Holbøll’s Red-necked Grebe revisited: are they doable in the field?
The Red-necked Grebe presents two isolated subspecies: grisegena in western Eurasia and holbollii in Far Eastern Asia and America. Although holbollii is primarily a Pacific taxon, birds breeding in central Canada and the US are pushed east when the interior lakes freeze, and they commonly winter along the American Atlantic Coast, making them prone to vagrancy to Europe. Indeed, there are several accepted records—see Riddington et al. (2024) for a review—in Iceland, Norway, Sweden, and France, but all these records were of birds found dead, and the identification was based on measurements.
In February 2024, a holbollii Red-necked Grebe was found in Shetland, UK. The bird was suspected to belong to this subspecies when first seen in the field and was later found dead, allowing its identity to be confirmed through both genetic analysis and measurements. While the authors were reasonably confident that their candidate was a true holbollii based solely on field observations, it may not have been accepted by the BBRC without measurements. Thus, the challenge of identifying an acceptable holbollii in the field remains. A short paper by the finders describes the circumstances of the sighting and provides an overview of the identification challenge, although it mainly focuses on measurements, which somewhat limits the discussion for future (living) candidates.
In January 2025, a Red-necked Grebe was found in Cantabria, northern Spain, not far from where I currently live. The bird appeared long-billed and especially long-necked, prompting some discussion about its subspecific identification. It showed up after a strong gale that hit the Spanish coast from the central Atlantic. However, the other remarkable birds that arrived the same day were of undisputed European origin (Black-throated Divers, Velvet Scoters, etc.). This sighting led me to revisit the subject and undertake a more systematic study of the variability in plumage and structural features in both subspecies, exploring their diagnosability in the field—since, currently, the only discriminant character described in the literature is bill measures, which obviously requires measurements.
Plate 1. Holbøll’s Red-necked Grebe (left), Scotland, February 2024 (Roger Ridington), and European/Holbøll’s Red-necked Grebe (right), N Spain, February 2025 (Juan Manuel Pérez de Ana). The bird on the left, confirmed as holbollii by measurements and genetics, is a good example of a typical (though not particularly extreme) holbollii, with a long, slim, and pointed bill, a long neck, and darkish flanks. The Spanish bird is similarly long-necked and structurally bulky, with only a slightly shorter bill—could it be a holbollii?
In this post, I present the results of this quantitative characterization of feature variation with the goal of finding a combination of features that is diagnostic enough to identify holbollii in the field. The analysis reveals the importance of the flank pattern, which is dark in many holbollii and white in most grisegena, as an identification feature that, when combined with differences in bill shape, can lead, in my opinion, to the correct assignment of holbollii in a vagrant context. Based on this criterion, I will discuss the identity of some candidates from western Europe found through virtual screening in eBird.
* An important warning: in the pictures that accompany this post, I have overrepresented the “identifiable holbollii” phenotype to help solidify it in the minds of European birders, but there is considerable variation. I have included numerous external links (underline text) to help understand this variation and to showcase some extreme examples.
Figure 1. Red-necked Grebe subspecies identification (© Marti Franch), showing plumage and structural variability within ‘identifiable’ individuals of holbollii and grisegena, and ‘unidentifiable’ birds from the overlapping range of variation. Map based on Birds of the World: https://birdsoftheworld.org/bow/species/rengre/cur/introduction
Some thoughts about the concept of ‘diagnostic’
holbollii and grisegena consistently differ in size—holbollii is considerably larger, and this is reflected in all measurements (bill length, bill depth, wing length, etc.—e.g., see Pyle 2007), with some overlap between male grisegena and female holbollii, but negligible overlap between birds of the same sex. The holbollii found in Shetland, for instance, had a bill length of 36.0 mm, whereas Pyle describes ranges of 27–33 mm for male grisegena (n=49) and 30–43 mm for male holbollii (n=88).
The question, then, is: could we assign this bird to holbollii based on measurements alone? Let’s make some simple calculations, trying to overcome the very inconvenient fact that the measurements are given as minimum–maximum ranges instead of mean and standard deviation, which would obviously provide a more rigorous description of the feature distributions. Pyle states that the ranges are derived from the 95% confidence intervals, meaning the mean is roughly the midpoint (30.0 mm for grisegena and 36.5 mm for holbollii), and the standard deviation should be 25% of the range (1.5 mm for grisegena and 3.25 mm for holbollii). Taking these estimated values and assuming a Gaussian distribution, we find that only 0.03% of male grisegena have a bill length of 36 mm (or longer), whereas as many as 43.89% of holbollii do.*
0.03% is certainly a small proportion, but considering that the European population is estimated to be 55,400–95,800 mature individuals (BirdLife International 2015), there might be more than 20 individuals in Europe with a similar bill length—likely too many to use this measurement alone to identify a holbollii on this side of the pond. In a vagrant context, the probability of a feature being expressed should be weighted against the frequency of each taxon in the region.
