The Avian Egg: Mass and Strength

Abstract

Paired values of yield point force F (the load required to initiate egg crushing when applied to both egg poles), and eggshell thickness L of 47 spp. of birds ranging in mass between approximately 1-1500 g, are reported. An equation describing F (g) as a function of egg mass W (g) was obtained: F = 50.86 W0.915. L (.mu.m) for the same eggs was formulated as L = 56.65 W0.458, not significantly different from the general equation obtained for 3434 eggshell thickness values gathered from the literature: L = 54.06 W0.448. Egg strength as measured by yield point force is correlated with thickness squared. Thus, F/L, force per unit thickness, is proportional to shell thickness among species. This ratio may be used in resolving influences of treatments within a species: DDE-treated Streptopelia risoria lay eggs which are weaker because their shells are not only thinner but also of lesser quality. The calculated constant of F/L2 = 1718 kg/cm2, estimates the yield point stress of the typical avian egg and is employed to predict F from eggshell fragments. Given a bird mass B, a safety factor against egg breakage in the nest may be designated as S = (F/B)-1. Its dimensionless value in different species tends to decrease with increasing egg and bird mass, indicating that large bird eggs are more susceptible to accidental breakage in the nest and would be more influenced by environmental contaminants.