Birds are a fascinating animal species, and they have a lot of exciting features, songs, and behaviors.
Even with our advanced technologies, there is still a lot we have to learn about birds, their habits, and more of their mysterious lives.
One of these mysteries is the color of eggs, especially blue eggs. Most eggs are white or with spots, so you must be shocked when you come across blue eggs in the forest. So let’s get into the details and see just why these eggs are blue and the birds that lay them;
What Birds Lay Blue Eggs
There is a long list of birds with blue eggs, but the most common ones are the American robins, blackbirds, song thrush, Dunnock, starlings, magpies, and house finches.
The depth of this blue color varies between the species, and some are dark blue while others are light blue and almost white. Even birds in the same group have different shades of the eggs, but they all have blue eggs.
What Makes The Eggs Blue?
Humans are naturally curious, so you have to be dying to know about what makes these eggs blue.
Understanding the reasons behind everything in nature is a good way for us to understand and make advancements in various fields so let’s get into it;
Most birds in the thrush family lay blue eggs. The eggs are blue because of a pigment called Biliverdin. The birds secrete this pigment, which is added to the eggshell while developing in the bird.
Just like humans have varying levels of hormones and body fluids, the birds have a different concentration of this bile pigment. The higher the concentration in a bird, the more Biliverdin will be deposited in the eggshell.
This means that the eggshells come out with varying depths. Some will have light blue eggs, while others will be dark blue like the American Robins. The shell gland is placed right before the cloaca, as the mammal’s uterus would be placed.
Smaller eggs and those laid first in the batch always have a deeper blue color since they are developed when the concentration of Biliverdin is high in the mother’s blood. Small birds like Robins all have deep blue eggs because of their tiny sizes.
Larger eggs tend to have a duller blue color since there is a limited supply of Biliverdin. They have a lighter coloration that resembles blue ice, but the first eggs in the batch are darker blue. Biliverdin is also present in humans and butterflies.
For butterflies, it is responsible for the bluish-green colors, while in humans, it causes the discoloration of wounds to bluish-green. It has strong anti-inflammatory properties, which makes it an essential part of the healing process.
Why Are The Eggs Colored Blue
While we are sure about what makes the eggs blue, scientists are still puzzled about why some birds lay blue eggs.
We know that birds tend to lay eggs that look similar to their nests or backgrounds for camouflage.
However, this idea does not hold up with blue eggs because the nests are not lined with anything blue.
If anything, the blue color makes the egg more conspicuous and more easily spotted by predators. So if this isn’t the reason, what is?
There is no exact answer for this, but scientists have come up with some convincing theories. Some of these theories are supported by the bird’s behaviors so let us look at them to see which one makes the most sense;
1. A representation of the mate quality
Some ornithologists believe that the blue color indicates the quality of the female that lays the eggs. By this theory, darker blue eggs are laid by better females.
This theory is supported by the fact that females that lay deep blue eggs are often in better physical shape than those who lay lighter blue eggs. This means that deep blue eggs indicate the female is of higher quality and is worth investing in.
This theory goes ahead to explain that deep blue eggs produce hatchlings of better quality. This is seen in the keen care and attention that the male birds give to offspring from blue eggs. The superior quality of the female leads to a better quality offspring.
Field studies support this idea since birds like the pied flycatcher deliver more food to the females that incubate deep blue eggs. Even American robin males tend to spend more time around deep blue eggs rather than light blue ones.
2. The blackmail hypothesis
Most birds that lay blue eggs nest in pairs, so they take care of the eggs together. The female is the one that incubates the eggs most of the time, but she needs to leave now and then to get food or stretch.
Some scientists believe that eggs are blue as a means to blackmail the male into being more present.
The idea is, since the eggs are brightly colored and more visible, they are more vulnerable to pests and predators.
The male will feel like the nest is more threatened since all the predators and dangers can see it as they pass overhead.
This will force the male to give additional attention when the female is away; thus, the eggs will be safer.
3. Protection from the sun
This theory says that the color has nothing to do with the quality or with the males, but it deals with the sun. It suggests that the blue color protects the embryo from UV radiation from the hot sun.
UV radiation can penetrate through a standard eggshell and harm the egg. Most members of the thrush don’t hide their nests inside crevices or tree trunks; instead, they build them in open areas. These eggs could get exposed to the sun when the parent is away.
In such events, the Biliverdin in the eggshell will help reduce the amount of UV rays that get to the bird, which will make the eggs more viable.
This seems solid, but it has one problem, increased blueness in the shell increases infrared radiation absorption by the egg.
This would warm up the eggs, and it’s not good. In the end, the egg needs to have a balance. It should not be too blue or too light, depending on the lighting and location.
As we do more tests, we learn more about the advantages of having blue eggs. Some studies suggest that Biliverdin makes the eggs tougher.
More studies give us unprecedented advantages of blue eggs, and we now understand why so many bird species evolved to lay blue eggs.
How Does Oxygen Get To The Embryo In An Egg?
The beauty of eggs is not the only fascinating thing about them; their structure and physiology are equally fascinating.
When you look at an egg, it seems to cut off from the world, but the embryo needs oxygen to develop. So, where does it get the air?
The embryo is connected to the mother for humans, and she supplies oxygen to it through the umbilical cord. Eggs have no link to their mother, so where does the embryo get its oxygen? Let us look into this;
All the other biological needed to form a chic are in the yolk and the white, but there is no oxygen supply in the egg. When you magnify an eggshell a thousand times, you will see the calcium carbonate crystals that make it up, and in the small space, you will see tiny holes.
These holes are a thousandth of an inch across; they allow air to filter into the egg, as the content of the egg is kept in.
The embryo doesn’t have working lungs, so it can’t breathe this air during its early developmental years.
A few days after fertilization, a network of blood vessels grows out of the embryo’s abdomen. This network will develop and go to the membrane of the egg. This gives the blood in the veins close contact with the membrane.
The oxygen in the membrane diffuses into the blood while the carbon dioxide from the embryo diffuses out.
This closely resembles human physiology, and when the chick is ready, it will crack the eggshell using the air in its shell.
There will be no more liquid in the egg when it’s about to hatch, so the bird cannot use diffusion. There will be a small air pocket in the eggshell, and the chick will puncture it and use it to get out of the shell.
Many bird species have blue eggs. The color is most common in birds of the thrush family, like the American Robin. The deposition of Biliverdin brings about the blue color in the eggshells during development.
The reason for the eggs becoming blue is not clear to ornithologists at this point. The bird’s behaviors support numerous convincing theories. These advantages keep growing with more research, and blue eggs make more sense to us.