Whilst spending sometime looking through the gulls on the boating lake, it struck me how varied the level of moult was amongst the 1st calendar year (Juvenile) birds. An early moult of juvenile mantle and scapular feathers is meant to be a feature shown by young Yellow-leg and Caspian Gulls, but as can be seen in the shots below, even by 2nd week of September there are some Herring Gulls with a very progressed moult throughout the mantle and others with almost wholly retained juvenile feathers. I saw most/all of these individuals in flight and there was nothing about the tail pattern nor inner primaries that suggested anything other than Argenteus Herring Gulls. I’m guessing by this time of year that feature becomes less relevant, with all things relating to the ID of large immature gulls, it’s about a combination of features rather than one alone.
A few feathers moulted but mostly this individual has dark centred juvenile mantle feathers.
A very juvenile looking Herring.
Another Herring with just a few new feathers beginning to show.
In contrast the Herring above has moulted all of it’s mantle feathers from dark centred juv. ones to paler, arrow headed “1st winter” ones. Emulating a feature of both 1st Cal. Yellow-legged and Caspian Gulls.
And a final Herring with lots of moult going on in the mantle.