The evolutionary drivers underlying marked variation in the pigmentation of eggs within many avian species remains unclear. The leading hypotheses proposed to explain such variation advocate the roles of genetic differences, signalling and/or structural integrity. One means of testing amongst these hypotheses is to capitalise on museum collections of eggs obtained throughout a broad geographic range of a species to ensure sufficient variation in predictors pertaining to each hypothesis. Here we measured colouration and patterning in eggs from 271 clutches of Australian Magpies (Cracticus tibicen) collected across most of their geographic range of ca. 7 million km2; encompassing eight subspecies, variation in environmental parameters, and the presence/ absence of a brood parasite. We found considerable variation in background colour, as well as in the extent and distribution of patterning across eggs. There was little evidence that this variation was explained by subspecies. However, we found marked differences in relation to the contemporary presence of a brood parasite. Measures of maximum temperature, relative humidity, leaf area index and soil calcium all contributed to variation in egg appearance, although their explanatory power was relatively low. Our results suggest that multiple factors combine to influence egg appearance in this species, and that even in species with highly variable eggs, colouration is not readily explained.

Note: An error in data manipulation was identified and corrected. A correction has been applied to this article (April 2024), approved by RSOS.