Birds Eggs Guide

Bird eggs guide

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There are so many types of birds and each of them lays a unique kind of egg. Understanding more about them can help to identify them as well as expand your knowledge of our feathered friends.

How Do Birds Fertilize Their Eggs?

Black-necked Stilts copulating at the edge of a California pond
Black-Necked Stilt (Himantopus mexicanus)

Before any fertilization can take place, the female must find a mate. Males are always vying for the attention of the females using various techniques such as nest building, showy dances, bringing food, and so many more. This, of course, varies according to the particular bird species.

Birds mate in a very different way to humans and other mammals. Each bird has a body part known as the cloaca, which contains the bird’s reproductive organs. The testes of the male can expand up to 1000 times when mating season comes around as his hormones run wild.

When it’s time to mate, the birds will join their cloacas together, and the male will shoot sperm into the female. This sperm travels to the egg, and it is possible for the female to ‘hold’ the sperm until she is ready to reproduce, a trait also seen in many reptiles.

The sperm will then penetrate through the egg, and fertilization occurs. After this, the eggshell develops before the chick.

Anatomy of a Bird Egg

Anatomy of a bird egg

We really only ever see what goes on on the outside of a bird egg. But what’s taking place inside? The anatomy of a bird egg is truly fascinating so let’s take a closer look.


The shell is the outermost part of the bird egg and what’s amazing is that it is covered in tiny pores; some eggs can have up to 17,000! The shell is made from calcium, and it’s actually possible for moisture and air to pass through to the inside. However, the shell has a thin outer layer called the cuticle that stops any bacteria or dirt getting through.


With a bird egg, there are inner and outer membranes that are made from protein and are largely see-through. The purpose of these membranes is to stop bacteria from getting into the egg, and while they don’t look like much, they’re incredibly robust thanks to their high keratin content.

Air Cell

Inside the egg, there is a small air space which is created once the egg is laid and the inner content cools down. This air cell is normally between the two membranes at the larger end of the egg. As the egg gets older, this air cell gets bigger.


You would normally call the albumen the egg white, and this is a layered system with four layers packed with proteins as well as water. Its main purpose is to protect the yolk.


The main reason for the yolk is as a way of ensuring that the chick receives nutrients while inside the egg. They’re usually round in shape as floating in the albumen.

The yolk does not have as much water as the egg white but has far more protein, fat, and vitamins plus other essential minerals. You’ll normally find that they are yellow in color but some can be much darker orange if they contain higher levels of fat.

Amniotic Liquid

The chick is surrounded by amniotic fluid, very similar to what would be seen in human pregnancy. This provides the embryo with protection and cushions them against bumps.

Allantoic Fluid

The allantoic fluid sits inside a sac and its main purpose is to help with excretion and nutrition. This reservoir is used to contain any nitrogen waste as well as ensuring a good oxygen supply to the chick which comes in through the shell.

Egg Size

Ostrich egg
Ostrich Egg

Bird eggs come in an array of different sizes depending on the species. The biggest bird eggs come from ostriches which happen to be the largest type of bird in the world. Typically, these eggs weigh around 3 lb, but the largest recorded weighed in at a whopping 5 lb 11.36 oz!

On the other end of the scale is the hummingbird which lays the smallest egg. In most cases, they don’t measure more than an inch and weigh only a third of a gram.

For some birds, the eggs are quite large compared to the size of the female. Take the kiwi (Apteryx), for example, that lays an egg up to 15% the size of her own body. The reason for this is so that the babies are born larger and can become independent more quickly.

Egg Shape

The shape of bird eggs is just as diverse as the range of sizes.

The shape of bird eggs is just as diverse as the range of sizes. Some eggs have a teardrop or peardrop shape. In days gone by, it was believed that this was to prevent the eggs from rolling away and while this may be the case, scientists now believe that it may have something to do with the flying ability of the species. The pointier the egg, the more streamlined the bird and therefore the better it flies.

However, there are also birds such as budgerigars and owls that lay more circular-shaped eggs.

Egg Color & Markings

American robins eggs
American Robin’s Nest

If you look at bird eggs, you will notice that the markings and colors vary dramatically. The most common color is white, and this is what you would describe as the natural color of a bird egg owing to the presence of calcium carbonate. But there are very few birds, aside from things like the blue-throated hummingbird and the Eurasian collared dove, that lay all white eggs. But what are the reasons for the varying colors?


The color of a bird egg could help to protect it from UV light which could be damaging to the embryo inside.

What’s interesting is that colored eggs also tend to be stronger. When a bird doesn’t have high enough calcium levels, the shell will be thinner which makes it more likely to break. The addition of color can improve this.

