Eggshell pigment composition co-varies with phylogeny but not with life history nor with nesting ecology traits of British passerines

No single hypothesis is likely to explain the diversity in eggshell coloration and
patterning across birds, suggesting that eggshell appearance is most likely to
have evolved to fulfill many nonexclusive functions. By controlling for nonindependent phylogenetic associations between related species, we describe this diversity using museum eggshells of 71 British breeding passerine species to
examine how eggshell pigment composition and concentrations vary with phylogeny and with life-history and nesting ecology traits. Across species, concentrations of biliverdin and protoporphyrin, the two main pigments found in
eggshells, were strongly and positively correlated, and both pigments strongly
covaried with phylogenetic relatedness. Controlling for phylogeny, cavity-nesting
species laid eggs with lower protoporphyrin concentrations in the shell,
while higher biliverdin concentrations were associated with thicker eggshells for
species of all nest types. Overall, these relationships between eggshell pigment
concentrations and the biology of passerines are similar to those previously
found in nonpasserine eggs, and imply that phylogenetic dependence must be
considered across the class in further explanations of the functional significance
of avian eggshell coloration.