Apparently the black and white swans behave in a similar fashion towards their young? Seen this often with Mute Swans.
In this instance the cygnet on the back only became obvious when the little darling put its head above mum's feathers. So perhaps this behaviour is more common but not so easily observed. Wiki notes that there is a difference between the black and white swans in this regard: "Cygnets may ride on their parent's back for longer trips into deeper water, but black swans undertake this behaviour less frequently than mute and black-necked swans.".
It seems that there are quite some differences in the behaviour of the mute vs the blacks wrt to aggression and social grouping...an English friend in the neighbourhood scientifically summed it up by describing the blacks as "darlings" but the whites as "bastards".