In these circumstances, birders typically rely on the combination of two rare features—a strategy pioneered by innovative gullers Peter Adriaens and Chris Gibbins to confirm difficult identifications, such as distinguishing Common Gull subspecies or American/European Herring Gulls by analyzing two or more aspects of the primary pattern. This approach is only valid when the two features are independent—for example, we cannot use two measurements at the upper range of variation, as large individuals tend to be proportionally large across all features. Similarly, if assessing the level of barring in the underwing of a shorebird, we cannot simultaneously use barring in the rump as a separate factor, since both are ultimately determined by the same genes and may be correlated.
Thus, the challenge is to identify a combination of weakly differentiated features (which cannot serve as diagnostic on their own) that become diagnostic when present together. This, in my view, is the statistical definition of "doable" when dealing with two difficult-to-separate subspecies!
* This is a simplified calculation to illustrate how we often use measurements in bird identification. These numbers should not be taken literally, as a more comprehensive dataset would be needed for a solid analysis.
Red-necked Grebe subspecies: identification features
Whereas it is difficult to separate grisegena and holbollii with confidence in a vagrant context, the average holbollii is quite a different bird from the average grisegena. Structurally, holbollii has a longer bill, longer neck, and a more robust appearance—sometimes recalling Great Crested Grebe, or, in the case of the largest males, even a small diver. In terms of plumage, holbollii is typically much darker than grisegena, especially in the flanks, which are usually concolorous with the back in holbollii, and white in grisegena. holbollii also shows a darker head (i.e., an extension of dark areas around the cheeks). All these features, however, are often quite difficult to assess: the bird’s structure changes constantly as it dives, and a bird can look quite long-necked and long-billed or suddenly change and look pretty average within seconds. The plumage features also strongly depend on the light conditions or when the bird is wet.
Other features that appear to show slight differences in proportion between the subspecies include bill color, with holbollii more frequently displaying an almost entirely yellow bill—though this still represents a small percentage of the overall population. Additionally, the iris color of first-winter birds tends to be pale yellow more often in holbollii than in grisegena.
Plate 2. European Red-necked Grebe P. g. grisegena (Delfín González – N Spain) and Holbøll’s Red-necked Grebe P. g. holbollii, Massachusetts, December 2016, USA (Christopher N. Ciccone). A comparison between two typical individuals of each subspecies. grisegena is often strikingly small and short-billed, with a compact, short body. holbollii is generally much bulkier and longer-necked, giving it a more diver-like structure. Note also the more extensive black on the face and the solid dark ashy flanks of the Nearctic subspecies.
To evaluate feature variation, I have focused on four features that, on the one hand, are easy to check in field conditions and, on the other, seem to be the best indicators of the subspecies: bill length (proportional to the head), color of the flanks, extension of dark areas around the cheeks, and the presence of black below the gape. For each feature, I categorized it into three representative groups and counted the feature distribution from the eBird galleries. The groups are obviously subjective, but I believe the exercise is robust enough to characterize key feature variation.
For grisegena, I selected birds from central Europe (i.e., filtering by ‘Europe’ and then removing birds seen in the UK, Ireland, and Iceland to avoid biases from potential undetected holbollii; most birds are from Germany, Poland, and the Czech Republic). For holbollii, I filtered by the US. In both cases, I only studied birds photographed between October and March and removed those still in juvenile plumage (in October) and molting adults (in March).
Bill Length:
As the measurements suggest, holbollii usually appears very long-billed, often strikingly so (around 50% of individuals). In contrast, grisegena typically has a short and petite bill, almost Little Grebe-like (~60%). This fact may have obscured our perception of the extensive variation within grisegena, as some individuals (although rare, 2–3%) exhibit a bill length comparable to the average holbollii—though certainly not reaching the upper quartile of the holbollii range.
These extreme grisegena, likely males, are also the most robust and long-necked, with bills that are long but usually very thick-based and thus its shape looks triangular (see eg birds in Plates 7 and 8, and other examples here, here and here). In many holbollii, the bill is long but thinner, appearing more elongated and pointed, forming a more angular profile with the steep forehead. In some individuals, the bill is also slightly curved downward. This characteristic bill shape (seen, for instance, in the birds in Plates 1-left, 4, 5 and 6), with a low thickness-to-length ratio, is never exhibited by grisegena.
