The evolution of egg shape in birds: selection during the incubation period

Abstract

A recent broad comparative study suggested that factors during egg formationin particular 'flight efficiency', which explained only 4% of the interspecific variationare the main forces of selection on the evolution of egg shape in birds. As an alternative, we tested whether selection during the incubation period might also influence egg shape in two taxa with a wide range of egg shapes, the alcids (Alcidae) and the penguins (Spheniscidae). To do this, we analysed data from 30 species of these two distantly related but ecologically similar bird families with egg shapes ranging from nearly spherical to the most pyriform eggs found in birds. The shape of pyriform eggs, in particular, has previously proven difficult to quantify. Using three egg-shape indicespointedness, polar-asymmetry and elongationthat accurately describe the shapes of all birds' eggs, we examined the effects of egg size, chick developmental mode, clutch size and incubation site on egg shape. Linear models that include only these factors explained 70-85% of the variation in these egg-shape indices, with incubation site consistently explaining > 60% of the variation in shape. The five species of alcids and penguins that produce the most pyriform eggs all incubate in an upright posture on flat or sloping substrates, whereas species that incubate in a cup nest have more spherical eggs. We suggest that breeding sites and incubation posture influence the ability of parents to manipulate egg position, and thus selection acting during incubation may influence egg-shape variation across birds as a whole.

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.…”

“…We did not find any of these factors to be a significant predictor of egg shape variation at a global scale across the more than 1200 species for which data were available. Therefore, our results (Stoddard et al 2017) at the global scale do not necessarily contradict the more taxonomically focused analyses of Birkhead et al (2019) but rather suggest that different rules emerge at different taxonomic scales. All of these factors were included in our comparative models, which used backbone phylogenies based on Jetz et al (2012) and Prum et al (2015).…”

“…First, we wish to clarify that we did include factors related to incubation in our global analysis (Stoddard et al 2017). Birkhead et al (2019) point out that 'as powerful as comparative studies can be, broad-based analyses like that of Stoddard et al (2017) can mask effects that differ from taxon to taxon'. We did not find any of these factors to be a significant predictor of egg shape variation at a global scale across the more than 1200 species for which data were available.…”

“…Birkhead et al (2019) have focused on egg shape in two extreme seabird families. Birkhead et al (2019) have focused on egg shape in two extreme seabird families.…”

“…Alcids, which include cliff-nesting guillemots (Uria spp.) Birkhead et al (2019) emphasize the importance of incubation site and the role of the incubating parent's posture as selection pressures acting on egg shape. 1)particularly in species nesting on rocky cliff-ledges.…”