Emu. This Australian ratite has no keel on its sternum and cannot fly.		Like many bird species, the mallard exhibits pronounced sexual dimorphism between the sexes.		Like most birds, Laysan albatrosses have specific mating behaviors, such as this courtship ritual.		The barn swallow is a perching bird (order Passeriformes ); its toes can lock around a branch or wire, enabling them to rest in place for long periods.