Thus, when encountering a long-billed Red-necked Grebe, it is always crucial to assess the bill shape. This has been noted in the literature in the past, and for instance Ridington et al. describe that they computed this ratio from photographs of the Scottish holbollii before the bird was found dead. However, as they describe in their paper, the difficulty of accurately measuring lengths from photographs makes this method somewhat unreliable. Here, we will assess bill shape qualitatively rather than attempting a precise quantitative measurement. Other sources have tried using the bill-to-head ratio, but due to the same uncertainties, the results are similarly unreliable.
Flanks
Surprisingly, as far as this feature is not mentioned in the literature, despite having the most divergent distribution between the two subspecies. grisegena typically has white flanks that contrast sharply with the back. While some individuals show variable greyish flecking, giving the flanks a slightly greyish appearance or sparse white-and-dark striping (Plate 9), the contrast remains.
In many holbollii, however, the white feathering ends at the lower breast, and the flanks are a dark ashy-brown, blending with the back and creating no contrast - like in breeding plumage (see Plates 2-right, 3-left, 4 and 5). That said, some variation exists, and a few holbollii do present white flanks similar to grisegena (some examples to illustrate the variation of pale-flanked birds: here, here, here and here). However, grisegena individuals with entirely solid dark flanks are extremely rare—if they exist at all: see this individual, although the dark is not as solid in typical holbollii.
It is important to note, however, that flank color can change significantly depending on whether the feathers are wet or dry. grisegena individuals with very white flanks when dry may appear to have darker (though not solidly dark) flanks when wet. Therefore, it is essential to observe how a bird’s flank color varies in different conditions, as demonstrated in Plate 10.
The flank pattern is also evident in flight: in grisegena, the white flanks appear as a fine line beneath the wing, whereas in most holbollii, they form a broad band—reminiscent of the difference between Black-throated and Pacific Divers. Compare this grisegena with this holbollii.
It is also worth noting that by late winter, some birds have already molted into breeding plumage, during which their flanks become darker. Additionally, flank feathers seem to wear over time, and in some first-winter holbollii, worn flanks appear lighter by the end of winter.
Face pattern
Although this may be the most variable and least reliable feature, holbollii often appears very dark-faced, with the dark coloration extending well onto the cheeks and auriculars. In contrast, grisegena is usually paler, with cleaner cheeks. However, there is considerable variation, and dark-faced grisegena are regularly observed in Europe (10–15%).
This feature is also often difficult to assess. Most birds have fine grey streaking on the cheeks, which can appear quite dark when wet or, conversely, nearly invisible under strong lighting. This variability makes it somewhat challenging and subjective to draw firm conclusions.
Extension of black below the gape
This is essentially a subfeature of the previous characteristic, but if a bird is seen close enough, it is easier to assess and less subjective than the overall face pattern. In most grisegena (~79%), the dark cap ends precisely at the midpoint of the gape, whereas in holbollii, the black usually extends further, surrounding the gape (71%).
Interestingly, the few grisegena individuals with extended black gapes do not necessarily have particularly dark flanks, suggesting that these two melanin-related features are not strongly correlated—contrary to what one might expect.
Understanding the feature variation in pictures
Plate 4. Holbøll’s Red-necked Grebe, January 2025, Massachusetts, USA (Nick Teague). The perfect holbollii, this bird shows two features never present in grisegena: solid dark flanks and a long, slender, pointed bill. The combination of both is, in my view, fully diagnostic, though it may represent only around ~25% of all Nearctic holbollii. Note also the darkish face, although not particularly so for holbollii standards.
Plate 5. Holbøll’s Red-necked Grebes, February 2025, Louisiana, USA (Caleb Persia). The bird in the previous plate is not exceptional—compared to typical grisegena, a significant proportion of holbollii are truly eye-catching. These three birds are all long-necked, long-billed, and have extensive yellow on the bill. Notice that while all three are darker-flanked than grisegena, the degree of 'solidness' varies, with some showing lighter, more washed-out flanks.
Plate 6. Holbøll’s Red-necked Grebe, December 2015, Massachusetts, USA (Jeremy Coleman). Not all holbollii present solid dark flanks - this holbollii does not appear particularly dark-flanked or long-billed (although its slender, pointed bill is somewhat eye-catching), and it is unlikely that it would raise suspicion if seen in Europe. However, note that it might be very large—compared to the Canada Goose—whereas most grisegena are smaller than Coots!
