Bird egg

Bird eggs are laid by female birds and come in many different sizes and numbers. Some birds, like condors, lay just one egg, while others, like the grey partridge, can lay up to seventeen eggs at a time. The number of eggs a bird lays can also change depending on where they live.

Birds sometimes lay eggs that are not fertilized, which means they won't hatch into baby birds. These eggs are often called wind-eggs. Pet bird owners might sometimes find these infertile eggs in their bird's nest.

Eggs are an important part of how birds reproduce and continue their species. Understanding bird eggs helps us learn more about the lives and habits of different birds.

Anatomy

Bird eggs are amazing little packages that help baby birds grow. Inside each egg, there is an embryo, which is the tiny developing chick. The egg also has a special liquid called amniotic fluid inside a membrane called the amnion, which protects the embryo from bumps. Another part called the allantois helps the embryo get oxygen and take care of its waste.

The egg's yellow part, called the vitellus or yolk, gives the baby bird food and important nutrients. Around the embryo is a clear liquid called albumen, made of proteins that help the egg stay together, though scientists aren't exactly sure what its main job is. Finally, the egg is wrapped in a shell with tiny holes that let air in while keeping out anything harmful.

Colors

Bird eggs are usually white because of the calcium carbonate in their shells. However, many birds, especially passerines, lay eggs with colors. Pigments like biliverdin and protoporphyrin create green, blue, red, and brown shades. Some birds, like Charadriiformes, sandgrouse, and nightjars, have white eggs for camouflage since they nest on the ground.

Birds that nest in trees often have blue or green eggs, while those nesting on or near the ground usually have speckled eggs. The color of eggs is influenced by genetics and is passed down from the mother. In some species, like the common guillemot, each female's eggs look different, helping them recognize their own eggs in crowded nesting areas.

Shell

Bird eggshells come in many different types. For example, cormorant eggs are rough and chalky, tinamou eggs are shiny, duck eggs are oily and waterproof, and cassowary eggs are heavily pitted. Tiny pores in a bird eggshell help the embryo breathe. A domestic hen's egg has about 7500 of these pores.

Shape

Bird eggs come in many different shapes. Some, like those of the little bee-eater, are almost round, while others, like those of the common murre, are pointy and cone-shaped. The familiar chicken egg is in between these two shapes.

Scientists used to think that the shape of an egg was determined as it passed through a bird's body, but we now know that the shape is set before the shell hardens. Birds that nest on cliffs often have very pointy eggs so they don’t roll away, while birds that nest in holes usually have more round eggs. The shape of an egg can also be linked to how the bird flies, with stronger fliers tending to have more elliptical eggs.

Size

Bird eggs come in many different sizes, depending mostly on how big the bird is. The tiniest bird, the bee hummingbird, lays eggs that weigh about half a gram, while the huge ostrich can lay eggs that weigh as much as 1.5 kilograms. Some birds, like kiwis, lay especially large eggs — up to 20% of the mother bird’s weight. These big eggs help kiwi chicks hatch ready to find their own food just a few days later.

Number

The number of eggs a bird lays in one nesting is called its clutch size. Most birds lay a small, fairly consistent number of eggs each time. Some birds will lay a new egg if one is lost by accident, while others stop laying if there are too many eggs. Scientists group birds into two types based on how they react to changes in the number of eggs: determinate layers, who lay a set number regardless of changes, and indeterminate layers, who may adjust their clutch size. Birds that usually lay just a few eggs are often indeterminate layers.

Predation

Main article: Egg predation

Many animals eat bird eggs. For example, raccoons, skunks, mink, otters, gulls, crows, and foxes often eat the eggs of birds like the black oystercatcher. Small animals such as stoats and weasels may take eggs from ducks. Large predators like mongooses, vultures, and hyenas can eat the big eggs of ostriches. Some snakes, like those in the genera Dasypeltis and Elachistodon, specialize in eating eggs.

Some birds practice brood parasitism, where they lay their eggs in the nests of other birds. This tricks the host bird into raising the parasite's young. Birds such as cowbirds, honeyguides, and many cuckoos are known for this behavior.

Evolutionary history, dinosaur eggs

Bird eggs have hard shells that are unique among animals today. However, this feature first developed in the group of animals that includes birds: dinosaurs. Because of this, a bird egg can also be considered a dinosaur egg. Eggs with the modern structure, including special layers, appeared during the Jurassic period and are linked to fossils of the Maniraptora.