Birds cannot breed where air is thinner because they cannot take in enough oxygen, so fewer species are found at high elevations. Polar and temperate regions are relatively species-poor because glaciation events repeatedly forced organisms toward tropical latitudes. Because penguins cannot fly, they are forced to remain in the Arctic, and there are few species of birds that can survive such harsh winters. Birds prefer to live near oceans, and proximity to the ocean has a great effect on bird diversity. Because polar bears prey on birds, there are few bird species that can live in the Arctic.