Avian egg shape: Form, function, and evolution

Abstract

21Avian egg shape is generally explained as an adaptation to life history, yet we currently lack a global 22 synthesis of how egg shape differences arise and evolve. Here, we apply morphometric, mechanistic and 23 macroevolutionary analyses to the egg shapes of ~1400 bird species. We characterize egg shape diversity in 24 terms of two biologically relevant variables, asymmetry and ellipticity, allowing us to quantify the observed 25 morphologies in a two-dimensional morphospace. We then propose a simple mechanical model that explains 26 the observed egg shape diversity based on geometric and material properties of the egg membrane. Finally, 27 using phylogenetic models, we show that egg shape correlates with flight ability on broad taxonomic scales, 28suggesting that adaptations for flight may have been critical drivers of egg shape variation in birds. 29 30

Evolution of avian egg shape: underlying mechanisms and the importance of taxonomic scale

“…Contrary to Birkhead et al (2019), who argue that their new findings regarding incubation provide an opposing explanation for egg shape variation in birds, we see no conflict between the results of Birkhead et al (2019) and Stoddard et al (2017), which were performed at different taxonomic scales. Here, we expand on this point and emphasize the complementaryrather than contradictorynature of our joint findings.…”

“…In a new study in Ibis, Birkhead et al (2019) investigated egg shape in 30 species belonging to two avian familiesthe alcids (Alcidae) and penguins (Spheniscidae). They present these findings as an alternative to some of the results we reported in Stoddard et al (2017), suggesting that 'selection acting during incubation may influence egg-shape variation across birds as a whole'. They present these findings as an alternative to some of the results we reported in Stoddard et al (2017), suggesting that 'selection acting during incubation may influence egg-shape variation across birds as a whole'.…”

“…At a broad taxonomic level, we previously examined egg shape in 1400 species (Stoddard et al 2017). At a broad taxonomic level, we previously examined egg shape in 1400 species (Stoddard et al 2017).…”

The pyriform egg of the Common Murre (Uria aalge) is more stable on sloping surfaces

“…pear-like or pointed), and one of the most extreme is the pyriform egg of the Common Murre (Uria aalge; Birkhead 2017, Birkhead et al 2017a, 2017b, Stoddard et al 2017. The adaptive significance of this shape, and indeed of the shapes of all other birds' eggs, is unclear.…”

“…In a wide-ranging comparative study of avian egg shape, Stoddard et al (2017) suggested that ''flight efficiency'' and thus adaptations for flight have been ''critical drivers of egg shape variation in birds, '' with species best adapted for highpowered flight producing more elongated, more asymmetric eggs. However, only 4% of the variation in egg shape across the~1,400 species studied by Stoddard et al (2017) is explained by the hand-wing index (their measure of ''flight efficiency'') analyzed in that study. An alternative hypothesis is that egg shape evolves in response to adult posture during incubation and the type of substrate on which eggs are incubated.…”