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.
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.
I spent this morning with the Stanford Ringing Group, which is currently enjoying a record year, having ringed more than 5000 birds since January! Ringing totals for most (not all) species have exceeded those of previous years making Stanford one of the top inland ringing sites in the UK. Among today’s list of trapped species were Redwing, Goldcrest, Blackcap, Reed Bunting, Linnet, Goldfinch and Greenfinch. During an apparent influx of migrants, double figures of the latter were trapped, each individual being 3-4 grams below the expected weight, indicating that they had burned off some of their fat reserves through migration. Of further interest, however, was the half a dozen Lesser Redpolls trapped, one of which was strikingly different to the others. This individual, a first-winter, had the red poll replaced by a gold one and this is the first time I have encountered a redpoll exhibiting this exceptional crown colour.
It is not without precedent, however, as two among 118 Lesser Redpolls trapped and photographed by Horsham Ringers during the first week of this month also showed this crown colour, as did an apparent Mealy Redpoll photographed in northern England in March 2010. For comparison, here is a ‘normal’ first-winter Lesser Redpoll.
It would be good to hear from anyone who can shed any light on what these ‘Goldpolls’ are or where they come from!
Pitsford causeway provided the stage for a brief, early afternoon performance by a species not seen in Northants since October 2000! Twite – that unassuming upland counterpart of our more familiar Linnet, more uniformly coloured, less white in the wing, rich buff throat, a barely discernible pink rump and, the bit which gives it its scientific name, flavirostris, in winter, a yellow bill. Most of these features are nicely illustrated in finder Adrian Borley’s image below. In fact it doesn’t seem so very long ago that I was walking around Pitsford Reservoir counting them. Nowadays you’d be incredibly lucky to find one in Northants, although until the early 1990s they were recorded annually in small numbers as scarce winter visitors and the ‘big side’ at Pitsford was definitely the place to find them. Cycle-tracks, ice cream vans and gaudily-clad Joe Public were, of course, things of the future. ‘Progress’ not withstanding, where are they now and why don’t we get them any more?
Traditionally, Twite has occurred in Northants in late autumn, with November being a prime month for arrivals and a build up to wintering. However, since the last few records of odd ones and twos in autumn, Twite in Northamptonshire has assumed vagrant status. I put the graph below together (for the Northamptonshire Bird Club Newsletter in 2003) to illustrate just how many were found in the early to mid-1970s, most of which were long-staying, wintering birds (figures from the Northamptonshire Bird Report 1969-2001). Indeed, it was possible to locate a wintering flock in excess of 40 individuals at Pitsford on some occasions. The UK Twite population has declined considerably with an estimate of only 9,950 pairs in 1999 (Langston et al) with around 100 of these in England. Here it is believed the decline has resulted from decreased breeding success associated with changes in feeding area habitat from hay meadows to silage production, resulting in less available food and fewer second broods (LBAP, 2001).
Langston, R.H.W., Smith, T., Brown, A.F. & Gregory, R.D. The Status of Twite Carduelis flavirostris in theUK in 1999.