Our paper “Pigment concentrations only partially predict avian eggshell colour mimicry in a polymorphic host–brood parasite system” has been published in the journal Biology Letters. This paper considers the relationship between eggshell colour the two pigments thought to largely be responsible for generating eggshell colours in birds. These are the blue-green pigment biliverdin and the red-brown pigment protoporphyrin.

We studied the tawny-flanked prinia and the cuckoo finch (see https://www.africancuckoos.com/study-systems/cuckoo-finches/), because both of these species have evolved an amazingly diverse array of eggshell colours (see image, with the outer ring of eggs being prinia eggs and the inner ring cuckoo finch eggs). Cuckoo finches parasitise prinias, and so match the colours of prinia eggs fairly well. Despite this, we found that cuckoo finches deposit more pigment in their eggshell – so they are actually able to generate similar colours to prinias despite using different amounts of pigment. Having more pigment might benefit cuckoo finches by making their eggshells stronger – and thus harder for prinias to puncture and reject.

We also found that the relationship between the biliverdin:protoporphyrin ratio and eggshell colour was complicated. Across both species, pigment ratios predicted colour well for bluer eggs, but less so for browner eggs, which might be because brown pigments play other roles – such as in spotting patterns or in eggshell strength. Overall, even though birds tend to only use two pigments in generating amazingly variable eggshell colours, there is much to learn about how these pigments are used in complex ways.

You can read the paper at https://doi.org/10.1098/rsbl.2025.0112 (please contact us if you cannot access it).