Argentine ruddy duck

Oxyura vittata. Immature male.
The flamingos were my last stop on this trip to Chester. I was surprised when this little chap bobbed to the surface as no stifftails were mentioned on the labels. It was so dark in colour that my first thought was 'Musk duck!' - but I knew that had to be wrong for a South American aviary. The hint of blue on the bill wasn't visible on the live bird, but it was a clue and the breadth of the bill was also important. ZTL confirmed the ID.