North-facing slopes and shady ravines are suitable for northern birds such as Blackburnian warbler, Canada warbler, and brown creeper. Southern birds such as fish crow, cardinal, mockingbird, blue-grey gnatcatcher, tufted titmouse, and turkey vulture are near the northern limit of their ranges, as are opossum, fence lizard, and eastern mudminnow. Goshawk and golden-winged warbler, birds of the north, are near the southern limit of their ranges. Prickly pear cactus grow on dry sunny outcrops, while glacial relics such as tundra bog moss and subarctic black spruce grow on top of the heights, at the southern limit of their ranges. The river channel through the Highlands is deep and turbulent.
Strong currents wash over the rocky bottom.
This deep channel is the major spawning area for Hudson River striped bass. The only other such area on the east coast of the United States is in Chesapeake Bay. Half of the year, during spring and early summer, the Highlands stretch is freshwater. Come late summer, salt water intrudes in a wedge shape upstream. At different times of year one finds either freshwater, anadromous, or marine fishes, or all of them at once.