Number of Clutches

Something really fascinating is that it has been noted that birds that have more than one clutch per season tend to have more intensely colored eggs with each clutch. For example, the second and third clutches will be far more pigmented than the first owing to the lower levels of calcium from the female.


It is essential that eggs are well hidden from predators, especially for birds that nest on or near to the ground. For this reason, these eggs may have different coloration and patterns to keep them camouflaged. But once you move up into the trees, you will notice that these dull colors turn brighter with tree birds having greenish or blue colored eggs.


If the eggs are not exposed to light, then there is more chance that they will be white. This applies to birds who keep their eggs out of sight or those that remain sitting on the eggs at all time.

Clutch Size

Cardinal's eggs
Cardinal Bird Eggs

Much like other aspects of bird eggs, the number in each clutch will vary between species. There are some birds, like the laysan albatross that only lay one egg each time, whereas other birds can lay a clutch of indefinite numbers.

Scientists created an experiment in which they removed eggs from a nest once they have been laid. Surprisingly, many birds including the house sparrow, continued to lay new eggs to replace the ones that had been taken. This shows that some species will be ‘required’ to lay a specific amount which scientists believe is in relation to the size of their brood patch.

Incubation Period

Gentoo penguin (Pygoscelis papua) incubating eggs
Gentoo Penguins (Pygoscelis papua) Incubate Thier Eggs for 34 to 37 Days

How long the eggs take to hatch largely depends on the size of the species. Some smaller birds’ eggs may only incubate for around ten to fifteen days, while larger birds could take up to a month. Of course, there are exceptions to this rule.

Wandering albatross eggs can take an incredible 85 days to hatch while the white eye zosterops hatches in just nine days!

The downy woodpecker (Picoides pubescens) is one of the most common garden birds in the USA, and these can take around three weeks to hatch owing to their slightly larger size. Look at most of the common songbirds in your yard, and you’ll see that they don’t usually take longer than two weeks to hatch.

How Does a Bird Hatch from an Egg?

How does a bird hatch from an egg?

If you have ever been lucky enough to watch a chick emerge from the egg, you’ll know what a special sight this is. But how does this process happen? When it’s time for the chick to hatch, they have to expend a surprising amount of effort since the egg shell is incredibly tough.

They have what is known as an egg tooth at the tip of their beak which they use to chip away at the egg from the inside. Depending on the bird, this could take anywhere from a couple of hours to a few days. Once the chick is born, this egg tooth will typically fall away within the course of a few weeks.

What Time of Year Do Birds Lay Their Eggs?

What time of year do birds lay their eggs?

Many birds will lay their eggs during the spring. However, this isn’t a hard and fast rule and can be affected by many factors.

For example, while September is the month where the least birds lay their eggs, there are still some that do so, such as the American goldfinch (Spinus tristis) and the pine siskin (Spinus pinus).

You also have to consider that a mating pair will not breed until they have found the perfect location. The pair will find a suitable location and begin building a nest together. It is only after this that they will begin to lay their clutch.

There are several factors that determine when a bird will lay, including the length of the day which can affect hormones as well as the fitness of the bird and the latitude. You see, birds in more northerly regions tend to have less time to breed compared to lower latitude birds.

Unique Bird Eggs

With such a wide variety of bird eggs in terms of shape, size, and color, it’s not surprising that some stand out more than others. Let’s explore some of the most remarkable and unique bird eggs.

1. Ostrich

Ostrich eggs

The ostrich egg is the biggest bird egg on the planet, and that’s what makes it so special. Compared to the bee hummingbird (Mellisuga helenae) whose eggs are tiny, the ostrich egg weighs the same as 4700 of these small eggs!

Ostrich eggs are creamy colored with a glossy texture and are pitted across the entire shell. The females lay in a communal nest, although the dominant birds will remove eggs from weaker birds if there is not enough space. They have quite a long incubation period of around 35 to 45 days.

While the ostrich egg is the biggest, in relation to the size of the mother, it’s actually the smallest. These birds are found in Africa and humans have long used the eggs for decorative purposes.

2. Hummingbird

Hummingbird eggs

While the ostrich has the largest egg, the hummingbird boasts the smallest. While there are several different types of hummingbird, none lay very large eggs. However, the bee hummingbird lays an egg that is no larger than a coffee bean; now that’s tiny!

Usually, hummingbirds will only lay a small clutch of around two eggs at a time. They are white in color with no markings and the females guard them through the entire incubation period of up to 18 days.

3. Emu

Emu egg

Like the ostrich, the emu is a flightless bird that is found primarily in Western Australia, although they also used to be prevalent on the east side of the country. What’s so unique about the emu egg is that it is the second largest. They can be up to five inches in length and weigh as much as three pounds.

