Following my further attempts to discover the origin of ‘Goldpolls’ Hein Van Grouw (Curator, Bird Group, Department of Zoology at The Natural History Museum, Tring) kindly commented on the Stanford individual as follows:
There is a genetic cause (mutation) known that ‘changes’ the colour tone
expression of carotenoids in the plumage from red into yellow.
I’ve seen that mutation in many species, including Redpolls, and in my
opinion the crown colour of this particular bird is also caused by that
mutation. It is of course possible that the relevant gene for ‘yellow carotenoid’
is widely spread amongst certain Redpoll populations. Perhaps all the
yellow-crowned birds seen so far in the UK belong to the same
geographical population and are therefore more or less genetically
related to each other. I’ve seen several museum specimens with this
aberration which were collected more than 100 years ago all over Europe
so the gene is not uncommon in the species.
In his detailed online redpoll identification paper Worcestershire Redpolls and a Guide to their Separation, Andy Warr states:
First-winters and female Lesser, Mealy and Arctic Redpolls generally show a crimson cap, with varying degrees of brightness, though it is not uncommon for some to show orange, copper, yellow, even brownish caps, or any combination of the aforementioned. These colour variations are exceptionally present on adult male Lesser and Mealy, though is reported as common in captive bred birds, but more regularly encountered in adult male Arctic.
The paper includes a range of photos to illustrate the variation in cap colour.
So, mystery solved? Maybe … In the meantime here’s a photo of one with a not so obvious yellow crown taken at Aylesbury in neighbouring Buckinghamshire by Mike Wallen in March 2006. The worn, buffy tips to the outer greater coverts point to it being a first-winter. It was in a flock of about 50 in an urban area.