Large reservoirs like Harlan County Lake provide habitat for aquatic birds, many of which eat fish such as small gizzard shad and carp. To avoid competing for food and territory, these birds forage in different parts of the reservoir, at different water depths, and in different seasons. Great blue herons are wading birds that feed on small fish, insects, and frogs in shallow water and nest in tree colonies called rookeries, including one in large cottonwoods west of the highway bridge; they are easiest to see at Harlan County Reservoir in summer, when the lake has been drawn down and shallow feeding water is abundant. Double-crested cormorants catch fish by diving and chasing prey in deep water, and because their feathers absorb water to help them dive, they are often seen perched with wings spread to dry; they are viewed here mainly in spring and fall. White pelicans feed by scooping fish from just below the water's surface with their large bills, taking in water and fish in the bill's pouch and then squeezing out the water; they feed in groups, herding small fish into shallower water, and are most often seen here in spring and fall as they migrate to and from nesting colonies farther north. Bald eagles are relatively abundant here in winter, feeding on fish and wintering waterfowl; they traditionally roost in trees downriver of the dam, and at sunset begin moving toward the roost, passing along the route near the pull-offs by the dam. Large reservoirs that do not freeze over are ideal wintering habitat for common mergansers, which are diving ducks that feed mostly on fish; they can often be seen here in fair numbers during December and are quite abundant in spring.