Not so colourful after all: eggshell pigments constrain avian eggshell colour space

Area of Science:

• Zoology
• Biophysics
• Evolutionary Biology

Background:

• Bird eggshells exhibit remarkable color diversity, a trait often assumed to be highly variable.
• Previous research identified two primary pigments, protoporphyrin and biliverdin, but their concentrations poorly predict observed eggshell colors.

Purpose of the Study:

• To quantify the actual diversity of avian eggshell coloration within the full perceivable color space.
• To develop and validate predictive models for eggshell coloration based on pigment composition and substrate properties.
• To establish proximate links between eggshell chemistry and perceived color for future functional studies.

Main Methods:

• Analysis of eggshell coloration across the full phylogenetic diversity of birds.
• Development of color mixing models incorporating protoporphyrin and biliverdin concentrations.
• Integration of calcium carbonate matrix reflectance into the color prediction models.

Main Results:

• Avian eggshell coloration occupies a small fraction (0.08-0.10%) of the total avian perceivable color space.
• Color mixing models accurately predict eggshell colors, with improved accuracy when accounting for matrix reflectance.
• Pigment concentrations alone are poor predictors of eggshell color variation.

Conclusions:

• The diversity of bird eggshell colors is constrained, not as vast as previously assumed.
• Color mixing models provide a robust framework for understanding the biophysical basis of eggshell coloration.
• This approach facilitates research into the evolutionary functions of eggshell colors by linking them to underlying chemistry.