シロガシラアジサシ Sterna trudeaui

Attractive but often rather local tern of southern South America. Favors shallow coastal waters, but also ranges inland locally along rivers and lakes. Often associates with other terns on beaches and at river mouths. All plumages have diagnostic white head with narrow blackish mask and reddish legs. Breeding adult has an orange bill with a black-and-yellow tip. Nonbreeding birds have a black bill with a small yellow tip. Immature birds have a black bill with little or no pale tip.