Variation in avian egg shape and nest structure is explained by climatic conditions

Daisy Englert Duursma,Rachael V Gallagher,J Jordan Price,Simon C Griffith

doi:10.1038/s41598-018-22436-0

Abstract

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.

Highlights

The classic ovoid egg shape of birds is one of the most familiar of all shapes, and yet it remains surprisingly poorly understood[1,2]
We examined how avian nest type and egg shape vary across the Australian continent in relation to two environmental variables important for understanding climatic stress: average vapour pressure deficit (VPD) and leaf area index (LAI)
To assess the spatial pattern of variation of average egg elongation across Australia, we found the mean elongation for species with cup-shaped nests and species with domed nests for each 100 km × 100 km grid cell species assemblages

Summary

Introduction

The classic ovoid egg shape of birds is one of the most familiar of all shapes, and yet it remains surprisingly poorly understood[1,2]. Egg shape can vary among species due to the co-evolution of avian brood parasites and their hosts[5] These analyses considered a range of biophysical, life history and ecological parameters, previous comparative studies have not considered the abiotic environment into which an egg is laid, which includes a wide range of factors that may be detrimental to the development of an embryo. Birds breeding in deserts, at high altitudes, or at high temperatures have a relatively lower shell conductance, limiting water loss whilst permitting gaseous exchange[9,10]. The regulatory effects of enclosed nests on nest microclimate may be of critical importance in hot arid environments

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