Bird eggs are the reproductive structures laid by female birds. They come in various shapes, sizes, and colors, depending on the species. Most bird eggs have a hard shell made of calcium carbonate, which protects the developing embryo inside. The average incubation period varies, with some eggs hatching in just a few days, while others may take weeks. Bird eggs are often found in nests, which provide safety and warmth. The number of eggs in a clutch can range from one to over a dozen, depending on the bird species. Some well-known examples of birds that lay eggs include sparrows, eagles, and penguins.