That's the part we call egg white. Albumin has several functions. First, it provides our little chick with most of the water it needs and a fair amount of protein. Second, it acts as a shock-absorbing cushion around the embryo. There are also two bungee cord-like fibers that run through the albumin. These are called calaisi, and they help suspend the corian and the embryo inside.

One calaisa connects to the pointy end of the egg, and the other connects to the blunt end. end. Amazingly, the calaisy twist as needed so that the embryo is always right side up. This suspension system keeps the growing bird in the perfect position to get oxygen and to get warmth from the bellies of its brooding parents. The third function of the albumin is to protect the chick against harmful microbes. Microscopic beasts like bacteria, viruses, and fungi from the outside world are a constant threat. They'd love to get a taste of that juicy little bird floating in the egg. But to a bacterium or virus, the albumin is a vast and dangerous barrier.

It's chock full of antimicrobial proteins. Biologists have discovered over 100 of these helpful proteins in the albumin. As the embryo grows, it uses up water and protein from the albumin. So the albumin shrinks until there's not much left of it when the little bird busts out of the shell. As development carries on, the Alantuus and the Corian fuse to form a membrane that expands outward. It eventually covers most of the inside of the shell. This is the Corio-Alantoic membrane. Yes, that's a mouthful.

The corio-alentoic membrane is criss-crossed by tons of capillaries. So this membrane's function is to absorb oxygen from the outside and also to get rid of carbon dioxide. You've heard about the placenta in mammals, right? Well, the Cori-Alantoic membrane is the bird's version of a placenta. Just inside the shell, on the blunt end, there's an air sack. The sack grows as the chick develops and becomes important just before hatching. We'll come back to that in a few minutes. There are even more membranes lining the inside of the shell,

but we're just going to ignore them for today. I know, I know. You want to learn about each and every membrane in excruciating detail. But kids, we just don't have time to cover all the membranes. We're moving on to talk about the shell itself. The shell is made of mostly calcium carbonate crystals, with some proteins in the mix, too. The crystals are arranged like parallel columns, pressed tightly together. The special microstructure of the shell allows it to be strong when pressure is applied from the outside,

like when a big fat bird is sitting on it. At the same time, the shell is weak enough to be cracked open from the inside by the hatchet chick. Egg shell is not as solid as you might think. I've been saying that there's air moving in and out of the egg, right? The way it does that is through microscopic pores. These are spaces between the columns of calcium carbonate that connect the inside of the egg with the air outside. Each pore is about one one thousandth of an inch wide, and there are about 10 to 20,000 of them perforating an egg's shell. These are what allow gases in and out of the egg. And water vapor can escape the egg, too. A growing chick makes water in its cells through

metabolism. That water has to go somewhere, so it diffuses out of the egg as vapor. But the shell is generally waterproof on the outside, at least in some birds. The shell works like a Gortex jacket. It keeps rain out, but lets water vapor inside escape. Biologists have figured out that eggshells get their waterproofing ability from countless microscopic spheres on the surface. The spheres are made of calcium phosphate, and they can give the egg a chalky or powdery texture. They force water on the egg's surface to bead up and roll away, like water off a duck's back. you could say. Having a waterproof shell is important for bird species living in wet habitats like wetlands or in rainy environments. You got to keep the water out so that the chick can get all the

oxygen it needs. Also, bacteria and other nasty microbes like to swim around in water. Having a waterproof shell is important for keeping them out too. Okay, quick review. We've got our adorable embryo, floating serenely in the middle of the egg. Maybe it's dreaming, about flying through the clouds, or about pooping on a car that just got washed. Who knows? Blood vessels from the chick's belly extend outward to line the inside of the egg's shell. They form a network in the Corio-Alantoic membrane. This membrane is most developed when the chick is close to hatching. The embryo also has the yolk sack attached to its belly. The corian is a membrane that forms a bubble around the chick and its belly attachments. Outside of all that mess is the albumin, the egg

white, and the egg shell is riddled with pores, and at least in some bird species is coated with countless microscopic spheres on the outside. Now let's follow the path of an egg on an exciting ride as it twists and turns through the dark tunnel that is the avian oveduct, starting at the ovary and all the way down to the cloaca, where the egg pops out, hot and fabulous. This is like an assembly line in a factory, where parts are added one after another to form a complete egg. Female mammals have two ovaries, but birds have only one, on the left side of the body. The other side of the female avian reproductive system shriveled up and became vestigial long ago,

like over 125 million years ago. The ovary produces the egg cells. The technical name for one of these cells is ovum. The female bird's contribution of DNA is, packaged into each ovum. As an ovum is getting ready to head down the tube to become a shelled egg, it swells with yolk material. This is a slimy mix of fats and proteins made in the female's liver. A single female egg cell is the yolk plus a tiny bundle of genetic material. The whole thing is enormous compared to your average cell. In fact, the single biggest individual cell

