Extreme Bird Eggs, pt. 1

Eggs! We’re all familiar with them, and they’re a crucial part of the biology of most animals, including every single one of the 10,000 or so species of birds alive today. Despite their commonality, the bird egg is still an incredible structure, and every bird has their unique egg. Among all these eggs, a few have gone to extremes in terms of color, size, or shape, creating some very unusual eggs indeed. Here are a few of them!

What better place to start than with probably the most famous eggs on the list, those of the ostrich? For most of history, it was thought that the ostrich was only one species, Struthio camelus. As of 2014, however, the species was split into two with the introduction of S. molybdophanes, the Somali ostrich. The information in this article is directed specifically at S. camelus, now known as the common ostrich, but much of it can apply to the Somali species as well. 

A female common ostrich (Struthio camelus) at the National Zoo in Washington, D.C., who is capable of producing the largest eggs of any modern bird. Image by Donovan Snell.

An ostrich egg with a human hand for scale. Despite how big it is, this egg is actually a bit smaller than what one would expect for a bird of this size. Photo by the Smithsonian Institute.

As most readers probably already know, the ostrich lays the largest egg of any modern bird. Including extinct birds, the ostrich egg is only surpassed in size by the now-extinct “elephant birds,” two (or possibly three, depending on who you ask) genera of large ratites that inhabited Madagascar until around 1200 CE or so, and the largest of the moas, a diverse group of flightless ratites that once lived in New Zealand. In fact, many sources claim that the egg of the ostrich is the largest individual cell of any animal, however, this is debatable and the title likely belongs to the egg cell of the whale shark instead. Healthier, larger females tend to lay larger eggs, as is true of any bird, and thus exceptionally large eggs can weigh almost four and a half lbs, though on average eggs are closer to around 3 lbs and, in volume, equal to about 24 chicken eggs. Despite its large size, the ostrich actually lays the smallest egg of any bird proportional to body size, equivalent to only about 1-2% of the female’s body weight. This follows a general trend in birds for larger species to lay proportionally smaller eggs, and vice versa. 

During the breeding season, the male ostrich will mate with a number of females but forms only a strong bond to the most dominant female in the flock. The male constructs a modest but large, around 9 to 10 feet in diameter, nest in sandy soil, where his ladies then come in to lay their eggs. Less dominant females lay their eggs first and lay them closer to the outside of the nest, where they are both more susceptible to predation and less likely to receive as much warmth from the brooding parent. The bonded female, the most dominant, is the last to lay, placing her eggs in the center of the nest and often pushing out some eggs from the other females. This whole process results in a clutch of around 20 or so eggs.

The male and the bonded female take turns incubating the clutch, with the female tending to sit on the eggs during the day with the male taking the night shift. This is likely an effort to try and conceal the nest a bit more, with the female’s duller feathers being more camouflaged during the day whereas the black feathers of the male are more apt for nocturnal nesting. The ostrich has an unusually short incubation period for a ratite at only 40 or so days, which is likely an anti-predator adaptation. Ostriches supposedly live in a habitat with more potential egg-thieves than other large ratites like emus and rheas and thus it’s optimal for the eggs to hatch as soon as possible. 

Another anti-predator adaptation is the remarkably thick shell of the ostrich egg, which can reach up to 1.6 mm thick. Few predators are able to crack through this tough armor. It also helps protect the eggs from the immense weight of the parent when they’re brooding on them. This thick shell has also been useful for many human cultures both past and present who use the ostrich egg as an artistic medium. Carved and painted ostrich eggs are known from ancient cultures across the Mediterranean, continued to be popular in Europe throughout the Middle Ages and the Renaissance (becoming quite popular in reliquaries), and the art is still practiced by some skilled artisans to this day. Interestingly, genetic evidence from many carved ostrich eggs from medieval Europe implies that these eggs were collected from the wild from various separate populations instead of coming from captive birds.

The smallest egg of any bird is more difficult to determine. It certainly belongs to some kind of hummingbird, but which exact species holds the title is disputed. The reproductive details of many hummingbird species are poorly known and, within many species, there is much variation in egg size, making a true “smallest egg” hard to determine. The bee hummingbird (Mellisuga helenae), the smallest bird known from both the modern and fossil record, is often cited as having the smallest bird egg, but this is dubious. They do likely have the shortest egg of any bird, measuring a mere 0.25 inches and weighing around half a gram. But in terms of weight, the Guinness Book of World Records lists the vervain hummingbird (Mellisuga minima), also known as the Jamaican emerald hummingbird, as having a slightly longer yet lighter egg, measuring 0.39 inches but weighing 0.365 grams. There is a known record of a captive canary laying an exceptionally small egg, weighing only 0.027 grams, but for the sake of this article, the vervain hummingbird will be treated as having the smallest egg of any bird. 

