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The Biology of Swans

This section will mainly deal with the basic biology of Mute Swans and will reveal some rather surprising facts about these birds. Also, many of the biological ideas below are facts about birds in general, not just Mute Swans – I just present them in the context of a swan.

It’s All About Flight

The first thing to realise about swans is that their whole body structure is designed around flight. Feathers are an integral part of this design. In fact, swans have about 25,000 feathers on their body – the vast majority of these are tiny, little feathers situated round the head and neck.

Although individual feathers are very light in weight, when you take the collective mass of feathers over the entire body, they weigh surprisingly more than what most people would think. For example, in some species of birds, the overall weight of the feathers is more than twice the weight of the dried bones.

Another aspect of the feathers is to do with how they are laid over the bird’s body. For example, although the feathers cover most of the body’s surface, they are not attached to the skin in a uniform manner. They are, in fact, grown in tracts, or channels. The way those feathers then lay out over the swan’s body give the bird complete, or near complete, surface coverage. 

Hollow Bones

The bones of a Mute Swan are designed with flight in mind, too. A lot of them are actually hollow. The bones of a bird are hollow because they’re designed to be light in weight, which makes flying easier. This is especially useful to a swan because, being such a large bird, it has to have huge chest muscles to enable enough power to be transmitted to its wings to facilitate flight.

If the bones were heavier, flight probably would not be possible and swans would not be so buoyant in the water. The more buoyant they are, the less energy it takes to move themselves through the water because they will ‘sit’ higher in water, so less of their body will be submerged, which means the bird will be more hydro dynamically efficient. 

Birds, relative to their size, have fewer bones in their bodies than similar sized reptiles or mammals. They achieve this by having better structural designs – another avian feature designed to keep weight to a minimum.

No Dental Armoury   

Swans have no teeth, so how do they chew their food? Instead of having teeth and a relatively heavy jawbone to hold them in place (which would make the head too heavy to be supported by its neck), they have a gizzard.

The gizzard is essentially a muscular organ in a bird that performs the same action as the molar teeth in a mammal, such as a cow. 

It grinds the food down into a pulp which makes it easier for the swan to digest. And it’s very powerful, too. For example, swans at Abbotsbury Swannery are regularly hand fed grain, such as wheat. The wheat being a relatively hard food that needs to be ground down into a pulp before it can be fully digested by the swan. So, the gizzard needs to be strong enough to do that. To help, swans deliberately swallow small particles of grit, which then reside in the gizzard and help masticate (chew) the food.

The lack of teeth also has the advantage of reducing the weight of the bird, another feature to help reduce the energy needed for flight.

Amazing Breathing System!

When it comes to breathing, swans are far more efficient than is found in mammals, like us humans. Birds are much more efficient in their design, again, this is partially linked with flight because during flight birds need to breathe up to 10 times faster to enable sufficient oxygen to be delivered to the muscles to enable flight to occur.

This increase in efficiency over mammals occurs for a couple of reasons:

Firstly, the density of tissue inside the lungs is far greater than that of a mammal, meaning more blood can flow through them, which means a greater rate of gaseous exchange, compared to a lung of similar size found in a mammal.

Secondly, in a bird’s lung, the air only flows in one direction, rather than in a mammal’s lung where the air meets a dead end and so has to flow into and out of the lung along the same path.  Birds have a breathing system where the air passes through the lungs – air entering the lung goes along one path, air leaving the lung, passes out along another path.

This is how it works:

1st Step

When a bird inhales through its nares (nasal openings on the bill), the air flows down into a chamber called the posterior air sac.

2nd Step

Shortly afterwards, the bird exhales (air from a previous breath) and when this happens, the air that’s residing inside the posterior air sac moves into the lungs.

3rd Step

Next, the bird inhales (this time new air moves into the posterior air sac from the atmosphere), the air that’s inside the lungs completes its one way path through the lung into another air chamber, called the anterior air sac.

4th Step

Finally, the bird exhales again, this time pushing the air inside the anterior air sac, back out into the atmosphere (at the same time, air taken into the posterior air sac during the third step passes from the posterior sac into the lungs).

This ensures that the lungs of a bird gets a near 100% flush of air with each breath, rather than having some air residing inside the lung, as is the case for a mammal, like a cat.

Therefore for an animal like the swan, the lungs optimise their contact with fresh air, meaning more oxygen can be extracted per unit volume of air during each breath (and more carbon dioxide removed from the body), compared to that of a mammal.

Hot Blooded Swans

Birds are warm blooded, like us mammals, but birds have their temperature even higher than ours. The body temperature of a healthy human being is around 37 degrees Celsius, but for a bird like a swan, their healthy body is around 40 degrees Celsius.

When we think about it, it makes sense that swans need to be warm blooded and have a relatively high normal working body temperature. Swans live in relatively cool environments (some species, like the Bewick Swan, live in the Arctic) and if they were not warm blooded they’d either freeze to death, or, have incredibly slow reactions.

Think about non-warm blooded creatures, like lizards, and how slowly they move in the early morning, before the Sun’s heated their bodies and you’ll see what I mean.

Where does the body heat come from? It certainly does not come from the temperature of its food – their food is stone cold and it actually needs heating up (by the bird’s body) before the swan can properly digest it. So, where does the warmth come from?

It comes from the chemical breakdown of the food.