in the world is the ovum of a common ostrich. The ovum eventually breaks free from the ovary and moves into the top end of the tunnel, the oviduct. At about this point is when the ovum gets fertilized by sperm. After mating, many female birds store sperm further down towards the cloaca. There are little tube-like structures down there, designed specifically for storing sperm. Some sperm cells are eventually released and they travel up towards the ovary. The first stop for an egg on the assembly line after leaving the ovary and getting fertilized is the magnum. This is where the first layers of gelatinous albumin are squirted out to cover the ovum. Then our egg moves further along until it reaches a hard-to-pronounce section of the tube called the itthmus.

You know and love the word itthmus as what we call a narrow strip of land with oceans on either side, a sort of bridge between continents or whatever. You know, like the isthmus of Panama. Who comes up with these words, man, if itthmus. Anyway, the itthmus, I can't even say it. The itthmus of oviduct is where a bunch of membranes are wrapped around the egg, and more albumin is added. At this point, our egg has already taken on its final three-dimensional shape, even though the hard shell hasn't been added yet.

So it's not the shell, but the outermost membranes that actually give the egg its shape, elliptical, pyriform, or whatever. In the final crucial stage, our egg reaches the uterus. This is where the calcareous shell is secreted onto the membrane. If the egg's color is destined to be just no-nonsense white, then the egg might be ready to shoot out into the world as is once the shell is complete. But for many bird species, pigments are added to the shell's surface in the uterus. Colors are sort of sprayed onto the egg by cells lining the uterus,

sort of like a battery of tiny spray paint cans. Muscular contractions of the oviduct often cause the egg to rotate as pigment is being laid down. That's how biologists think those crazy, squiggly lines are drawn on. The pigment keeps flowing out like paint as the egg rotates around. Pigments are usually added moments before the egg is laid. Once the paint job is done, the egg is ready to roll off the assembly line. Mom squeezes it out of the cloaca and the egg finally sees the light at the end of the tunnel.

How long did all of this take from the release of the ovum from the ovary to laying? For birds like chickens, woodpeckers, and most songbirds, it takes about 24 hours. But the bigger the egg, the longer it usually takes. Many types of birds lay an egg every other day because for them it takes more like, 48 hours to make an egg.

Birds in this category include ostriches, herons, gulls, doves, and hummingbirds. Birds of some species lay just one egg per clutch, and then they work hard to raise their single chick. An only child chick is lucky because not only does it get all the food, it all the food, it all also gets all the presents every year at Christmas, and it's the sole inheritor of its parents' gold and jewelry. Examples of birds that lay only one egg per clutch are kiwis, common murs and other ox, albatrosses, some penguins, and some turns. Other types of birds, such as hummingbirds, typically lay two eggs per clutch. But some small songbirds, like the Great Tit in northern Europe, can lay more than a dozen eggs per clutch.

Why is there this variation in the number of eggs per clutch across different types of birds? At a deep genetic level, clutch size variation from one species to another and one bird family to another is a result of natural selection. Evolutionary forces over the eons have optimized the average number of eggs per clutch for each species. The optimum number of eggs that make sense for a duck is different from the number that makes sense for a falcon, for example. Some things that have probably been involved in the natural selection of clutch size include the body size of a bird species, the number of chicks the parents can successfully raise, the way the bird nests, what it eats, its predators, and its habitat. Ornithologists have gathered tons of data on clutch sizes across species and among individuals.

Collecting this data is relatively easy since you basically just count the number of eggs in a nest, right? Yeah, I know you have to find the nest first, and that can be challenging. But there's a reason that an ornithologist usually takes the gold medal every year at the Easter Egg Hunt World Champion. Clutch-sized data are discrete. They come in nice whole numbers, like two, seven, or ten. Counting eggs is much easier than trying to collect data that's more fuzzy, more nebulous. Like, what if you were trying to measure the hopes and dreams of a bird? Or what kind of data would you collect if your research question is Would a Peruvian screech owl

Megasops Roberatus make a good late-night talk show host? I mean, yes, probably, but how do you quantify good? It's so subjective. I might think the owl is killing it, doing a fantastic job. But another person watching the same show might take offense to the owl's political jokes

and then just leave the room. Better to stick with counting eggs So clutch size varies among species And among families and whatnot There are some broad patterns we find here For example, cavity nesting birds tend to lay larger clutches than birds that nest out in the open Also, birds that breed in the temperate zone

Have larger clutches than tropical species And birds that have to feed their chicks lay fewer eggs than those whose chicks can simply feed themselves. The number of eggs in a clutch also varies from one individual female to the next, and clutch sizes for a single female can vary from one season to the next or from one clutch to the next. This sort of variation can come from differences in food availability and or stress. When the land is overflowing with juicy bugs and berries, a female might produce 10 eggs in a clutch.

