Black-necked Swans

The South American Black-necked Swan, Cygnus melanocoryphus, only develops the full black neck on its first adult moult. These swans are quite vocal and have a soft musical whistle. Though less aggressive than their closest relatives, the Mute and Black Swans, they are pugnacious in the breeding season.
Black-necked Swan cygnets
Black-necked Swan cygnets — Trudy Robertson