Even though they are kept clean, feathers become worn and are usually replaced at least once a year. This process is called molting. Birds lay hard-shelled eggs made mostly of calcium carbonate. The hard shell keeps an egg from dehydrating and allows parents to sit on the eggs during incubation. Even though bird eggs are hard-shelled, they possess microscopic pores which allow oxygen to pass into and carbon dioxide to exit the shell.
Eggs come in a variety of colors and patterns.
Colored and speckled eggs are laid in areas where they need to be camouflaged. This makes them less visible in the dappled sunlight. The shape of the egg depends on where the bird nests. The most common shape for eggs is oval. Birds that lay many eggs typically have eggs that are pointed.