Bird eggs display a wide array of colors and patterns. Beyond simple white, these shells show a spectrum of hues and markings, prompting questions about their origins and purpose.
The Spectrum of Colors
Bird eggs present a wide range of colors, from common whites and various shades of brown to vibrant blues and greens. Examples include the familiar blue of a robin’s egg, the speckled brown of a thrush’s egg, or the plain white of a chicken’s egg. Many eggs feature additional markings like spots, speckles, blotches, or streaks. These patterns can appear against any background color, sometimes even covering the entire surface of the egg.
How Eggs Get Their Color
Egg coloration arises from the deposition of two primary pigments onto the eggshell during its formation within the female bird’s oviduct. Protoporphyrin, derived from hemoglobin, creates reddish-browns, reds, and yellows. Biliverdin, also related to hemoglobin breakdown, produces blue and green colors. The specific color results from the concentration and mixture of these two pigments.
These pigments are synthesized and deposited in the shell gland, also known as the uterus, during the final stages of egg development. The timing and method of pigment deposition determine the final appearance. If the egg remains still during this process, spots or speckles form, while movement of the egg through the oviduct can create streaks or scrawls.
Why Egg Colors Vary
The diverse coloration of bird eggs serves multiple adaptive purposes, shaped by evolutionary pressures for survival. Camouflage is a primary reason, helping to hide eggs from predators. Ground-nesting birds, such as nightjars and plovers, often lay eggs with cryptic patterns and colors that blend seamlessly with their surroundings, like soil, pebbles, or foliage. Conversely, birds that nest in enclosed spaces, like tree cavities or holes, frequently lay plain white eggs because concealment is less necessary.
Egg coloration can also offer protection against harmful ultraviolet (UV) radiation. Darker or more heavily pigmented eggs may provide better shielding for the developing embryo, particularly for eggs exposed to direct sunlight. Research suggests that darker eggs tend to be more common in colder regions, potentially aiding in heat absorption, while brighter eggs are found in warmer areas where heat absorption is less desirable.
Specific colors and patterns can also aid in species recognition and communication. In colonial nesting environments, distinct egg patterns allow parents to identify their own eggs among many others. This is relevant in defense against brood parasites, such as cuckoos, which lay their eggs in other birds’ nests; host birds can sometimes reject eggs that do not match their own species’ pattern.
Some pigments contribute to the structural integrity of the eggshell. Protoporphyrin, the pigment responsible for brown and red hues, has been linked to increased shell strength. This pigment may compensate for areas of reduced shell thickness, which can occur due to calcium deficiency in the female bird’s diet.