This was where it all started. Or was it? Which came first, the bird or the egg? Whatever the truth is, nature has identified a strong, complex shape and design most suitable for birds. I have heard it named ‘nature’s miracle of packaging’. All avians have one thing in common: they all lay eggs; some have even been known to lay unfertilised eggs.
Although some mammals create eggs, most incubate their young ones within their bodies. This would never have worked out for birds. Birds must be very light to be able to fly. Their skeletal design is adapted for flight by being strong yet very lightweight. Their wing bones are even hollow for this reason.
Imagine if birds had to incubate their eggs inside their bodies. They wouldn’t be able to fly. They would be too heavy.
This article will examine egg formation, shapes and sizes, egg colouring, family planning, incubation and chicks breaking out of eggs. I will also give some hints on trying to identify the eggs of a bird if you are lucky enough to find a nest with eggs in it.
Birds’ sperm banks
A bird’s eggs can be fertilised in the first few minutes of a male mating with her. But if she is not ready to lay her eggs, she can delay this by a few days. She has her very own ‘sperm bank’ called sperm storage tubules. Hornbills have been known to fertilise their eggs three weeks after being sealed in their nests by the male.
Egg formation
It takes a whole day from when the eggs get fertilised to when the eggs are formed in smaller birds. In all bird species, females can only lay one egg at a time. A full day might pass before they lay a second egg, and so on. Raptors, being bigger birds, create bigger chicks. The chicks need much food. So, they will take a few days between laying eggs.
The eggs themselves are very complex structures. They house the ever-growing embryos, which get all their nutrition from the yoke.
The egg’s journey
Female birds have a reproductive tract. In her ovary, a mature ovum starts filling with a yoke with the germinal disc on top, which contains all her DNA and genetic material to create an embryo. Only when the yoke is ready is she able to drop the yoke into the Oviduct.
From there, the following happens.
- Egg Fertilized – Embryo formed.
- Protective membrane layers applied (inner and outer).
- Egg white filled for water and nutrition.
- Shell (calcium carbonate) forms with numerous pores to let in oxygen and expel Carbon dioxide.
- The eggs get painted here, but more about that later.
- Once the painting has been finished, the egg will sit and wait for its last journey into the world outside.
Egg shapes and sizes
Birds’ eggs have different shapes and sizes for various reasons.
Shapes
The main shapes, with an example, are:
- Oval – Starling
- Pyriform(pear-shaped) – Jacana
- Elliptical – Nightjar, murre
- Spherical(round) – Swift, hawk owl
- Sub elliptical – Kingfisher
There are two schools of thought regarding why eggs have different shapes and sizes.
- The types of nests dictate the shape and size of the eggs; for example, nests of the Jacana species are made on little islands of vegetation on a lake or dam, so an egg cannot afford to fall out of the nest. These eggs are also neatly placed with the sharp ends facing inward, so the clutch would be best positioned to incubate. Their eggs are pyriformed (sharper at one end). This helps if the eggs are accidentally kicked; they roll towards the middle and stay in the nest.
The more spherical the egg, the more chance it comes from a closed-up nest like the Hawk Owl, so it wouldn’t matter if it bumped. They are very secure in these nests.
- The other train of thought is that birds that spend their lives flying must have a more aerodynamic body shape. In other words, the size of their pelvis and oviducts would be smaller, therefore, longer, narrower eggs.
Just remember that these are all hypotheses still being analysed and studied.
Sizes
The egg sizes are related to the size of the birds. If you compare the length of the gigantic eggs of the Common Ostrich (about 15 cm long) to those of the hummingbirds, which are about 13mm long, you will notice the difference.
Also, the sizes are related to where the eggs are nested. Let’s look at two birds of the same size but nest in different areas.
- Tree-dwelling nesting birds – White-browed Robin-Chat (Cossypha heuglini), 18 cm long.
Tree-nesting birds’ eggs are safer by having their nests in trees. These eggs will not need much camouflage, so the colour will be much plainer. The robin-chats eggs are almost the same colour as chicken eggs. The eggs are also smaller, so the chicks hatch sooner.
They have a smaller volume-to-surface ratio, so they will heat up sooner and need to hatch quicker. Earlier means they would be underdeveloped to be more helpless (altricial). They are usually born naked and very dependent on their parents.
- Nest – deep opened, neatly made cup shape. Made with plant material, especially rootlets and leaves.
- No. of eggs – 2
- Egg size – 23 mm
- Incubation period – 15 to 17 days
- Ground-nesting birds – Three Banded Plover(Charadrius tricollaris),18 cm long.
The eggs of these birds are much larger as they need to be incubated longer. When the chicks break through, they are ready for life (precocial). They hatch fully developed, feathered and able to walk. They can fly in about three weeks. Little assistance is required from the parents.
- Nest – Simple scrape in the ground.
