Testing the structural-function hypothesis of eggshell maculation in the Great Tit: An experimental approach

The function of eggshell pigmentation has long been the subject of much speculation. Most of the proposed hypotheses have been based on a signalling function. According to the recently proposed structural-function hypothesis, birds use the pigment protoporphyrin to strengthen the eggshell in response to dietary calcium (Ca) shortage. The aim of the study reported here was to test this hypothesis in a Ca-provisioning experiment involving Great Tit Parus major populations breeding in Ca-poor coniferous and Ca-rich deciduous Estonian forests, respectively, over a 2-year period. Ca-supplemented birds laid eggs with 'darker' pigmentation than birds in the unsupplemented (control) group, which is at odds with the structural-function hypothesis. Although eggs were smaller and had thinner shells in Ca-poor habitat, eggshell maculation did not differ between the habitats. Eggshells were more pigmented in the year when they were also thinner, but their thickness was not related to their maculation within each year. Although the results of our study neither prove nor disprove the structural-function hypothesis, they do suggest that the relationship between eggshell pigmentation and Ca availability cannot be explained simply by this hypothesis.