Percentages vary considerably by habitat and geographic location.
Many exotic bird species were introduced to the United States by European colonists who missed the familiar birds of their homeland and tried to establish populations of familiar Old World species. Farmers also saw opportunities for pest control by birds such as starlings and house sparrows, but they did not anticipate the degree to which these exotic species would outcompete native birds for nesting sites. Most introductions, however, were by sporting or hunting organizations and state game departments that wished to provide more hunting opportunities. Competition between exotic and native species has been particularly severe on islands. In the Hawaiian Islands, introduced songbird species far exceed native ones.