The mouth of the river and harbor provide waterfowl and waterbirds a sheltered winter environment from the gales of Lake Ontario. Breakwalls serve as resting places for gulls where approaching predators may be quickly detected, while shielded water at the river's mouth lets diving ducks harvest food. Large flocks improve social interactions that help birds find mates for the summer, making this a safe winter place to prepare for the next breeding season. By mid-January, courtship begins in many ducks, and on warm sunny days several male Common Goldeneye may display for a single female, pumping their heads back and forth to outdo rivals. The large flocks of gulls and waterfowl also attract predators including Bald Eagle and Snowy Owl. An adult Bald Eagle may flush a flock of ducks, hover over a sick or injured bird, and force the weakened bird to dive until exhausted before taking it from the water. These predators help prey species by removing sick birds and limiting the spread of disease, while sustaining themselves through the winter before migration and breeding. Four common gulls here are Great Black-backed, Herring, Ring-billed, and Bonaparte's; as Bonaparte's leave in early winter, rarer Arctic gulls including Glaucous and Iceland Gull arrive, with pure white immature plumages and adults lacking black wing tips. Gulls often steal food from diving ducks, and larger species such as Glaucous and Great Black-backed may take small waterfowl and gulls. Diving ducks such as Common Goldeneye, Bufflehead, Greater Scaup, and Long-tailed Duck predominate in wintering flocks, while dabblers including Mallard, American Black Duck, and Gadwall also occur, though the fast river water and deep harbor better suit diving ducks that can reach depths of 20 or more meters where food is readily available.