Why are avian eggs ovoid, while the eggs of most other vertebrates are symmetrical? The interaction between an egg and its environment likely drives selection that will shape eggs across evolutionary time. For example, eggs incubated in hot, arid regions face acute exposure to harsh climatic conditions relative to those in temperate zones, and this exposure will differ across nest types, with eggs in open nests being more exposed to direct solar radiation than those in enclosed nests. We examined the idea that the geographical distribution of both egg shapes and nest types should reflect selective pressures of key environmental parameters, such as ambient temperature and the drying capacity of air. We took a comparative approach, using 310 passerine species from Australia, many of which are found in some of the most extreme climates on earth. We found that, across the continent, egg elongation decreases and the proportion of species with domed nests with roofs increases in hotter and drier areas with sparse plant canopies. Eggs are most spherical in open nests in the hottest environments, and most elongate in domed nests in wetter, shadier environments. Our findings suggest that climatic conditions played a key role in the evolution of passerine egg shape.