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.
Many species including raptors that breed further north migrate through this area in spring and fall. Over half of the species breeding in the North Cascades are migratory species. Hummingbirds, flycatchers, vireos, swallows, thrushes, warblers, tanagers, and grosbeaks are among the species that return annually in spring. These migrants fly thousands of miles from their winter homes in the southern United States, Mexico, Central America, and even South America to breed in the park. May through July, male olive-sided flycatchers, warbling vireos, Swainson's thrushes, Wilson's warblers, and western tanagers sing from conspicuous perches to attract females and defend territories, all part of their annual breeding cycle. In August and September, having raised another generation of offspring and molted new feathers, they migrate south, returning to their winter homes. Birds reflect changes to our environments. By monitoring their populations, distributions, and such demographic attributes as productivity and survival, birds can serve as "early warning signals" for environmental problems occurring in and around the North Cascades.