A prodigious cover for a perfect machine

The egg – and specifically its shell – is one of the most extraordinary works of biological engineering that nature gives us. On one hand, it must be strong enough to support the weight of the incubating bird, on the other hand, it must be fragile enough to allow the chick to break it from the inside when hatching. And all this while little by little the shell thins as the embryo uses shell’s calcium to form its own skeleton.

Moreover, it must isolate the richness in nutrients that it possesses from the external attack of bacteria and fungi that would destroy the embryo. And it has to do so without impeding the gas exchange essential for the development of the future chick. Besides, the color of the cover and the shape – more or less elliptical – play key functions in the survival of the different species of bird.

The egg is the biological structure that contains and covers the ovule and that supplies the nutrients that will make the development of the embryo possible until the moment when it breaks the shell and begins the independent life as a chick.

To produce an egg, the ovule is surrounded by the yolk in the bird’s own ovary, from there, as it descends through the reproductive system, it will superimpose new layers such as the yolk membrane that covers the yolk, the white (fluid part and dense part) , the membranes that separate the latter from the shell and finally the calcareous cover itself that will incorporate different pigments that will give the final hue to the egg (the absence of pigment will result in white eggs).

Figure 1: Process of egg formation

The pigmentation of the shell is determined by the bird’s genetics and not by its diet, contrary to the yolk whose color is dependent on the greater or lesser presence of carotenes in the female’s diet.

Figure 2: Shell color depends on genetics of the bird

The color variety presented by the eggs shells is so wide that since ancient times it has been the object of desire by collectors and scholars.

Figure 3: Shapes and hues, all have a reason

But beyond the aesthetic value that for some they may have, there must be some evolutionary reason that causes the tones to oscillate from white to mottled through the bright blue of some of them. In nature, nothing is done for its own sake, so if birds invest resources in coloring their eggs, there must be good reasons for it.

There are two pigments that give color to birds’ eggs: protoporphyrin IX and biliverdin. The first is responsible for the brick hue – typical of brown chicken eggs that we all know – and the second gives a blue-green color (biliverdin is a derivative of the heme group, the same pigment that gives greenish color to bruises). The absence of pigment will result in the presence of white eggs. The combinations of all these elements give rise to the enormous panoply of tones and patterns that we find in the setting of the different birds.

Most birds that lay in hidden places (a tree hole or a gallery of dirt) lay white eggs. Why invest in expensive pigments when there is no need for camouflage? Reptile eggs are also white as they hatch underground or under piles of vegetation. As every rule, it presents its exceptions, so the ostrich egg is incubated in a nest on the surface. The reason for the absence of pigment in this case is that a colored egg under the African sun would reach temperatures that would kill the embryo. The white color reflects light and keeps temperature at a low level.

Figure 4: If eggs are hidden why to invest on pigments. 

On the other hand, we have eggs that present a little less than an infinite variety of specks. In some cases, this color is due to the need to camouflage the setting of possible predators, such as the plover’s or the whistling puffin’s.

 Figure 5: Perfect camouflage


Another reason can be found in the need to recognize the egg itself. This is needed in areas where thousands and thousands of birds nest. This is the case of the common murre (Uria aalge) also known as the Galician penguin due to its black and white plumage, although it has nothing to do with the Antarctic bird. Studies carried out by Tim Birkhead show that after laying the egg, the female spends a few minutes observing it before incubating it. The researcher concludes that this time is used to memorize the pattern of the egg so that the bird does recognize it from others, on a cliff populated by thousands of other birds.

Curiously, some eggs have a color that makes them very conspicuous, perfectly distinguishable and showy. This coloration seems to defy evolutionary laws themselves since these eggs, being easily visible, would be more exposed to predators. There are various theories that explain this fact, the most accepted postulates that, since the eggs are so visible, the male of the couple is forced to have a more active participation in their care, incubation, as well as in the feeding of the female. That is, the risk of this color itself becomes a competitive advantage by having the two members of the couple involved in their surveillance and breeding.

Figure 6: Sometimes the best strategy is to lay conspicuous eggs

Another explanation, which does not exclude the previous one, refers to the protection that this colored shell provides against ultraviolet radiation, that is, the color would act as a shield that would protect the embryo from this radiation. An additional hypothesis suggests that a color that stands out is a sign of a strong female, with good reproductive capacity, which would make her more attractive as a couple.

