Data from: The shape of avian eggs: assessment of a novel metric for quantifying eggshell conicality

Studying avian egg shape and other aspects of its morphology has recently undergone a renaissance. Yet, most studies rely solely on two metrics for the quantification of egg shape: elongation and asymmetry. The difficulty of additionally quantifying the curvature of an eggshell has yielded many attempts including those with complex equations and spatial modeling techniques based on digitized images. These have lacked an independent single-variable metric, hampering comparative studies. We propose a metric for one common quality of egg shape, conicality, which is notably variable in diverse species' calcareous shells including shorebirds and non-avian theropods. This metric utilizes multiple measurements of the slope along an eggs profile to produce a distribution of angular measurements, which can be analyzed with a Kurtosis (K) value. This metric was tested with sets of computer modeled and 3D printed egg forms, where elongation, the percentage of conicality, and the relative curvature of the shell profile were controlled. For applicability to natural eggs and their diversity across taxa, the Kurtosis value was used to quantify the gradient of conicality across a focal avian family, Alcidae, where the Kurtosis value successfully identified the most conical eggs using qualitative descriptions from well-established literature. Given the significance of egg morphology and profile curvature to structural integrity, surface-area to volume ratio, egg mobility/stability, nesting behavior, and embryonic development, our proposed measure of conicality could prove a useful variable to the study of avian and non-avian egg-producing species.