As is typical of all hummingbirds, the vervain lays two white, elongated eggs in an adorably petite nest. The nest, again, typical of a hummingbird, is constructed from soft material, such as mammal fur or downy plant fibers and surrounded in a shell of moss. Finally, it is anchored down to a branch via spider’s silk. The female broods and raises the chicks alone, an endeavor that, in total, is usually over in less than two months. Despite how small the eggs are, they are proportionally rather large, usually around 13% of the mother’s body weight. 

The most extreme egg of perhaps any bird belongs to one of the strangest. The kiwis (Apteryx) are a genus of five small, nocturnal ratites from New Zealand that are typically cited as having the largest egg proportional to body size of any bird.

An ancient Mycenaean rhyton (a container for liquids) made from an ostrich egg, carved with some kind of fish or cetacean on it. Image by Schuppi on Wikimedia Commons. 

The tiny bee hummingbird (left), the smallest dinosaur to have ever lived. However, the smallest eggs likely belong their close relative, the vervain hummingbird (right). Left image by Ekaterina Chernetsova on Wikimedia Commons and right image by ZankaM on Wikimedia Commons.

The minute nest of the vervain hummingbird. Image by Claude Fletcher on Flickr.

The strange little pseudo-mammals that are the kiwis are the gifts that keep on giving in the bird world. At left, an illustration of a dissected southern brown kiwi (Apteryx australis), and at right, a little spotted kiwi (Apteryx owenii), the smallest of the kiwis, foraging. Left image by James Erxleben and right image by Judi L. Miller on Wikimedia Commons.

Which exact species has the largest proportional egg is up for some debate, with some sources citing it as the southern brown kiwi (Apteryx australis) and others claiming the honor goes to the little spotted kiwi ((Apteryx owenii). Semantics aside, reproduction is pretty similar across the five species, and thus most of the information in this article is about kiwis in general and not any one particular species. Depending on the species, the egg of the kiwi is somewhere between 18-25% of the body weight of the mother. To put that into perspective, despite the kiwi being around the same size as a domestic chicken, they lay an egg that is six times larger. 

The size of the kiwi’s egg is something that can really only be appreciated in a photo. This x-ray of a gravid female, close to laying, demonstrates how extreme this egg really is. Image by the Otorohanga Kiwi House.

Due to how strange the egg itself is, other unusual aspects of kiwi reproduction are often overlooked. Unusually for a ratite, kiwis are highly monogamous, with pair bonds that may last for decades. They lay their eggs in an underground burrow and, besides a few birds of prey, they are the only bird with a functioning pair of ovaries. In most birds, only the left ovary produces eggs, with the right one being largely useless. The male incubates the egg after it is laid for up to 92 days, one of the longest incubation periods of any bird, only surpassed in time by a few species of albatross. The chicks are highly precocial; they are fully feathered and able to see and walk right after hatching. The large yolk of the egg, which can take up to 60% of its volume, is so nutritious and energy-rich that it can feed the chick for several weeks after hatching. 

At only a day old, this very cute kiwi chick looks essentially like a miniature version of the adult, which is very unusual in birds. Image by Mike Dickison at the West Coast Wildlife Centre on Wikimedia Commons.

So why does the kiwi have this massive egg in the first place? The original hypothesis was that this large egg was just an evolutionary holdover from the kiwi’s larger, moa-like ancestors. This idea has come under much scrutiny. Producing this massive egg takes up a lot of the female’s energy, forcing her to have to eat three times the amount of food she would normally during the thirty or so days the egg is inside of her. During the final few days before the egg is laid, however, she completely fasts. It’s possible that this is more of a necessity than a choice, as it is believed that, at that point, the egg has grown so large that it crushes part of her digestive system and she is temporarily unable to process food. Clearly, this trait has a lot of disadvantages, which implies it must have some kind of important function. The final nail in the coffin for the holdover hypothesis was some recent investigation into the evolutionary history of ratites, which revealed that kiwis almost certainly never directly descended from particularly large ancestors. Assuming the egg is functional, given, again, the obvious disadvantages of this reproductive method, its function must be something pretty important. The primary proposal now is that it’s an adaptation for the unique situation the kiwi is in regarding predators. Prior to the introduction of invasive species like feral cats and stoats, New Zealand had no terrestrial predators, with the only threats to kiwis coming from flying, predatory birds. For most birds, the egg is a highly vulnerable point in their life cycle, but without any ground predators that would enter their burrows, the kiwi can maximize the time the chick spends in the egg, allowing them to grow big, fast, and pumped full of nutrients from the large yolk, to make their time as a chick as short and safe as possible. When the chick leaves the burrow, they are more vulnerable than they were as an egg since they now have to worry about aerial predators. Essentially, having a big egg means the chick can be as precocial as possible, meaning they are more able to escape predators that don’t target eggs but do target chicks. An interesting theory!