A swan’s body is made from billions of cells, and living cells need fuel to enable them to work – that fuel is the food it eats. The job of the digestive system is to take the food and make into a form that the cells of the body can use, as a fuel to enable the cells do their job.

When the cell performs its function, it releases some of the energy from the fuel (i.e. food) as heat, as a by-product. Although, the heat is described as a ‘by-product’, that heat is essential to the health of the cells; if they get too hot or cold, they’ll die.

So, in a nutshell, that’s the origin of a swan’s body heat; the heat released by chemical reactions inside the swan’s living body cells, as they burn fuel to perform their function.

The advantages to a swan in having a relatively high body temperature include:

Faster reflexes; for every 10 degrees Celsius increase in body temperature, there’s an increase in the speed of nerve transmissions of 1.8 times. Therefore this explains why birds have such fast reactions to their environment – the actual signals down the nerve fibres travel faster as a result of their body temperature being high.

More powerful muscles; for every 10 degrees Celsius increase in body temperature, the muscle power contractions increase by 3 times. This is clearly very important for a heavy bird like a Mute Swan. Mute Swans have an average weight of about 11kg (slightly more for males, slightly less for females) and getting this large mass off the ground requires a lot of power. In fact, the design of a swan (and the Great Bustard, heaviest flying bird in the world) is at about the upper limit in which a bird could conceivably fly. If the bird was any heavier, the wings would have to become disproportionally larger to enable lift off.

There’s one major disadvantage to having such high body temperature – the swan needs to eat a lot of food to sustain this, hence the large amount of time a swan needs to spend every day to foraging.

Swans Have Very Good Eyesight

Good eyesight plays an important part of a swan’s life. For example, they need to eat a lot everyday and to a very large extent, swans locate most, if not all, of their food items, by sight.

A swan’s sight also forms a key element when it comes to recognising other swans; their mates, offspring and intruders to their territory. I have seen many times where two partners are swimming towards each other from a large distance away, but they have not recognised each other. They then adopt an aggressive raised wing display as their body language, rather like they were dealing with an intruder. However, when they get close to each other, there’s a characteristic head-bob nod, in friendly acknowledgment and they join each other in a harmonious manner, with some more head nodding and looking at each other in the eye.

Swan’s eyes are located on the sides of the head, which are very well suited for spotting predators and other dangers, but the price to pay for good sideways vision, is poor forward sight. It’s this poor forward sight which makes them very susceptible to flying into overhead power lines, the largest manmade cause of death and injury.

The structure of the eye makes swans very able to see into the ultra violet part of the electromagnetic spectrum.  So, it’s entirely possible that a swan’s view of the world look quite different to that of a human. For example, some objects are very good at reflecting ultra violet light, but we can’t see them because our eyes are insensitive to that radiation. But a bird, like a swan, could be able to see them because they have the necessary cells in the retina (the back of the eye is called the retina).

The feathers of many birds are very highly reflective to ultra violet rays. Could it be that although all swans seem to look the same to you and I, seen through the eyes of an ultraviolet viewer, they may look very different because each different bird’s feathers reflects the ultra violet light to a marginally different extent? Who knows? It’ll certainly be very interesting if research throws some light on the matter (no pun intended!).

One surprising aspect of a swan’s eye is that it has what we call a nictitating membrane. This is essentially is an extra eyelid that is transparent that acts just like a pair of goggles underwater. As you know, when swans dip their head below the water surface to feed, they use vision to locate their food. If the swan didn’t have this clear window (i.e. the nictitating membrane) in front of the eye when feeding underwater, their vision would be very blurry, just the same as if we opened our eyes under the water. But when you put goggles on, everything is so much clearer – it’s just the same with swans, they have their own built-in goggles.

Other creatures that have a nictitating membrane include sharks, crocodiles, seals, polar bears and diving birds.


Like all birds, swans lay eggs. The eggs laid by a swan are some of the largest found in the animal kingdom.

An ovum is released from the ovary and it takes at least 24 hours to pass down the oviduct and out of the bird’s vagina and cloaca.

The cloaca is the opening also used for excretion of waste and releasing eggs. The male swan also has a cloaca; this is used for excretion of waste and allowing the phallus (normally contained in his body) out to enter the female’s cloaca. The phallus is only used for placing sperm inside the female because swans don’t excrete urine –  the urine produced by the kidneys is mixed with the solid waste, so all their waste comes out in solid (or semisolid) form. The waste products in the urine are made into uric acid, which forms the white crystals that makes up the familiar colour of bird waste. The water from the urine is reabsorbed into the blood in the lower intestine.

Brainy Birds

Have you ever heard of the term, 'bird brain' – a derogatory term meant to express someone, or something, as being stupid? The implication being that birds have little brains and as a result, are somewhat simple and unintelligent. Well, nothing could be further from the truth.

The reality is that birds have relatively large brains – anywhere from 6 to 11 times larger than a similar sized reptile. And they need it, too. Swans exhibit complex social behaviour and make a variety of sounds used to communicate to other birds and animals - even Mute Swans, mute by name, but not by nature.

On top of those, the swan needs a high capacity brain to cope with all the other things that go on. Complex tasks such as flying and maintaining a constant temperature day and night, even when the air temperature is up and down like a yo-yo. 

Back to Swan Information page.

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