Plate 7. First-winter Holbøll’s Red-necked Grebe, Oregon, USA, December 2023 (Gregory W. Courtney) & juvenile European Red-necked Grebe, Czech Republic, September 2023 (Veronika Švestková). Large males of both subspecies can have impressive bills, and such bills tend to correspond with the longest-necked and bulkiest individuals. In extreme cases, grisegena may overlap in bill length with average holbollii, but its massive bill is typically thick at the base and very triangular, conserving the typical shape of smaller birds—different from the slender, pointed bill shape of many holbollii. Note that the dark flanks of this grisegena are due to the juvenile plumage as it has not moulted yet to winter plumage.
Plate 8. European Red-necked Grebe, January 2023, Germany (Hugo Schlenker). An example of a long-billed, bulky-bodied grisegena that could raise eyebrows if seen in the UK (another example here). However, such clean white flanks seem to be very rare in holbollii. Also, note that the bill is thick at the base and not as elongated as it tends to be in many holbollii (compare, for instance, with the otherwise similar holbollii in Plate 6).
Plate 9. European Red-necked Grebe, January 2023, Czech Republic (Veronika Švestková). An example of a darkish grisegena, with black extending below the gape midpoint and grey flanks—though still far from the classic solid dark of holbollii. Structurally, it is quite typical for a European bird.
Plate 10. European Red-necked Grebe, N Spain, February 2016 (Fran Montaño, left, and Delfín González, right). A good example of the variation in flank coloration within a single individual: this bird had white flanks with some brown feathering, and under certain conditions (when wet and in poor light), it appeared quite dark—though not as solid and dark as in holbollii.
So, are they doable? Some candidates from Western Europe
From the previous analysis of feature variation, the ideal holbollii candidate should have:
a long, narrow, and pointed bill;
solid dark flanks, proven to be truly dark in several situations (not only when wet); and
as a supportive feature, a dark face with black extending below the gape.
I believe that birds with this combination are identifiable as holbollii even in Europe, but they likely represent only a fraction of the population (<20–30%, according to the data presented above). In my view, birds with not extremely long bills or some white feathering on the flanks can still be identified in a vagrant context, though this is certainly open to debate. Looking retrospectively at the Scottish holbollii, for instance, I think it falls on the identifiable side, but it is not a perfect candidate: its bill is long and pointed, though not exceptionally so, and its flanks are dark but show some white feathering that could raise doubts.
The Spanish bird mentioned in the introduction, on the other hand, is clearly far from that "identifiable line" and might be grisegena. In some pictures, its flanks appear quite white, though they look darker when wet. The bill, while certainly on the longer end of the grisegena range, is thick at the base and triangular in shape, suggesting it may be a large male grisegena.
Plate 11. European Red-necked Grebe (same bird as in Plate 1, now re-identified), N Spain, February 2025 (Juan Manuel Pérez de Ana). Note how the flanks change in appearance depending on light and condition: on the right, when wet, they look quite darkish, whereas they appear snowy white when dry.
After using data from Central Europe to estimate the distribution of each feature in grisegena, I once again turned to eBird to search for long-billed, dark-flanked birds in Europe. Only a few caught my attention, presented here as solid candidates for holbollii:
This bird from Iceland (pictures below, Plate 12) with a very long, very thin, pointed bill - holbollii without doubt.
This bird from Ireland, not the most long-billed individual but with the right combination of long bill, black face, and apparent dark flanks - at least with those pictures it looks like a pretty good candidate.
This bird from the UK, that looks quite long-billed, long-necked, dark faced and with relatively dark flanks also might be holbollii, although the pictures are a bit distant and it would be safer to evaluate the flank color when the bird was dry.
And this last bird from Spain (pictures below, Plate 13), best left unidentified in my opinion, as the flanks look whitish in some pictures (although compatible with the palest end of the range of holbollii).
Plate 12. Holboll’s Red-necked Grebe, Iceland, March 2024 (Ingvar Atli Sigurðsson). A striking bird, with a long and pointed bill that perfectly matches the typical shape of holbollii. The bird is also black-faced, with black extending below the gape. The flank pattern, however, is a bit tricky to interpret: the feathers appear very worn and may look paler due to abrasion. It also shows some black patches that could be the first pre-nuptial feathers.
Plate 13. Red-necked Grebe ssp, Spain, December 2020 (Juan Manuel Perez de Ana). A confusing bird: powerful, long-billed, and dark-faced, it generally looks like a strong holbollii candidate. However, its white flanks fall at the palest end of holbollii variation, making it best left unidentified.
Acknowledgements
I would like to thank Marti Franch for creating the artwork that illustrates this post and truly helps in understanding the feature variation. Roger Riddington kindly shared his thoughts on this subject with me. I would also like to thank all the photographers who allowed me to share their photographs here.
