section_path: Eggs of different animal groups/Bird eggs/Shell
section_name: Shell
paragraph_text: Shell
  
  Bird eggshells are diverse. For example:
  
  Tiny pores in bird eggshells allow the embryo to breathe. The domestic hen 's egg has around 7000 pores.
  
  Some bird eggshells have a coating of vaterite spherules, which is a rare polymorph of calcium carbonate. In Greater Ani Crotophaga major this vaterite coating is thought to act as a shock absorber, protecting the calcite shell from fracture during incubation, such as colliding with other eggs in the nest.