In a lean year, however, that same female might lay only five eggs. The last thing I want to mention about clutch size is the difference between birds that are determinate layers and those that are indeterminate layers. Determinate laying species are hardwired by their genes to lay a specific number of eggs. For example, the spotted sandpiper, like most members of its family, lays four eggs, pretty much every time. If an egg from a determinate layer's clutch is lost or is removed by a sneaky ornithologist,

the female does not lay another egg to replace it. Maybe you can guess then what happens with indeterminate layers. These birds replace any eggs that are lost or stolen. chickens are a great example. Take an egg away from a hen, the next day there's another one to replace it. Of course, chickens have been bred for this sort of thing for thousands of years. But there are also plenty of wild species that are indeterminate layers. Back around the turn of the last century, there was an experiment performed with a northern flicker.

In case you don't know, this is a woodpecker species common in North America. The Northern Flickr normally has a clutch of seven or eight eggs. The researcher in this experiment removed all but one egg from the Flickers' nest every day. One egg was always left behind in the nest to make sure the female wouldn't just give up and abandon her nest. The female laid about an egg a day to refill her clutch. This went on for 73 days. In that time span, the Flickr ended up laying 71 eggs. At this point, we know the basics of egg anatomy,

of how eggs are formed, and of variation in clutch size. We've got a nice little pile of eggs in the nest. So now what? Now it's time for our parent bird or birds to incubate those bad boys. The optimum temperature for incubating bird eggs is 98.6 to 100.4 degrees Fahrenheit, which is 37 to 38 degrees Celsius. If the temperature of a developing egg gets too far below or above these values, the chick is in trouble. So parent birds work hard to maintain the optimum temperature for their babies. In many bird species, both mom and dad share in the incubation duties.

They take shifts. But in other species, the female alone does all of the incubating. Males do all of the incubating in relatively few species. Body heat is transferred from a brooding parent to its eggs through the brood patch. This is an area of bare skin on the belly or breast of the adult bird. The brood patch fills with fluids during the incubation period, and it has many arteries and capillaries carrying warm blood close to its surface. In hot environments like deserts, the biggest challenge might be keeping the eggs cool.

Rather than sitting on its eggs, a parent bird will often just stand over its eggs to create some shade when the sun is beating down. Some birds go a step further to cool their eggs down in the heat. The Namaqua sand grouse and the Egyptian plover of Africa soak up water in specialized feathers on their breasts. They fly back to the nest and use the water to cool their eggs. The number of hours per day that a parent bird sits in the nest varies among species. But the brooding parent has to eat at least every once in a while. Without a helpful partner around to deliver food or to take a shift incubating the eggs, a solo parent will have to take breaks to feed itself. That said, some females may spend almost all day, every day incubating their eggs for weeks. This is what

happens in birds like Canada geese and common iders. These females are fasting during incubation and they lose as much as 30 to 40% of their body mass. The incubation period lasts from about 11 days in small species up to about 80 days for larger birds like albatrosses. Brewing parents warm or cool their eggs as needed, and they also turn them regularly. Turning, rotating the eggs around in the nest is thought to make sure each growing embryo gets enough albumin.

Remember that albumin provides most of the water a developing chick needs while in the egg. You'd think that a chick in its cozy little camper van isn't really aware of what's going on beyond the shell. It's warm in there, it's got all the albumin it needs, it's blissfully oblivious to world events. That may be true, but scientists have figured out that at least some chicks in their eggs can gather information from the outside. Take superb fairy wrens in Australia. Studies have shown that fairy wren chicks in their eggs listen to the calls and songs of their mother. The chicks end up incorporating snippets of what they heard

into their own repertoires after they hatch. And only a few years ago, researchers in Spain made the amazing discovery that baby yellow-legged gulls communicate with each other while still in their eggs. In a controlled experiment, the researchers subjected a test group of gull eggs to recorded alarm calls. These are the sounds adult yellow-legged gulls make when a predator is on the scene. Yeah, something like that.

