In 1962, five mute swans escaped from captivity and began a breeding population in Chesapeake Bay. Today, there are over 4,000 mute swans living in the bay. Each year they eat approximately 10.5 million pounds of aquatic grasses. These grasses provide habitat for waterfowl and crustaceans, improve water quality, decrease erosion, and increase dissolved oxygen concentrations in the bay. The birds are also aggressively territorial, and have been known to trample nests of other birds (e. g., least terns and black skimmers) and drive away native birds like tundra swans and black ducks from feeding and roosting areas.