The shell of a bird egg is hard yet full of tiny invisible pores.

The longer version

Shell is mostly calcium carbonate laid as interlocking crystals around a protein framework, which is why it resists a squeeze from outside yet stays brittle enough to be broken from inside. Running through it are thousands of microscopic channels that let water vapour out and let oxygen and carbon dioxide reach the embryo.

Those pores explain why an egg loses mass as it sits and why humidity matters so much during incubation. Too dry and the chick loses too much water; too damp and it cannot lose enough. A thin outer cuticle blocks most bacteria. A shell thick enough to feel safer would suffocate the embryo, so the balance is tight.

Where this entry sits

Here you will find feathered trivia about flight, migration, nests, eggs, and the songs birds sing. Expect short surprising facts on penguins, owls, hummingbirds, and other familiar birds from gardens, forests, wetlands, and distant coastlines. The full list sits on the Birds page, where every entry is listed in order.