Finally, it should be noted that some birds modify the speckling of the eggs to avoid some parasitic birds. The cuckoo lays its egg in the nest of other birds, in such a way that, when it hatches, it discards the eggs of the true owners of the nest and is fed by them until it starts to fly.

Figure 7: Egg specks can help to spot parasitic birds who lay on the same nest

To avoid this, the birds susceptible to being parasitized modify the mottling of their laying, but the cuckoo also adapts it to these changes, so that a real endless race is established.

Another interesting aspect of the calcareous cover of the egg is that, in the same laying, not all the eggs have the same tones. Usually the last egg (and sometimes the first) are lighter in color.

Much has been speculated on the causes of this chromatic variation. Some experts point out that the different pigmentation serves to distinguish the own egg from the one that a parasitic bird could lay at any given time. The most accepted theory today, however, postulates that it helps the female to know which egg was the last to be laid. In this way you can arrange the arrangement of the eggs in the nest so that the one that has been laid last is in the central area of ​​the nest, which is where heat is best kept, so that, even if laid later, it is it develops faster than its brothers and the hatching of all the eggs is synchronized.

Figure 8: Depending on laying order, eggs change their tonalities a little

The wonders of the calcareous cover of the egg do not end here. As we pointed out at the beginning of the article, the eggs must allow the embryo to exchange gases. For this purpose, the shell has microscopic pores that will allow the transit of oxygen to the interior and the exit of CO2 and water vapor to the exterior. A chicken egg has 10,000 pores and an emu egg has 30,000. Studies carried out by Dr. Rahn of the University of Buffalo confirm that as the height at which a bird lays increases, the porous surface of the egg decreases so that water loss also decreases. With height, the atmospheric pressure decreases, if the steam output is not modified, the egg will dehydrate. The author explains that birds that nest in height lay larger eggs, thus reducing the surface area in relation to the total volume and thus increasing the total amount of water present inside the egg.

Another amazing characteristic of the shells of some species is their self-cleaning capacity. Some birds lay in very steep places, in which thousands of them compete for a very limited space, for example, on a cliff. In these circumstances, it is very easy for the egg to become stained with feces. Well, evolution has endowed the eggs of these birds with the ability to clean themselves: the calcium carbonate in the shell is assembled in the shape of small conical structures that increase the surface tension, in this way, the water does not wet the egg but form small droplets on the shell that will roll down cleaning it. In addition, these structures increase friction and prevent the egg of the common murre from slipping and tumbling down rocks.

Another aspect that draws attention to this point that concerns us today is the surprising variability in the shape of the eggs of different birds. Thus we have the almost perfectly spherical egg of the eagle owl or ostrich, compared to slightly elliptical ones such as that of the common hen or the practically conical and asymmetrical ones such as that of the common murre.

Figure 9-10 Eggs shapes are determined by the flying ability of the birds

Many have been the hypotheses that have been formulated on the causes of these morphological differences: some pointed to the need to turn on themselves in a small circle of some eggs of birds that lay on cliffs or vertical walls with almost no space for the egg not fall into the void. Others suggest that the more elliptical shape maximizes the contact surface with the bird that incubates the eggs.

A study published in the journal Science barely a year ago in which more than 50,000 eggs of some 1,400 species were studied indicates that those birds most adapted for a more powerful flight sacrifice abdominal volume in order to have a greater muscle mass that allows them a longer, vigorous flight. This lower abdominal capacity forces the egg to adapt to this limitation, which will lead to the laying of more elliptical and asymmetric eggs – almost conical in some cases. Although this rule has exceptions, it does seem that there is a fairly high correlation between these factors that would explain the shape of the eggs: https://science.sciencemag.org/content/356/6344/1249

We will close this article with another very interesting piece of information: it has been verified in some species – for example, in a type of quail (Colinus virginianus) that chicks, before hatching, emit sounds that alert their sister eggs that they are ready. to leave. Hearing this sound, they all prepare to hatch simultaneously. So much so, that the authors of this study reproduced the sound artificially, obtaining the same answer: the egg that was exposed to this sound hatched soon after.

It is surprising how many adaptations eggs can have to be more efficient and have a better chance of survival. They may seem like a simple biological tool, but they can be tremendously complex.

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