Swans Incubating Eggs

​Swans lay on average around six eggs per clutch, but there’s a lot of variation, up to thirteen is the maximum I’ve reliably heard about. But as you can imagine after reading the section Swans’ Eggs, they are not all going to be laid in one go.

This section will discuss the pattern that Mute Swans follow in laying their eggs, why they follow that pattern and what they do during the time between laying the eggs and the baby swans (called, cygnets) hatching.


Synchronised Hatching


As you know, it takes around one to two days for a swan to create an egg, after mating has occurred – see sections Swans’ Eggs and Swans Mating Ritual. Only one egg can be produced at a time, so as you can imagine, it could take two weeks for a complete clutch of six eggs to be laid by the pen.

The main problem with that is that if the first egg started to develop from the moment it was laid by the pen (i.e. embryo starting to grow and getting ready for hatching), the hatching from the first egg would emerge two weeks prior to the cygnet from the last egg hatching. This staggered hatching would be a disaster – the initial baby swan would need to leave the nest and start to find food, before many of the other eggs were ready to reveal their hatchlings.

The end result would either the initial babies dying because of them not being able to find food etc..., or if the parents followed the inclinations of their first young, the remaining ripe eggs would not bear their fruit because the incubation would not be complete and the embryos would die in the eggs.

The solution that Mute Swans employ is the lay the whole set of eggs and then incubate them when the clutch is complete. That way, the hatchlings will (in principle!) all emerge from their eggs at the same time. That way, the parents will be able to care for them properly since they will all have the same needs at the same time.

During this time when the pen is producing and laying eggs, she eats enormously more than the male swan. This is because the process of making and laying eggs is very nutritionally and energetically demanding.

As you saw in the section Swans’ Eggs, a lot of material goes into the final egg because its job is to not only protect the embryo from the outside world, but also to provide all the food it will need to grow from a tiny speck of life, into and a fully functional baby swan, in just over a month. And this life support package can only come from its mum – so she needs to feed herself heavily to get into prime condition.

The normal pattern we see in the female swan will be feeding on the close by vegetation whenever possible and the cob will sit on, or around, the eggs to protect them from predators. The amount of heat he applies to the eggs is not zero, but is not thought to be significant enough to raise the temperature enough to cause them to start incubating.

During this time, he will spend quite a lot of time on the eggs and building up the nest from nearby vegetation. When the eggs are briefly left unattended, the bird will normally cover the eggs with some of the loose nest material – probably to reduce the chances of them being found by another animal and predated on. Foxes, otters and mink have been known to eat swan eggs.

In the mean time, the pair will engage in more mating and courtship, so as you can appreciate, it’s a demanding time for the pen.

How the swans ‘choose’ the number of eggs to lay is not known at this time but is likely to be affected by the availability of food, prior condition of the birds, age of the pair and how far advanced into the season the egg laying takes place. See the section, Swans Breeding.

Colour Of Swans’ Eggs

When the egg first emerges from the female swan, the colour could best be described as a bluish-grey. This colour is due to pigments are that added during the time when the shell was being formed, at the end of the oviduct. After the egg has been in the nest for a while, constant shuffling around in the cup of the nest by the pen, turns the egg more of an olive colour.

Eggs that have been in the nest for a while also exhibit many little scratches in their surface. These are due to them being stood on and being caught by the pen and cob’s claws on their webbed feet. The thick shell of the egg is easily strong enough to take the weight of an adult swan.

Incidentally, if an egg were to get broken, depending on the stage of development, the liquid contents are likely to get eaten by either parent the resulting empty shell being physically removed by the nest. Although, I’ve never seen this happen, there are accounts of this method of disposal being recorded.  

When the Incubation Starts

Once all the eggs have been laid, she will start her long vigil of the incubation.

What is incubation?

Essentially it is bringing the temperature of the eggs up to a sufficient level to enable the development of the offspring inside the shell, and keeping that temperature at that level.

To do this, the pen brings her body into thermal contact with the egg’s shell to enable heat to be transferred to the growing baby inside. If the temperature does not reach a high enough level, the necessary chemical reaction inside the egg will not occur. This temperature is about 38 degrees Celsius. The swan’s body temperature is about 41 degrees Celsius.