Emu eggs take a very long time to hatch; often around 50 days. But that doesn’t stop these birds from breeding at a shocking rate, with most females laying around 40 to 50 eggs every year! The eggs have a unique coloration that can be anywhere between green and black, which is likely why they’re also used as decorative items.

4. Killdeer

killdeer's eggs

The killdeer is a shorebird that lays its eggs in a scratch nest lined with things like twigs and pebbles. To deter predators, the parents will act injured, but they also have perfectly colored eggs that help them to camouflage in the ground. They have a unique black pattern on a whitish-colored egg that makes them look like the pebbles within the nest. What’s more, this blends in with the color of the adult; they’re the ultimate hiders!

These birds are common across North America in coastal areas, and the eggs usually take up to 28 days to hatch.

5. American Robin

American Robin eggs

Looking at the eggs of the American robin, one of the first things you notice is the color. They are bright blue with black speckled markings. Although these eggs can range in color from green to purple as well.

These brightly colored eggs take their hues from a pigment called biliverdin which is secreted in the shell gland of the females. What’s more, the females will lay up to five clutches every year, starting early in spring and the eggs are so unique that they even have their own color named after them – robin egg blue!

6. Guillemot

Guillemot's eggs

The guillemot does not create a nest but lays a single egg on a cliff edge which has to incubate for 30 days. Sounds risky, right? But the pointed, pear shape of these eggs was long thought to have been a way of ensuring that they did not roll off into the sea. However, this has now been disputed.

There is now a suggestion, that while the shape does ensure the eggs don’t roll off the cliff, it also makes them more robust and so less likely to sustain damage from impact. Moreover, since these birds tend to poop a lot, the shape is thought to prevent contamination from excrement.

What’s more, it’s very unlikely that the eggs would ever be allowed to roll off since the guillemot guards it very carefully for the full incubation period.

7. Southern Cassowary

Southern Cassowary eggs

Another egg that is famed for its color is the green egg of the southern cassowary. These large birds are native to parts of Australia and Papua New Guinea as well as Indonesia.

The green speckled eggs contain the same pigment as the robin’s egg which is what gives them their deep color. The reason for this is that the southern cassowary lives in tropical forests, and the green eggs are easily camouflaged in this environment.

The females will lay three to five eggs per clutch, but it is the males that carefully protect them for the lengthy 50-day incubation period.

8. Kiwi

Kiwi has the largest egg in comparison to the size of the mother

The kiwi lays the largest egg in comparison to the size of the mother. These birds lay eggs that are up to 15-30% of their own body size and contain chicks that are much more developed at birth. Since they are flightless birds, this is important in becoming independent more quickly.

The kiwi is native to the forests of New Zealand and can live for up to 30 years in the wild, with females laying a single egg every year. But the incubation period is very long, up to 85 days, and it’s thanks to a large yolk that this is possible.

9. Great Tinamou

Great Tinamou eggs
Andres Cuervo / Flickr / CC BY 2.0

The great tinamou can be found in South and Central America, and there are around 47 species. What’s fascinating is that the females may produce up to five clutches every mating season, all from different males.

Once the bright green eggs hatch, which allows for better camouflage in their tropical environment, it is the males that will take care of them for the following three-week incubation period. But what’s really special is the iridescent tinge to the eggs meaning they change color and appearance depending on what angle you look at them from.

10. African Jacana

African jacana nest wth eggs

As you can probably guess from the name, the African jacana is found in Africa, sub-Saharan Africa, to be exact. This bird lays eggs that one might consider more of a work of art than an egg. They are cream in color with black markings that look similar to drizzled paint.

It is the males that take care of the eggs after they have been laid with females moving on to find more males to breed with. She has nothing to do with the raising of her chicks. The males build floating nests and slide the roughly four eggs per clutch under their wings while they incubate.

11. King Penguin

King Penguin incubating egg

The king penguin is largely found in Antarctica, but also found in the Falkland Islands and is the smaller cousin of the emperor penguin. But what makes their eggs so special? Well, it’s not the eggs as such but how the males incubate them.

Since the ground is so cold and there isn’t much to make a nest from, these birds will lay their eggs on the ground, and the males then quickly put them on their feet to keep them warm. They will pass the egg over to the female for a ‘shift’ which can last up to 18 days!

The egg is pale white in color, so has no especially amazing markings, and they take around 54 days to incubate. Even once the chick emerges, they will continue to huddle on their parents’ feet for the first few weeks of life.

Frequently Asked Questions

Nest with eggs and hatchlings

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