Birds are significant components of biological diversity within the North Cascades ecosystem. Over 200 species in 38 families can be found in park habitats that range from alpine meadows to low elevation forests and wetlands. Three species (bald eagle, marbled murrelet, and spotted owl) are listed as 'threatened' under the federal Endangered Species Act. The rivers, lakes and streams of the North Cascades attract breeding, migrating, and wintering birds. The Skagit River attracts one of the largest wintering concentrations of bald eagles in the continental United States. Clear, fast-flowing rivers and streams host breeding populations of Harlequin ducks.