Where does a bird egg begin?
In the ovary.
What we call the yolk grows around the female reproductive cell, or ovum. In most bird species, only the left ovary and left oviduct become fully functional as the female matures.
When a yolk is ready, it is released from the ovary. That release is ovulation.
At this point there is no hard shell around it. The familiar egg is assembled during the trip that comes next.
What does the oviduct do?
The oviduct is a long reproductive tube with several specialized regions.
Think of it as an assembly line, although a very biological one. Each section adds something different around the ovum and yolk.
In the upper oviduct, fertilization can occur if viable sperm is present. Farther down, albumen is added. Then come the shell membranes. The shell gland deposits the hard calcium-rich shell and, in many species, shell pigments.
By the time the egg reaches the end, it looks like an egg.
When is a bird egg fertilized?
Early in the trip.
If the female has mated and sperm is available, fertilization occurs around the infundibulum near the upper end of the oviduct. That happens before the shell is built.
This is internal fertilization. The embryo can begin its first cell divisions before the egg is laid, although most development waits for suitable incubation.
See how do birds mate? for the step before this one.
Where does the egg white come from?
The albumen, or egg white, is added in the oviduct.
It contains a lot of water along with proteins and other compounds. It cushions the yolk and, if the egg is fertile, becomes part of the developing chick's food and water supply.
The white is not sitting in the ovary with the yolk. It is layered around the yolk later.
How does the shell form?
The hard shell is added mainly in the shell gland, a region of the oviduct that is also called the uterus in avian anatomy.
Calcium carbonate is deposited around the shell membranes. This stage takes many hours and accounts for much of the total time needed to make an egg.
The shell has to be strong enough to protect what is inside, yet porous enough to let oxygen and carbon dioxide move through tiny pores during incubation.
That is a lot to ask from something that can crack on the kitchen counter.
Why are bird eggs different colours?
Pigments can be added to the shell late in formation.
Depending on the species, eggs may be white, blue, green, tan, reddish, or covered in spots and blotches. Pigment patterns can help with camouflage, strengthen parts of the shell, or simply reflect inherited egg chemistry.
Not every colourful egg has the same explanation.
How long does it take a bird to make an egg?
Around a day is a good rough figure for chickens and many other birds, but it is not a universal bird clock.
Egg formation time differs among species. Some wild birds also have delays between yolk development and laying that are much longer than the classic poultry example.
So if you see "24 hours" quoted as a rule for every bird, give it a little room.
How does the egg leave the bird?
Muscular contractions of the oviduct move the completed egg toward the cloaca.
It then passes out through the vent. The act of laying an egg is called oviposition.
The urinary and digestive tracts also connect to the cloaca, but the reproductive tract is organized so the egg can pass through without being coated in droppings under normal conditions.
Can a bird lay eggs without a male?
Yes.
A female's hormones can trigger ovulation and egg formation without fertilization. That egg will still have yolk, white, membranes, and shell.
What it will not have is a developing embryo.
This is why an egg is not proof that mating occurred, and it is one reason birds are not described as becoming pregnant in the mammal sense.
Do all birds lay eggs?
All living bird species reproduce by laying eggs.
Bird eggs vary enormously in size, shape, clutch size, nest site, and parental care. But no living bird gives birth to a fully developed chick the way a live-bearing mammal does.
A hummingbird egg and an ostrich egg could hardly look more different. The basic reproductive strategy is still the same.
What happens after the egg is laid?
For a fertile egg, incubation becomes the next big step.
Parents may begin sitting as soon as the first egg is laid or wait until more of the clutch is complete. That timing affects how closely together the chicks hatch.
Our page on how long do bird eggs take to hatch? follows the story from there.
Frequently asked questions
Where do bird eggs come out?
They pass from the oviduct into the cloaca and leave through the vent, the external opening at the rear of the bird.
Does laying an egg hurt a bird?
Normal egg laying is a demanding biological process, but it is part of the female reproductive cycle. Problems such as an abnormally large or retained egg are different and should not be treated as normal laying.
Can an unfertilized bird egg hatch?
No. Without fertilization there is no embryo to develop, even if the adult sits on the egg.
Do birds lay one egg at a time?
Eggs are laid individually. A clutch can contain several eggs, usually laid over a series of days rather than all at once.
Keep following the question
Egg laying leads straight into nesting. Read how long bird eggs take to hatch, learn why birds do not have mammal-style periods, or see why an adult bird has no visible belly button.