Avian Egg Coloration and Pigmentation Mechanisms

Bird eggs exhibit a remarkable variety of colours and patterns, yet the principal pigments involved are limited to two tetrapyrroles: protoporphyrin IX, which generates reddish-brown hues, and biliverdin, responsible for blue-green tones. Both pigments derive from haem degradation pathways in the shell gland and become deposited within the calcite matrix of the eggshell. Beyond aesthetic diversity, eggshell pigmentation fulfils multiple functions: it can modulate the amount and quality of light reaching the embryo, contribute to antimicrobial defence through pro-oxidant and free-radical scavenging activities, and serve as a signal in parent–offspring and inter-sexual communication. Environmental factors such as nest illumination, temperature and humidity interact with genetic determinants—including transporters and regulatory elements controlling pigment synthesis—resulting in intra- and interspecific variation. Structural components of the shell, notably the outer cuticle layer, further influence ultraviolet reflectance and pigment presentation. Together, these biochemical, genetic and ecological dimensions underscore the global significance of eggshell coloration in avian reproductive success, species adaptation and biodiversity monitoring.

Research from Nature Portfolio

Recent studies have experimentally tested the influence of nesting light conditions on eggshell pigmentation in a hole-nesting passerine. By comparing clutches laid in nest boxes with contrasting light exposure, researchers applied multispectral imaging to quantify spot brightness, size and chroma correlating with protoporphyrin content. Contrary to the hypothesis that low-light environments would yield paler eggs to enhance embryonic illumination, no significant differences in pigmentation traits were observed. This work highlights the complex interplay of genetic constraint and environmental plasticity in pigment deposition and suggests that factors beyond ambient light—such as predation risk or parental behaviour—may drive observed eggshell colour variation.

Avian Egg Coloration and Pigmentation Mechanisms publication trend

The graph below shows the total number of articles in avian egg coloration and pigmentation mechanisms across all publications each year (not limited to Nature Index journals).

Technical terms

Protoporphyrin IX (PP): Tetrapyrrole pigment producing brown and reddish-brown eggshell hues.

Biliverdin (BV): Tetrapyrrole pigment responsible for blue-green eggshell tones.

Chelation: Binding of metal ions by organic molecules, affecting pigment colour and activity.

Multispectral imaging: Technique measuring reflectance across multiple wavelengths to quantify eggshell colour traits.

Electron transfer: Chemical process transferring an electron between molecules, central to antioxidant activity of pigments.

References

1. The cuticle modulates ultraviolet reflectance of avian eggshells. Biology Open (2015).
2. Effect of light intensity in the nest site on eggshell pigmentation in a hole-nesting passerine. Scientific Reports (2023).
3. To Prevent Oxidative Stress, What about Protoporphyrin IX, Biliverdin, and Bilirubin?. Antioxidants (2023).
4. Metals Can Change the Colors of Eggshells but How Is This Related to Oxidative Stress and Antibacterial Capacity?. ACS Omega (2024).