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

Birds' eggshells are renowned for their striking colours and varied
patterns. Although often considered exceptionally diverse, we report that
avian eggshell coloration, sampled here across the full phylogenetic
diversity of birds, occupies only 0.08–0.10% of the avian perceivable
colour space. The concentrations of the two known tetrapyrrole eggshell
pigments (protoporphyrin and biliverdin) are generally poor predictors of
colour, both intra- and interspecifically. Here, we show that the
constrained diversity of eggshell coloration can be accurately predicted
by colour mixing models based on the relative contribution of both
pigments and we demonstrate that the models' predictions can be
improved by accounting for the reflectance of the eggshell's calcium
carbonate matrix. The establishment of these proximate links between
pigmentation and colour will enable future tests of hypotheses on the
functions of perceived avian eggshell colours that depend on eggshell
chemistry. More generally, colour mixing models are not limited to avian
eggshell colours but apply to any natural colour. Our approach illustrates
how modelling can aid the understanding of constraints on phenotypic
diversity.