- No. of eggs – 2
- Egg size – 30 mm
- Incubation period – 30 days
There is an exaggerated example of egg laying I would like to make as it is unique, and that is the eggs of the Maleo (Macrocephalon maleo) from the islands of Indonesia. Once the female is ready to lay her egg, she and her mate leave the forest and go to the sandy areas. They will dig a huge hole and lay the egg there.
Then, they will completely cover the egg, compress the soil, and walk away, never to return. They are large eggs as they have fully formed maleo by the time they hatch. This chick must escape from the eggs and the hole in which it was buried. The young fledgling must start looking after itself. The eggs are about five times the size of a chicken’s egg.
Colours
As discussed earlier, certain species have different coloured eggs with various patterns of splotches and lines. Some could find their way to expensive art galleries if they were on canvas. The main reason for the colouring is not so they will please the eyes, but more for camouflage.
The two basic colours for all species are blue or white when the shell is first put on. Then, two more colouring steps occur as the egg moves down the oviduct. All three of these steps are.
- When the shell is deposited. (white or blue)
- A yellow pigment is applied, giving the white egg a beige or brown colour. The eggs will turn darker blue if the basic colour is blue.
- If needed, more pigment is layered on the shell, together with lines and further markings. These are normally put on with blood and bile from a pigment gland.
Quite a few birds, such as the raptors and species with hidden nests, only go through the first steps of the colouring procedure.
- Raptors are large birds and will rigorously defend their nests. Predators will be too wary to approach the raptor’s nests even if they see these eggs.
- White eggs are easier to see for parents when they enter a dark nest like a deep hole burrowed in a sand bank (Bee-eaters).
The norm is that most of the tree-dwelling birds’ shells are blueish, as their nests are in less danger than the ground-dwelling birds. The tree-dwelling bird’s eggs also hatch a lot quicker. The ground-dwelling birds are beige or brownish, depending on the environment. The ground-dwelling bird tends to put more markings and lines so that their eggs are very well camouflaged when they are laid.
Smaller birds, such as doves, will lay white eggs, but in their case, both males and females share incubation duties, so the eggs will always remain covered.
There is one difference to the norm. There are species like the weavers, which lay different colours across the species, but each female will lay eggs of the same colour. The cuckoo mainly parasitises these birds, so it is theorised that it makes it more difficult for parasite birds to choose where to lay their eggs.
Numbers game
Over time, bird species have determined their strategies to ensure their survival. These change from large clutches to small clutches once a year. Some of the factors that influence these strategies are.
- Food availability – This is one of the main factors. When the rains are good, this leads to more food so that birds will have bigger and more clutches, but as soon as there is a bad year of rain or there is a prolonged cold spell, there is less food, so the clutch sizes are smaller (one to two)
- Risk of predation – This will result in two or more clutches laid in a year. (with a smaller incubation time)
- Parental care – The higher the care, the smaller the clutch sizes.
- Physicality – What can the female bird physically lay? Maybe the female is sick?
- Incubation size – The number of eggs the birds can cover when incubating.
- Longevity of species – Some species with higher longevity will only lay a single egg every few years (eagles, albatrosses). Birds with lower longevity will lay more clutches (robins, starlings).
Here are some of the egg-laying strategies.
- Put all the eggs in one basket – Guineafowls tend to do this by laying a large batch at once, up to about 15 eggs.
- Multiple small clutches – Some birds like to play it safe by laying one or two eggs thrice a year.
- Glueing eggs into position. Some birds will glue their eggs between broad leaves, which makes predatation difficult – African Palm swifts (Cypsiurus parvus)
- Polyandry, which is a form of polygamy where a female takes more than one male. The bird that depicts this well is the African Jacana ( Actophilornis africanus). The female jacana is a bit bigger than the male. The males will build their nests of floating vegetation., and start displaying, hoping to attract her. The female will then choose a male, get her eggs fertilised by him, and lay them on his nest. She would normally lay about four eggs. She will then move on to another male. The number of clutches could reach 10.
This strategy is important for the female Jacana as it ensures that some of her offspring survive. Nesting in an aquatic environment is very susceptible to predation, and the success rate of clutches is under 20%.
Conclusion
After reading this, did you determine whether the bird came first or vice versa? Birds spend much energy creating and incubating their eggs. We have looked at the creation, sizes and colours of eggs and parental family planning concerning the laying of the eggs. With the hatchling needing a lot of care and protection, I know where my thoughts lie regarding this theory.
Identifying eggs can be a hobby all on its own, but be aware of the vulnerability of the birds during this breeding time. The birds get very sensitive to any disturbances. The breeding time is a matter of life and death to them.
As we have seen above, all species have egg shapes and colouring specific to them. We know how to identify whether they are either tree-dwelling or ground-based. We also know from the size of the eggs whether the birds are small or large. So, information has made it easier to begin identification.
Go out there and enjoy looking for these nests and eggs, but please be cautious. Temper your need to see the eggs to the survival needs of the birds.
