At what age do mute swan cygnets get their black bill knobs?

[Squirrels]

Hi,

Can anybody tell me at what age mute swan cygnets get their black bill knobs? We are following the progress of a family of 2 parents and 2 cygnets at our local park. The cygnets are now 7 months old and still have grey beaks and lack the black bill knobs.

[Gill Catton]

The black markings get steadily bigger and more defined with age (also bigger in males)- within a year to a year and a half they should look like adults