Bird egg
A bird egg is the reproductive structure produced by female birds, consisting of a hard, calcareous shell that encloses nutrient-dense yolk, protective albumen, and embryonic membranes, providing a self-contained environment for embryonic development and hatching. Bird eggs range in quantity from one (as in condors) to up to seventeen (the grey partridge). Clutch size may vary latitudinally within a species. Some birds lay eggs even when the eggs have not been fertilized; it is not uncommon for pet owners to find their lone bird nesting on a clutch of infertile eggs, which are sometimes called wind-eggs.