Bird eggs

Foods, Animal source foods, Eggs

Animal family that lays them: Birds

Consumption area(s): Earth

Introduction

Bird eggs are laid exclusively by females, and their number can vary greatly between species—ranging from a single egg, as seen in condors, to as many as seventeen, like in the gray partridge. Even within the same species, clutch size may differ depending on latitude, adapting to local environmental and climatic conditions.

Some birds also lay unfertilized eggs, a behavior that is quite common. It’s not unusual for pet owners to find their birds nesting on a clutch of sterile eggs, often referred to as “wind eggs.” While these eggs cannot develop into embryos, they are still a natural part of the birds’ reproductive behavior.

Shape and morphology of bird eggs

The shape of bird eggs varies considerably across species, ranging from almost spherical to pyriform (pear-shaped) or even conical. The familiar shape of a chicken egg falls somewhere in between. Birds that nest on cliffs often lay conical eggs, which are less likely to roll off ledges, while burrow-nesting species tend to produce rounder eggs.

Morphologically, bird eggs consist of a protective shell that encloses the yolk and albumen, held together by thin membranes (2,3 in the image). The yolk (10,11) is suspended within the albumen (5,6,12) by one or two spiral bands known as chalazae (4,13), which are visible as the translucent threads anchoring the yolk when an egg is cracked open.

Here’s a closer look at the main components:

1. Shell (1): Made up of 95–97% calcium carbonate crystals, bound in a protein matrix that gives the shell both strength and flexibility.
   The base color of most vertebrate eggs is white, but many bird species—especially passerines—lay colored eggs due to the presence of pigments such as biliverdin (producing green or blue hues) and protoporphyrin (responsible for reddish or brown colors, often forming spots or background shades).
   Pigmentation isn’t just for camouflage or species recognition—it also plays a role in shell strength. Protoporphyrin spots, for instance, can compensate for calcium deficiency, which tends to increase in later-laid eggs as the female’s calcium reserves are depleted. Birds nesting in trees typically lay blue or green eggs, with or without spots, while ground-nesting or shrub-nesting species tend to lay speckled eggs for better camouflage.
2. Membranes (2,3): These are thin, transparent films that cling to the shell’s interior. They’re composed mainly of fibrous proteins, including type I collagen.
3. Yolk (10,11): This is the nutrient-rich portion of the egg containing the ovum. Its main role is to nourish the developing embryo, and it’s packed with fats and proteins.
4. Albumen (5,6,12): Commonly known as the egg white, this clear fluid surrounds the yolk and is formed regardless of fertilization. Composed of 90% water and 10% proteins, the albumen protects the yolk and offers additional nutrients to support embryonic growth.

History of bird eggs as food

Bird eggs have been a valuable food source since prehistoric times, used both by hunter-gatherer societies and later cultures where birds were domesticated. The chicken, originally native to the tropical and subtropical jungles of Southeast Asia and the Indian subcontinent, was likely domesticated primarily for egg production before 7500 BCE. By 1500 BCE, chickens had been introduced to Sumer and Egypt, and reached Greece around 800 BCE, where previously the quail had been the main source of eggs.

In Egypt, the significance of eggs is evidenced by artistic representations. In the tomb of Haremhab at Thebes, dating to around 1420 BCE, a man is depicted carrying bowls containing ostrich eggs and other large eggs, presumably from pelicans, offered in ritual contexts.

Classification of bird eggs

Bird eggs are classified according to the species of bird that laid them. Some of the most commonly known varieties include:

• Chicken egg
• Duck egg
• Ostrich egg
• Quail egg