I watched the pond in Shatin Park over a month or so as the lilies began to bloom and the resident waterhens moved around very protectively with their little chicks, around 3-4 of them at a time.
The adults do have a loud, insistent call.... I think it sounds like a car alam. The famous Indian naturalist writer Eha describes the call like this "It began with loud harsh roars which might have been elicited from a bear by roasting it slowly over a large fire, then suddenly changed to a clear note repeated like the coo of a dove." What do you think? have you heard it near you, as they are quite urbanised, and I often hear them near villages, and around me in Sha Kok Mei.