by Guillermo Rodríguez
The American/East Asian subspecies of Red-necked Grebe, P g holboelliii, has been observed in the WP with accepted records in the UK (a bird shot in Wester Ross in September 1925 and subsequently identified based on measurements), Spain (two winter records from Galicia in 1984 and 1987, which were identified in the field, although these records will likely be reviewed again in the near future by the Spanish Records Committee), Iceland (at least five), and single accepted records in Sweden and Norway, in addition to several other reports. Since they are quite common as wintering birds along the American Atlantic coast (where, for instance, Pacific Diver is scarce/rare), they should be expected to reach Europe regularly. But do they?
holboellii is known to be larger and darker, and to have more yellow in the bill than the nominate grisegena. According to Pyle (2008), the differences in size are likely to be significant enough to clinch the ID. For example, the wing length is 180-212mm in holboellii versus 153-188mm in grisegena, showing that there is limited size overlap. However, these differences would obviously require in-hand measurements for identification. It’s my understanding that separation of holboellii from grisegena is currently considered to be impossible under field conditions, and the validity of the pattern of yellow in the bill has also been questioned (because a few grisegena show yellow bills similar to holboellii). However, the longest-billed holboellii show impressive harpoon-like bills which, in my opinion, are clearly outside the range of variation of grisegena.
Another identification feature that’s not usually mentioned in the literature is the general body structure; holboellii is considerably more powerful, with a longer neck, a longer and stronger bill and a flatter forehead. The head often looks square rather than rounded (although it’s also important to consider the age-related variation of the head shape, since first-winter birds tend to show rounder heads in general). As a useful comparison, holboellii somewhat resembles Great Crested Grebe. The main problem is that structure is subject to interpretation, and any identification solely based on the jizz is usually disputable. Variability in body shape is also quite extensive, and in particular many American holboellii may look as small and delicate as European grisegena. On the other hand, the largest-billed grisegena are at the same time the biggest individuals, with a more powerful structure than average birds, altogether favouring the holboellii impression.
Still, I do think that the largest and most striking holboellii could be definitively identified in the field if one turns up in Europe. For instance, check out two examples of such extremely large birds here and here.
Many first-winter holboellii show a striking pale iris, which forms a contrasting ring around the dark eye that is very obvious with close views. I don’t know the variation in grisegena well (any feedback is welcome!), but my impression is that it isn’t always so obvious; perhaps the iris is on average paler in holboellii?.
The structural differences are easily noticeable even in distant birds at sea:
Sadly not all individuals are so distinctive; for instance the bird below – although it still looks large and long-billed – is probably still consistent with grisegena.
And actually many holboelli look much smaller, more delicate and round-headed:
As a rough estimate, I would say that on the east coast of the United States the proportion of birds that are 1) markedly large and powerful, 2) intermediate 3) small, grisegena-like is somewhere between 20-40-40(%) and 10-30-60(%). My impression from a winter trip to Korea is that Asian holboellii on average are even more obvious, but at the time I didn’t pay enough attention (see some examples from Japan here).
Some birds, particularly adults, are remarkably dark, especially on the flanks; in addition, the facial dark mask sometimes extends towards the cheek.
A couple of birds from Spain
One obvious problem with using structural features for identification is that they are strongly affected by the posture and activity of the birds. Take a look at this (presumed) grisegena from northern Spain photographed on two different days. I have the impression that birds at sea tend to look more like holboellii than birds on calmer waters, such as estuaries, where they tend to look more like grisegena. Presumably this is because the latter are more relaxed, but it’s difficult to say.
The amount of yellow on the bill of this bird is similar to that shown on all the holboellii in previous photos, extending over the entire lower mandible and reaching the upper mandible up to the nostrils.
In March 2015, an interesting Red-necked Grebe was found in Galicia, northwest Spain. The bird was remarkably dark on flanks and cheek and presented a substantially long neck, strong structure, and powerful bill, which accentuated its long-headed impression. This bird was probably within the size range of grisegena (at least it wasn’t one of the obvious and “identifiable” holboellii) and the reduced yellow in the bill was certainly against it being holboellii, but it still gave a Nearctic impression.