To facilitate the transfer of heat, she will fluff her feathers up on her breast and belly, just before she sits down on the clutch. What this does is to bring the eggs shell closer to her skin (or even touching it, sometimes) meaning the eggs will be heated more efficiently. As we all know, feathers are an excellent insulator (very poor at allowing heat to pass through), so moving them out of the way, makes the whole job so much more effective.

During this incubation time she will seldom leave the nest or her clutch of eggs, the only times she will do so will be to briefly feed and preen/wash herself.

This is another reason why female swans feed so voraciously before the incubation period starts. She eats only sparingly during the incubation time since she can’t jeopardise the growth of her babies by being away from the maturing eggs.

The male’s thermal contribution to the process is not as important as the pen’s. His job is more likely to be providing protection against predators and to stop the eggs from cooling significantly whilst the female’s heat is not being applied, when she’s away from the nest.

The reason the male’s presence on the eggs is less effective that the pen’s is probably because he doesn’t fluff/ruffle his breast and belly feathers before sitting on them, therefore less thermal energy is transferred to the eggs from the cob. But the feathers are such a good insulator, meaning he can effectively stop them cooling down too much, when the pen’s briefly away.

Swan eggs don’t need a constant temperature, indeed, the temperature of the eggs can vary quite a bit. Night time is probably the time when the temperature is at its highest level because it’s when they get the longest period of time when the pen is continually on the nest. Changeovers during the day mean the egg’s temperature may drop a little during the pen’s absence.

Swan’s are inclined to maintain the temperature at its optimum level since it will keep the incubation period to its shortest. Birds that leave the eggs fairly often have longer incubation times because the temperature will not be at its optimum level as often as it could be, hence the embryos develop at a slower rate and hatch later on.

Significant problems occur to the embryo if the egg temperature falls outside the range 35 to 40.5 degrees Celsius. Higher temperatures than this will kill it and temperatures in the region 26 to 35 degrees Celsius are likely to disrupt embryonic development.

During Incubation


Swans incubate their eggs over a lengthy time period – the average is around 35 to 36 days. During a lot of this time she will be dosing, preening, building/arranging the nest and turning the eggs.

It’s a long period of time to be sat on the nest, doing very little; she, and her clutch, are quite vulnerable. And because of that the cob’s main role is to protect her and his future offspring. And he will do so vigorously. Some male swans are more aggressive than others, but charging at boats and people who get too close, is very common. Most people are aware of the swan’s reputation for such behaviour and respect their space as much as they can.  

On top of protecting the immediate vicinity of the nest, he’s  also got to maintain his territory. Therefore the cob will regularly make patrols and chase off any intruders. Local ducks, geese and other waterfowl are well aware of the male swan’s bad temper at this time of year and steer clear, otherwise they will get chased and get attacked.

I’ve noticed that white ducks seem to be more frequently a target of his charges than other coloured waterfowl. Whether this is because they’re more easily seen, or maybe, the cob mistakes then for another swan is unknown, either way, it does not pay to have a white coat on during the swan breeding season!

As previously mentioned, the pen will leave the nest from time to time, and when this happens, the cob normally takes his turn on the eggs. During the change over, they often go through a display sequence.

It goes something like this:

The bird on the nest is typically asleep and the incomer will slowly approach the nest. The approaching swan will often have a bit of a preen and scratch, etc... and in the mean time, the swan on the nest would have woken-up and raised their head. The newly arrived bird will then frequently pick up a few pieces of vegetation and then add them to the nest pile.

Then they will approach the nest fluffing up their feathers and half raising their wings as well as lowering and raising their head. The one on the nest will reply in kind with similar movements and whilst doing this, both birds will often let out a low pitched rumbling noise.

After that, the bird doing the relieving will sidle up to the side of the nest and the swan on the eggs will then give way. The bird that is leaving the nest will often have a bit of preen, add a few more pieces of nest material to the pile and then paddle off.

During the course of the incubation, there will be numerous changeovers and this typically occurs when the swan doing the relieving, would have been in the water for a while beforehand.

Although, I have noticed that the birds do get into a daily routine with changeovers. For example, one pair of swans I tracked for a couple of years did a changeover most days at about ten in the morning, after that, she’d stay on the eggs normally through the rest of the day. They never did any changeovers in the evening.

Normally, the bird will have a preen before taking position on the nest and part of this will involve squeezing some of the water out their breast and belly feathers. But some moisture will probably remain on the bird and will be transferred to the eggs.