When they eventually hatched out, the chicks exposed to the alarm calls were little nervous Nellies. They were quiet and cautious, and they had more stress hormones flowing in their blood. These chicks from the test group had been conditioned to hatch out into a scary world full of ravenous predators. But I said that the chicks communicated with each other, right? Well, after the eggs that heard the recorded alarm calls were placed back in the nest, they were surrounded by eggs that hadn't heard those sounds. But all of the chicks whose eggs were in physical contact with the test eggs ended up being born extra cautious too, even though they hadn't been directly exposed to the recordings. The researchers figured out that the test eggs

vibrated more frequently. Those vibrations were picked up by the other eggs. So word had spread among the eggs about the dangers ahead. The chicks had shared information with each other. Pretty incredible, huh? I bet all those little buggers were relieved when they hatched out and things weren't so bad after all. At last, it's time for our chick to hatch. In most species, all of the eggs in a clutch hatch almost simultaneously within a short time of each other. But you might be thinking, okay, okay, sure, but if many birds lay only one egg per day and their clutches have like 10 eggs, how can they all hatch at the same time? Didn't the first eggs to be laid get a head start on their development? Shouldn't those eggs hatch days earlier

than the last eggs to be laid? Excellent question. Of course, I want to say excellent question, but I won't do that, even though I just did. Many bird species have synchronous hatching, where all the eggs hatch more or less at once. This works out because the parents don't start to incubate the eggs until the last one is laid. Before that point, the eggs laid earlier just sit there in a sort of suspended animation.

Only when mom or dad start warming the eggs against their brood patch, does development kick into high. gear. But I'm sure you've heard about species with asynchronous hatching. This is where chicks in a clutch bust out of their shells at different times, hours or days apart. Notable examples here include hawks, eagles, owls, pelicans, parrots, and cranes. Asynchronous hatching occurs when the parents begin brooding eggs as they're laid. The first chicks to hatch are larger and they tend to get favored by mom and dad. The younger siblings get bullied and they get less food. Sadly, late hatching chicks often die of starvation and neglect. This situation seems cruel to our human

sensibilities. But for birds, it's a way for the parents to adjust how many young they can successfully raise based on food availability. When there's plenty of food to go around, the family can be large and happy. But when food is scarce, the adults focus their attention on the chicks with the best chance of survival. That is almost always the oldest, largest chicks. Anyway, back to hatching. It's no easy thing for a chick to break free from its calcareous prison. By the time of hatching, the chick's contorted body fills most of the egg. It has rotated so that the head is at the blunt end of the egg, at least for species with asymmetrical eggs. Remember that there's an air pocket at one end of the egg.

The air pocket expanded as the albumin was absorbed by the growing chick. You've probably seen the air pocket in a hard-boiled egg, where there's that space between the shell and the egg white. Now, just before hatching, the baby bird punctures the membrane containing the pocket of It takes its first breath. The sweet, sweet oxygen in that breath is just enough to energize our little dude so that it can break the shell. The chick has a tool for cracking the shell from the inside, an egg tooth. This is a tiny horn-like structure on the upper part of the beak.

It's not a real tooth like the ones in the stinky mouths of reptiles and mammals, and at some point, after hatching, the egg tooth will fall off or get reabsorbed. Hammering against the shell, the chick eventually makes a small hole. It calls to its parents, and they stand attentively nearby. The process of pipping has begun. That's what this stage is called, pipping. The chick makes a series of holes, perforating the shell with its egg tooth until, at last, A section of the shell falls away.

The escape hatch is open. You're free! We know from commercials and cartoons that a baby bird is all fluffy, bright-eyed, and smiling the moment it hatches. There's no blood or slimy mess. Sometimes the chick even has a piece of eggshell on its head, like it's wearing an adorable little hat. In the real world, a freshly hatched bird is usually exhausted, and bedraggled. It just spent a lot of energy breaking free. It's still wet with goo as it flops out of the shell. But the incredible three-dimensional egg has done its job. It was a life

support system that provided the food, water, air, and protection the baby bird needed to get a good start in life. Welcome to the world, little buddy. Rest up while you can because it's going to be a wild ride. That was a lot of information, wasn't it? I hope this episode helped you understand eggs a little better. If you'd like to dig deeper and learn more about eggs, I'll put a link to a book I recommend in the show notes. A deep thank you to all my wonderful supporters on Patreon.

You're helping me to keep churning out new episodes like this. And here's a shout out to the newest members of my Patreon community. Mauricio Ugarte Lewis, Jennifer Montoya, Brian Campbell, and Annette Madjucha. You guys rock. Thank you so much. If this has become one of your favorite podcasts and you're not already a patron, consider having a look at my Patreon page. Over at patreon.com slash science of birds.

That's patreon.com slash science of birds. Science of Birds. I like to get emails from my listeners, and I do my best to answer them all. So shoot me a message if you have something to share. Like your thoughts about the show, or a better analogy for the egg, better than a camper van. Whatever it is, send your emails to Ivan at scienceofbirds.com. You can check out the show notes for this episode, which is number 54, or on the Science of Birds website, Scienceofbirds.com. I'm Ivan Philipson, and I'll catch you in the next episode. Cheers.