Yellow-legged Gulls, 1cy to Adult (arranged by age and month, rather than by year, to illustrate moult progression).

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Ageing of sub-adults is guided by studies of an individual Yellow-legged Gull which first appeared at Kingsbury Water Park in January 2010. It was then just into its 3cy (and in characteristic 2W plumage) and remained until April 2011, in its 4cy and having undergone a complete moult into 3W plumage during summer/autumn 2010. It has returned and undergone its complete moult in each year since, generally arriving in mid-July and departing in late January. Thus, it has been possible to track and document its appearance in its 3cy, 4cy and subsequently. See:

Studies of an individual Yellow-legged Gull: a photographic essay