Maybe this is meant to happen as part of the care given to the eggs from the parents, but I assume they try to make sure that not too much is not given to the clutch, otherwise, there could be a significant cooling effect on the eggs, jeopardising internal development.

Right the way through the incubation period, the pen will be adding to the nest from nearby vegetation. To help her, I’ve often seen the cob create a pile of twigs, strands of weed, etc.... within a foot or so of the nest, so the pen can easily reach out from her sitting position and grab some nesting material.

The female swan will frequently stand up and with her bill, push the point of it under the eggs and rotate her clutch so each egg gets the right amount of rotation for good development and is exposed to the optimum spot on her body for sufficient heat transfer.

It was during one of these times when a pen was rotating her eggs that I noticed her brood patch. This is small patch on her breast/underside where she has pulled out feathers to expose a small area of bare skin.

Hormones in the birds, called estrogen and prolactin, cause her to lose feathers (she also assists by pulling some of the  feathers out) on an area of the abdomen – breast and belly. This creates the small area of loose, flaccid skin, as well as being relatively soft to the touch. The skin is grey in colour and the folds/ridges that develop in this area mean that because it’s physically soft, the skin will mould itself around the egg, rather like a cloth.

This increases the area that her warm body is in contact with the egg’s surface and hence, improves the thermal contact between the two.

With increased blood flow to the area, she is able to very effectively warm any eggs that are in direct contact with the brood patch. So, when she moves the eggs around in the nest, she is looking to position eggs on the brood patch.

The pen will often change position every half an hour to make sure she gets round to heating all the eggs equally – this will be especially important in a large clutch with more than eight eggs.

The pen stays on the eggs all through the night, with the cob normally sleeping right beside her for protection. Sleeping on the land is not the usual place for swans to spend the night, though. They normally do their main night sleep on the water – they are less susceptible to predators , like foxes, whilst afloat.

End Of The Incubation

Spending more than a month incubating the eggs, feeding very little, is a very demanding task for the pen. Although, she’s not moving a great deal, a lot of the energy she expends everyday is used to simply keep her alive. Being warm blooded, swans burn a considerable amount of energy doing that job alone.

This is one reason why the female swan will eat so much before the incubation period starts, she instinctively knows that there are lean times ahead, so she will take on as much in the way of nutritional reserves as possible.

By the end of the incubation, she’s probably lost about one third of her normal bodyweight. She will be very weak and relies on the cob to do more for protection. In fact, the male will often spend more and more time around the nest and his mate, towards the end the incubation. He will know instinctively that she’s getting weaker by the day and the eggs will be hatching soon. Mute Swans have been known to sit for up to fifty days on the eggs, by the end of this time period, the pen would be close to starving to death.

The swans will know that the incubation period is coming to an end by the faint callings from the cygnets inside the egg shell. It has been said that they call within forty eight hours of finally breaking out and this could be used to synchronise their emergence.

In addition to that, the pen swan will often call very softly to its young still inside the egg, thereby familiarising the whole family with their voices, even before they have emerged from the egg.

Some observers have stated that they are able to tell, within a day or so, when the eggs are likely to hatch. It’s been said that she does a lot of extra preening just a short time before the cygnets emerge. This is because the babies may not have a totally water resistant coat of down when they arrive and to afford them a helping hand, she liberally coats herself in her own preen oil. Some of this will be transferred to her offspring, since they will spend a lot of time in the first couple of days, climbing over her, exploring their new world.

Hatching Swans’ Eggs

The definitive end to the incubation is the time when the baby swans appear from their eggs. These hatchlings can take hours to fully break out of the egg shell. As explained in the section, Swans’ Eggs, the shell of the eggs gets progressively thinner and thinner as the embryo develops inside.  This makes it easier for the tiny bird to break out.

The hatchling has grown a special tool on the end of its upper bill (the mandible) to help it crack open the egg shell. It’s a structure that looks similar to a small thorn; it lies along the long axis of the mandible and is normally grey in colouration. This is known as the egg-tooth.

The cygnet will push the egg-tooth along the inside of the egg shell, eventually creating a crack or a small crater like feature on the surface.  It does this by rotating inside the egg and scoring with the egg tooth, as the cygnet moves around and stretches.

Small cracks radiate out from the part of the shell where the main breakage lies. Slowly, over the next few hours, the cygnet will create many holes in the shell and the area of breakage will get larger and larger, until the bird is able to thrust its head outwards enabling the baby swan to be born as it finally breaks free of its egg.  

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