Chapter nine looks at how birds reproduce, through eggs. The various ways birds breed are described, as well as how climate change can affect their breeding success. Males usually compete for mates in various ways like getting territories, singing, visual displays and so on. Nests are built to hold eggs and range from simple one to elaborate houses. Birds may also build individual nests or nest in huge colonies. Eggs may also differ in size, colouration and shape, although we aren't certain why they differ. When eggs hatch, chicks may either be born masked or feathered, affecting how they are taken care of. Some birds, of course, are parasites and lay eggs in the nest of other birds.