The main way in which the relative densities of oil and water affect sea birds, aquatic mammals, and animals that occasional swim or hunt in bodies of water has to do with the effects of oil spills such as Deepwater Horizon or the Exxon Valdez. Accidents with ocean-based oil rigs and oil tankers cause oil to be spilled into the environment. Because oil is less dense than water, it floats on the surface of the ocean, often traveling long distances and spreading over large areas on the ocean surface. This affects sea birds and aquatic mammals and other species that come to the surface of the ocean. They get covered in oil and usually die from it.