So far, we’ve only looked at bird eggs with record-breaking sizes, but, in some species, shape can also become quite odd. The common murre (Uria aalge), also known as the common guillemot, is a large type of auk native to the polar regions of the northern hemisphere. They have some of the most variations in their eggs of any bird, with each egg having its own unique pattern of spots and splotches.

At left, a common murre, and at right, a small selection of the eggs they make. Note not only all the differences in color and patterning but also their unusual, pointed shape. Left image by Mike Pennington on Wikimedia Commons and right image by Didier Descouens on Wikimedia Commons.

Like many seabirds, murres reproduce and brood in large, cliffside colonies that can contain thousands upon thousands of eggs. The fact that each egg appears unique is likely the point- it’s believed that the differences between all the eggs help the parents find their egg among the masses of others. Common murres are monogamous and lay one relatively large egg, around 11% of the female’s body weight, without really building much of a nest. Instead, the female simply lays her egg on the rocky outcropping of a cliffside, with both parents brooding on the egg and assisting in the rearing of the chick. More unusual than their variability is the odd shape of the murre egg. Murre eggs are described as “pyriform,” with one end being quite rounded and the other being much pointier. The traditional explanation for this, around since the 1800s, is that if the eggs were to be blown by a gust of wind or accidentally pushed by the wing of a neighbor murre, the egg would spin on its axis and thus would be less likely to fall off of the cliff. However, there is no real evidence to support this claim and tests have shown this idea to be unlikely. 

The mystery still hasn’t quite been cracked (egg pun!) with many different hypotheses attempting to explain the pyriform shape. All-around murre expert Tim Birkhead has suggested two possible explanations for why the egg is this shape. First, the pointed shape of the murre makes it so that more of the egg touches the ground than a regular, egg-shaped egg. In those parts that do touch the ground, the eggshell is thickened and reinforced. As stated, murres nest in absurdly populous and crowded colonies, with potentially as many as 40 birds crammed into one square meter. The risk that one of your neighbors might accidentally trample your egg seems rather high. It’s possible that, by having more of the egg touch the ground, the egg is made stronger and more able to dissipate the force of another murre stepping on it.

The second part of his theory is that, despite how more of the egg touches the ground, the more rounded end actually becomes more raised in the pyriform condition. Most of the pores in the shell are also concentrated at this end of the egg. Raising the rounded end of the egg prevents the pores from getting clogged up and contaminated with the obscene amounts of guano produced by these colonies. It has also been independently proposed that the pyriform shape evolved as a way of keeping the egg large while also keeping the body of the gravid female streamlined, so she can continue to swim and hunt easily despite having a large, cumbersome egg inside her. 

A male Somali ostrich incubating his eggs. Note how the outside eggs, almost certainly from a less dominant female, are not fully covered by his body and thus not receiving as much heat. Photo by Christophe Eyquem on Wikimedia Commons.

The crowded conditions of a common murre colony, such as this one at Oregon Islands National Wildlife Refuge, cannot be overstated. This may have been a driver in the evolution of the pyriform egg. Photo by Roy W. Lowe on Wikimedia Commons.

For a swimming murre, such as this one, having a streamlined shape is important for catching prey and escaping aquatic predators. This could be a factor in their pyriform eggs. Image by Christopher Woo.

This article was intended to be much shorter than it actually ended up being, so stay tuned for part two! In the next part, we’ll explore birds with oddly colored and surprisingly conspicuous eggs. 

Sources

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“How Fast is an Ostrich? And More Fun Facts.” Smithsonian’s National Zoo and Conservation Biology Institute. March 3, 2022. https://nationalzoo.si.edu/animals/news/how-fast-ostrich-and-more-fun-facts

Harrison, Colin and Alan Greensmith. Birds of the World. Dorling Kindersley, 1993. https://archive.org/details/birdsofworld0000harr/mode/2up

“Smallest Bird Egg.” Guinness World Records. Accessed June 29, 2023. https://www.guinnessworldrecords.com/world-records/smallest-bird-egg/?fb_comment_id=570243936412137_695830443853485

“Largest Bird Egg Relative to Body Size.” Guinness World Records. Accessed June 29, 2023. https://www.guinnessworldrecords.com/world-records/83111-largest-bird-egg-relative-to-body-size

Kinsley, F. C. “The consistent presence of paired ovaries in the Kiwi (Apteryx) with some discussion of this condition in other birds.” Journal für Ornithologie, 1971. https://link.springer.com/article/10.1007/BF01640692

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Mitchel, J. Kieren et al. “Ancient DNA reveals elephant bird and kiwi are sister taxa and clarifies ratite bird evolution.” Science, 2014. https://www.science.org/doi/10.1126/science.1251981

Birkhead, R. Tim, Jamie E. Thompson, and Robert Montgomerie. “Pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces.” The Auk, 2018. https://academic.oup.com/auk/article/135/4/1020/5149021?login=false