Temperature, humidity and vegetation density affect eggshell pigmentation across a continent

The distinctive evolutionary shift from white reptilian eggs to pigmented eggs in a majority of avian species, has stimulated much thought and research, but has yet to be completely explained. Pigments in eggs may serve a variety of structural or signalling functions, which are not mutually exclusive. This research focuses on the variability of Australian Magpie (Cracticus tibicen) eggs, and the possible drivers of such variability. Using a dataset of 283 clutches from the Australian continent we used a range of methodologies to assess the degree of inter-clutch variation in egg pigmentation in relation to genetic divergence, environmental factors, and brood parasitism. We found little evidence for divergence between subspecies, however the Tasmanian subspecies did differ significantly from most of the others. The analysis of environmental parameters revealed that maximum temperature and the interactions between maximum temperature, relative humidity and leaf area cover explained variation for background colour patterning. The presence of a brood parasite in about one third of the magpie's distribution was related to a small degree, not significantly so, to the variation in colour and patterning. The results from our study add to the body of evidence that environmental drivers have an impact on pigment deposition.