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Female still in winter plumage?
I take no responsibility for her dress sense...
Seriously, I think it's quite interesting the variation in when birds of the same species switch plummage. I saw a bunch of Black Headed Gulls the other day with almost every stage from white head with small black spot to a nearly black head.
Some of the difference may be due to birds from different areas migrating into the same place, perhaps. Maybe age comes into it (that's only a guess). Perhaps it's just dimorphism - variation with the species. I don't know.
There must be triggers that set the change off - temperature, length of day, food availability, age all spring to mind as possibilities; but if there's a bunch of the same species in the same place, how come there's variety?
Tricky to establish with any certainty.
Lat/Lng: 52.2477, -0.24862
OS grid ref: TL196625