Allometry of Egg Size as a Factor Influencing Egg Shape in Birds

Abstract

Despite a huge diversity of egg shapes among birds, similar egg shapes occur in species that differ in habitats and are not phylogenetically closely related; this indicates the presence of factors limiting the egg shape variability. We assume that one of these factors is a general pattern of egg shape change across all birds. We tested a hypothesis of allometric patterns of egg morphometric traits (radii of blunt end (infundibular zone, ri), pointed end (cloacal zone, rc), lateral zone (rl) and maximum diameter (D)) scaling against egg length (L). Phylogenetic allometric analysis was applied. We also studied hierarchical distribution of variation in egg traits at different taxonomic levels in order to determine the degree of egg shape variability on each of them. We revealed allometric patterns in the scaling of egg traits: D and ri scaled against L with negative allometry, while rc and rl were positively allometric. A high degree of covariation was observed between the length and diameter, radii of the lateral and infundibular zones, i.e. traits that are associated with the overall size of an egg, the degree of its elongation, and the shape of infundibular zone. Evolutionary correlation between L and rc was rather low. The greatest variation of egg traits resided at the level of orders. The identified general trends in egg shape variation are realized on the basis of various groups of birds in phylogenetic and ecological sense that assume the influence of these factors on the bird's egg shape.