Swans create huge eggs – they are about 12cm x 7cm in size and have a weight of around 350g, although, they are nowhere near as heavy as the ones an ostrich lays, which weigh around 1300 to 1500g! But swans eggs are still very large, nonetheless.
What we will look at in this section is what happens after the swans have gone through the copulation process (see the section on the Swans’ Mating Ritual) and how these huge eggs are made.
Inside the Swan’s Body
At the time of year in which the mating takes place, hormones in the female’s body stimulate the release of ova from her ovary (think of the ovary as a bunch of grapes – the grapes are the eggs which are released from the ovary when ‘ripe’). The ovary releases a microscopic ovum, so fertilisation by the male’s sperm can occur. Fertilisation takes place in the space directly below the ovary, called the infundibulum.
This is what happens: The male’s sperm are injected into the cloaca, which then go into the bird’s vagina and then into the oviduct (a tube from the vagina to the ovary). Once in the oviduct, the sperm then rapidly swim up the tube into the infundibulum. If there is a ripe ovum there (one that’s just been released from the ovary), the sperm can fertilise the egg and an embryo (a soon to be baby swan) will be formed. If the ovum is not fertilised, it will still pass out of the swan as an unfertilised egg.
After fertilisation has taken place, the egg travels down the oviduct by muscular contractions, similar to the way that food passes through our intestines. As the egg travels down the tube, essential elements are added to the egg piece by piece, so the final egg that will be laid by the bird will be protected from the outside environment and will also contain all the nutrients to enable the embryo to grow into a hatchling.
Why Does The Swan’s Egg End Up So Large?
As the egg travels down the tube it gets larger and larger. This is because during its passage (it takes about a day or two to travel all the way down the tube) various parts are added in turn to give the egg its essential parts – very much like it’s moving along a factory conveyor belt.
The first layer to go onto the swan’s egg is the albumen. This is the ‘egg white’ and its job is to support the yolk and provide the protein, albumin, which is used for food for the embryo.
After the first addition of albumen, two membranes are added; first one for the egg (that will be the inner membrane) and then another for the shell. This second membrane will encapsulate the inner membrane, which contains the egg white, yolk and embryo. More albumen is added at this stage, too.
Finally, in the lower part of the oviduct, the greatly enlarged egg, will enter the uterus where the familiar hard egg shell is added.
The shell of the egg is porous because the growing embryo needs to exchange water, along with letting out carbon dioxide and taking in oxygen from the outside environment. (All of its food and other nutrients are contained inside the egg white and yolk.)
How The Embryo Grows Its Bones
The egg shell contains a lot of calcium carbonate – the growing embryo will absorb some of the calcium from the egg shell as it grows (the calcium is used to grow the baby swan’s bones), so the shell gets thinner as the day of hatching approaches. Which is useful because it will be easier for the hatchling to break its way out of the egg shell when it’s thinner, and permit more gaseous exchange as the growing embryo gets mature enough to be born.
The addition of the thick, hard shell, takes just under a day – after that, some colouration is added and the egg is ready to be laid.
One interesting point is that it has been suggested that cygnets hatched from bigger eggs stand a higher chance of surviving those critical first two weeks because the bigger eggs have larger nutrient stores in the yolk. When the hatchling is born, it absorbs the remains of the egg’s yolk into its body to provide much of its food for the first week, or so. Hence it has a richer food parcel to help it on its way than a cygnet hatching from a smaller egg.
Laying Swan Eggs
Finally, after all the various components to the egg have been added in the correct order to protect and nourish the growing embryo, the swan will lay the egg in a prepared nest.
She will normally lay these overnight or early in the morning because that’s when she’s least active and as a result, the lack of physical movement, etc... will minimise the risk to the large, fragile egg – that’s just inside her.
During the breeding season, female swans can produce an egg once every other day, or so. As discussed in the section Swans Breeding, the clutch size varies according to how late in the season the swan lays and this is related to the level of experience of the swan pair.
How Many Eggs Does A Mute Swan Lay?
The average number of eggs produced by a pair of Mute Swans is about six, but a range of four to eight is quite common. The highest I’ve heard of is thirteen eggs.
Eggs laid early in the breeding season commonly make up large clutches than those produced later in the spring/early summer.
More experienced pairs of swans, that’s those who have been together on a territory for a number of years, tend to produce their clutch earlier on the breeding season than inexperienced pairs. Hence their clutch, on average, tends to be larger. (See the section Swans Breeding.) In fact, inexperienced Mute Swans that are new to a territory, often don’t lay any eggs at all in the first year.