Whistle Register? Well here you go.
The Whistle Register, also known as the 'Flageolet Register', is the highest human vocal register. Its name is derived from the whistle sound that is produced when notes are sung in this register. A tone is classified as being a "whistle" note typically when it is a C6 or over.
It is also possible for males to sing in this register with one of the most famous being Adam Lopez. Listen to him sing some incredible high notes as he covers Mariah Careys Emotions.