Birds’ eggshells are primarily composed of calcite, an abundant polymorph of calcium carbonate (CaCO3). However, the eggshells of some species are coated with spherules of vaterite, a rare and thermodynamically unstable polymorph of CaCO3, the function of which remains unknown. We experimentally tested the mechanical and physiological effects of the vaterite coating on eggshells of the Greater Ani Crotophaga major, a tropical cuckoo. Vaterite removal did not affect vapour conductance rates across the eggshell, indicating that the vaterite coating does not influence gas exchange during embryonic development. However, nanoindentation revealed that the hardness and elasticity of vaterite is similar to that of calcite, and white light interferometry showed that the vaterite layer increased the total thickness of the shell cuticle by up to 10%. Furthermore, calculations of contact mechanics found that when two eggs come into contact, the depth of the surface deformation caused by the contact is far less than the thickness of the vaterite coating. These results suggest that the layer of vaterite spherules may act as a shock absorber for the underlying calcite shell, protecting it from mechanical damage caused by collision with other eggs in the nest and reducing the risk of eggshell fracture during incubation.