Many of you will have seen the possible Barrow's Goldeneye that has been present mainly in the Heron's Green Bay area since its discovery on 22 December 2007. Although it initially seemed a good candidate, as time has gone on it has become apparent that it shows a number of features that are not compatible with the species. In an attempt to resolve its identification, I visited Slimbridge on 5 January and had a good look at five female Barrow's Goldeneyes in the collection and took a lot of photos, some of which are reproduced here.
The Chew bird does indeed have a number of pro-Barrow's features:
Most obviously, it has a vertical forehead with a flat crown, producing the general impression of a large and rather bulbous head. It consistently shows this although of course the crown feathers are flattened to some extent when it is diving.
Secondly, the head colour is distinctly darker than Common Goldeneye, being chocolate brown.
The profile of the culmen is different in that the basal two-thirds of the bill appear more 'swollen' than the tip. When it is viewed exactly in profile, this creates a slight but distinct 'step', about two-thirds of the way down the bill. In addition, there is a subtle 'tubular' structure extending back from the nostril, reminiscent of the tube-nose of a storm-petrel. While it is apparent that not all Barrow's show this, I can find no evidence to indicate that Common ever does. Incidentally, I noted these differences in bill structure on several Barrow'sGoldeneyes that I saw in Iceland in 1997.
Although I haven't been able to see this myself, Andy Davis and Richard Mielcarek assure me that, in direct comparison, it is distinctly larger than Common.
The eye is whiter than Common.
Female Barrows' Goldeneye, Slimbridge WWT, 5 January 2008. Note the oblong head shape and the 'mane' at the back of the head of the swimming bird (left); the oblong head shape and the 'mane' at the back of the head is obvious on the sleeping bird (inset). Unlike adult female Commons, most Barrow's lack significant white on the forewing (right). (Keith Vinicombe)
The pro-Common Goldeneye features are as follows:
It lacks a strong bulge on the rear of the head. The Barrow's at Slimbridge had a conspicuous triangular bulge that was especially prominent when the birds were sleeping with their bills tucked back into their scapulars. When awake, the heads of the Slimbridge Barrow's looked quite oblong from front to back, with a bulging 'mane' at the rear of the head. The bulge at the back can virtually disappear when the head feathers are flattened, so you have to be wary of looking at photos of birds with their heads sleeked down (eg when anxious or about to take flight as in some of Richard Andrews' and Gary Thoburn's shots).
The brown of the head does not appear to extend down onto to the top of the fore-neck as it does on Barrow's.
There is a large white oval patch on the forewing which most Barrow's seem to lack. However, some Barrow's do show more white than others and I have found one shot of a female Barrow's on an American website with a large white forewing patch, although even that bird had grey feathering mixed into the rear line of feathers at the outer end of the patch. The Chew bird's upperwing appeared to be classic Common. Interestingly, Andy Davis discovered a paper that indicated that those female Common Goldeneyes with most white in the wing were the most reproductively successful. It seems possible that this might also apply to Barrow's, explaining the individual variation. It should also be noted of course that first-year goldeneyes will show less white than adults, particularly in the fore-wing. Because of this, it seems safe to assume that our bird is an adult.
The Slimbridge Barrow's had noticeably stubby bills - which reminded me of the shape of a clog! The Chew bird's bill, although small and rather triangular, seems to be too long.
'Goldeneye', Chew Valley Lake, 23 December 2007. This shot was taken into the light, so the colours are not very accurate, but note the extent of brown on the neck and the lack of any 'mane' at the bottom of the nape - typical of Common Goldeneye (Rich Andrews). January 12 2008 (inset, Keith Vinicombe).
'Goldeneye', Chew Valley Lake, 23 December 2007. This shot shows the extent of white in the wing well. Contrary to popular belief, the presence of white on the median coverts does not necessarily exclude Barrow's as can be seen in the bird here: http://www.pbase.com/image/37463162. That said, the amount of white on the Chew bird looks too much for even a well-marked Barrow's. (Rich Andrews).
So what is it? Given the apparent mixture of features, the most logical explanation is that the bird is a hybrid. Hybridisation between the two species is well documented although all the examples so far described appear to have been males. It is known that some Common Goldeneyes occur in Iceland and, conversely, there have been vagrant Barrow's seen in Britain and Ireland, as well as in Norway. Conversely, of course, it could be an escape, although Andy Davis is confident that it is not ringed.
After the recent presumed Redhead x Pochard hybrid, this bird is yet another disappointment, the latest in a long list that includes presumed Pacific x European Golden Plover, Caspian x Herring Gull and Canvasback x Pochard, plus another ten Aythya hybrids seen there in November!