An egg is an external womb that provides nourishment and protection for the growth and development of the embryo inside.

Creating an Egg
Eggs begin essentially as a food sac or yolk. The fertilized egg cell is part of the yolk and rests on its surface. It is the fertilized egg cell that grows into the embryo. As the yolk, or ovum, moves along the female bird’s reproductive tract, it is coated in albumen, or egg white, for protection. Then, shell membranes, the egg shell and the shell covering (cuticle), are added. The whole process takes about a day.

An egg is an external womb that provides nourishment and protection for the growth and development of the embryo inside.

Creating an Egg
Eggs begin essentially as a food sac or yolk. The fertilized egg cell is part of the yolk and rests on its surface. It is the fertilized egg cell that grows into the embryo. As the yolk, or ovum, moves along the female bird’s reproductive tract, it is coated in albumen, or egg white, for protection. Then, shell membranes, the egg shell and the shell covering (cuticle), are added. The whole process takes about a day.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.

Birds’ eggs, like the birds themselves, vary enormously in size. The largest egg from a living bird belongs to the ostrich. It is over 2000 times larger than the smallest egg produced by a hummingbird. Ostrich eggs are about 180 mm long and 140 mm wide and weigh 1.2 kg. Hummingbird eggs are 13 mm long and 8 mm wide and they weigh only half of a gram. The extinct Elephant Bird from Madagascar produced an egg 7 times larger than that of the Ostrich!

Relative Size
Although large birds lay larger eggs than do small birds, small birds actually have proportionately larger eggs. It takes 60 ostrich eggs to equal the weight of one ostrich but only nine hummingbird eggs to equal the weight of the Calliope Hummingbird. The birds that lays the largest eggs, relative to body size, are the kiwis of New Zealand. Kiwis are about the size of a chicken but lay eggs that are about 140 mm long and 90 mm wide, about three times the size of a chicken egg. It takes only four kiwi eggs to equal the weight of a kiwi. Fortunately, female kiwis lay only one and occasionally two eggs per year!
Birds’ eggs, like the birds themselves, vary enormously in size. The largest egg from a living bird belongs to the ostrich. It is over 2000 times larger than the smallest egg produced by a hummingbird. Ostrich eggs are about 180 mm long and 140 mm wide and weigh 1.2 kg. Hummingbird eggs are 13 mm long and 8 mm wide and they weigh only half of a gram. The extinct Elephant Bird from Madagascar produced an egg 7 times larger than that of the Ostrich!

Relative Size
Although large birds lay larger eggs than do small birds, small birds actually have proportionately larger eggs. It takes 60 ostrich eggs to equal the weight of one ostrich but only nine hummingbird eggs to equal the weight of the Calliope Hummingbird. The birds that lays the largest eggs, relative to body size, are the kiwis of New Zealand. Kiwis are about the size of a chicken but lay eggs that are about 140 mm long and 90 mm wide, about three times the size of a chicken egg. It takes only four kiwi eggs to equal the weight of a kiwi. Fortunately, female kiwis lay only one and occasionally two eggs per year!

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.

Eggs vary in size

Ostrich and Hummingbird eggs

Photos by Graham Mitchell-Lawson and Kim Chapman.
The Royal Alberta Museum, E.T. Jones collection.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.


Within a species, egg size may differ. For example, younger birds of a species tend to lay smaller eggs than older birds of the same species. Even eggs laid by a particular bird may vary in size, both within and between clutches. Food availability is a controlling factor. A clutch is the total number of eggs laid by one bird during one nesting. If you were advising a bird, would you recommend that it lays four large eggs or five smaller eggs? Can you imagine situations where one strategy is better than the other?
Within a species, egg size may differ. For example, younger birds of a species tend to lay smaller eggs than older birds of the same species. Even eggs laid by a particular bird may vary in size, both within and between clutches. Food availability is a controlling factor. A clutch is the total number of eggs laid by one bird during one nesting. If you were advising a bird, would you recommend that it lays four large eggs or five smaller eggs? Can you imagine situations where one strategy is better than the other?

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.

Eggs are not all the same shape as the chicken eggs that we eat. Even the shape of the domestic chicken egg is different from that of its wild ancestor, the Red Junglefowl. Some birds eggs are more pointed, or pyriform and, at the other extreme, some are more rounded or spherical.

Many Factors Affect Egg Shape Egg shape is determined by the internal structure of the hen. Her oviduct, distribution of internal organs and shape of her pelvic bones all affect egg shape. The Mallard’s egg at the top of this picture is subelliptical (not quite spherical). The Great Horned Owl’s egg is spherical and the Red-tailed Hawk’s egg, at the bottom, is elliptical.

Consequences of Shape Aristotle once suggested that males came from more pointed eggs and females from more rounded eggs. This of course, is not true. The shape of an eggshell does affect its physical properties, however. The general spherical shape of an egg maximizes shell strength while also conserving shell materials. More rounded eggs further maximize the volume of the egg for th Read More

Eggs are not all the same shape as the chicken eggs that we eat. Even the shape of the domestic chicken egg is different from that of its wild ancestor, the Red Junglefowl. Some birds eggs are more pointed, or pyriform and, at the other extreme, some are more rounded or spherical.

Many Factors Affect Egg Shape

  • Egg shape is determined by the internal structure of the hen. Her oviduct, distribution of internal organs and shape of her pelvic bones all affect egg shape. The Mallard’s egg at the top of this picture is subelliptical (not quite spherical). The Great Horned Owl’s egg is spherical and the Red-tailed Hawk’s egg, at the bottom, is elliptical.

Consequences of Shape

  • Aristotle once suggested that males came from more pointed eggs and females from more rounded eggs. This of course, is not true. The shape of an eggshell does affect its physical properties, however. The general spherical shape of an egg maximizes shell strength while also conserving shell materials. More rounded eggs further maximize the volume of the egg for theamount of eggshell as well as ensure heat conservation.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.

Egg Shapes

Examples of egg shapes

Photos by Graham Mitchell-Lawson and Kim Chapman.
The Royal Alberta Museum, E.T. Jones collection.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.


Egg shapes, over time, are often closely matched to their nest environment. Shorebirds normally lay four pointed eggs. In the nest, the eggs are oriented with their pointed ends towards the centre. This minimizes the amount of space needed to form the nest and increases the efficiency of the heat transfer from parent to egg during incubation.

Murres lay one egg on a bare nesting ledge. Their egg is very pointed. If bumped, the egg is more likely to roll in a tight arc than to roll off the cliff.
Egg shapes, over time, are often closely matched to their nest environment. Shorebirds normally lay four pointed eggs. In the nest, the eggs are oriented with their pointed ends towards the centre. This minimizes the amount of space needed to form the nest and increases the efficiency of the heat transfer from parent to egg during incubation.

Murres lay one egg on a bare nesting ledge. Their egg is very pointed. If bumped, the egg is more likely to roll in a tight arc than to roll off the cliff.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.

Egg Shape Ecology

Example of shore bird eggs

Photos by Graham Mitchell-Lawson and Kim Chapman.
The Royal Alberta Museum, E.T. Jones collection.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.


Murre Eggs

The shape of Murre eggs reduces the risk of them rolling off a cliff

Photos by Graham Mitchell-Lawson and Kim Chapman.
The Royal Alberta Museum, E.T. Jones collection.

© 2006 Government of Alberta and © 1996-2006 Royal Alberta Museum.


Learning Objectives

The learner will:

  • Become familiar with the basic biology of eggs, and their variation
  • Relate ecology to evolution and adaptation (relate form and function)

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