An 'Intermediate Brant' from Long Island, New York on 26 January 2003.
Introduction The extensive saltwater bay and marshes of Jamaica Bay on the outskirts of New York City host a large wintering and migrant population of Atlantic Brant (Branta bernicla hrota) and each year includes a few Black Brant (reviewed in Wilson and Guthrie 1999). In addition to these conventional looking Black Brant, a number of 'intermediate' birds have also been discovered that do not seem to fit the 'textbook view' of Black Brant. [Several earlier examples can be found by clicking here.] Although local birders have been looking at and actively discussing these puzzling individuals for several years, no clear consensus has been reached. A recent article in North American Birds also addresses this issue, favoring the idea that these are vagrant Gray-bellied Brant, a small and poorly known population that winters in the Pacific Northwest (Buckley and Mitra 2003). The relative merits of this and other explanations will be discussed below.
This web page focuses on the latest example of these puzzling birds. The featured goose was discovered in small flock of 50-60 Atlantic Brant feeding on lawns at the southern end of Floyd Bennett Field, a section of the Gateway Recreation Area by myself, Andy Guthrie, and four visiting birders from the Albany area. We had already noticed a typical Black Brant (see Figs. 6- 8 below) in this small flock and it was thrilling to find another interesting bird (Figs. 1- 5) a few feet away.
Fig. 1. The most obvious difference from the surrounding Atlantic Brant was the brown wash on the lower breast which extended up on to the shoulder. The division between the blackish neck sock and brown lower breast was obvious for all angles.
Fig. 2. Unlike the Black Brant in the same flock, the intermediate bird (left) the upperparts were only fractionally darker than those of the surrounding Atlantic Brant.
Fig. 3. The dark brown underparts of this bird (left) are very evident when compared directly to adult Atlantic Brant (right) and extend between the legs.
Fig. 4. The neck collar was not particularly prominent and showed a clear break on the front of the throat. While this occurs in female Black Brant, most have a more complete collar across the throat, with a more extensive network of webbing.
Fig. 5. View of the upperside of the neck. The brown wash across the upper flanks and mantle is very evident in this view.
Figs. 6-8. This Black Brant was present in the same small flock as the 'intermediate'. Regrettably, I couldn't manage any pictures with the two together without being one or both being blocked by other birds in this tight knit and actively grazing flock. This bird is obviously blacker than any of the other birds, including the 'intermediate', with a much more prominent collar (somewhat compressed by the lowered head). Contrast between the black neck sock and dark lower belly is quite evident in these photos, and is quite consistent with many Black Brant observed in Central California. Under strong and direct sunlight (typical of many Black Brant photographs on the web and in photoguides) this difference may become less obvious.
So what is the 'intermediate' bird?
During the past few years a number of 'intermediate-type' birds have been observed in the brant flocks that use Jamaica Bay (Kings and Queens Counties) as well as the vast areas of salt marsh that separate the south shore of Long Island from the outer barrier beach (Nassau County). It is not clear whether this reflects an increase in observer awareness or a newer phenomenon. It is worth noting, however, that there has been as also been a clear increase in the numbers of Black Brant recorded in the region (reviewed in Wilson and Guthrie 1999).
So what are these 'intermediate' birds? Three separate hypotheses seem worthy of our consideration: (1) that these are hybrids between Black and Atlantic Brant; (2) that these are examples of variation in Atlantic Brant, or lastly, (3) that these are vagrant Gray-bellied Brant. Given the lack of solid information (namely molecular analysis or banding studies) it seems prudent to keep an open mind. Regardless of which hypothesis wins out, these are delightful birds and their presence encourages us to look with a fresh eye at the flocks brant which spend a good fraction of the year along the Atlantic seaboard.
1) Hybridization in Brant:
This issue should resonate with observers in the UK, where goose watchers and the national rarities committee (BBRC) have been wrestling with a number of non-classical Black Brant. Much of the evidence suggests introgression between Black Brant (B. b. nigricans) and the common wintering form, Dark-bellied Brant (B. b. bernicla). Indeed, several mixed pairs of Black and Dark-bellied Brant, some with offspring in tow, have been documented in Britain, Belgium and Holland (Bloomfield and McCallum 2001). Moreover, careful observation of several apparent adult 'Black Brant' indicates that they are most likely hybrids between Black x Dark-bellied Brant (Martin 2002; Wynn 2002). This raises the intriguing possibility that the intermediate birds from Long Island are the western Atlantic equivalent, namely Black (B. b. nigricans) x Atlantic Brant (B. b. hrota) intergrades. More specifically, hybrids between Black Brant and Atlantic Brant from the Low Arctic population which winters in coastal New York.
Hybridization between vagrant Black Brant and the carrier species (Atlantic Brant in eastern North America and Dark-bellied Brant in Europe) should come as no surprise. Waterfowl as a whole are notorious for their tendency to hybridize, especially within the same genera and it is hard to believe that Atlantic Brant should be an exception to this otherwise very strong rule. Indeed, an apparent brant x canada goose hybrid at nearby Corona Park, Queens Co., New York provides a dramatic example of the potential to hybridize.
One interesting feature of the probable Black x Dark-bellied Brant described in Europe is that their appearance seems to change with time. In a bird studied in Hampshire England, the contrast between the neck and the body became stronger as the winter progressed. This may be a result of wear or bleaching, making the body look paler with time. The flank patches also became less intense due to abrasion of the pale tips to the flank feathers, revealing the darker feather bases. Unfortunately, the intermediate geese being seen each winter on Long Island are very mobile and may even be migrants from elsewhere. We do not know whether we are seeing the same birds in multiple winters, or even if we are seeing the same individuals on different occasions during a single season. A 'Black brant' in November may become an 'intermediate' by February? In other words, observations of a single individual over a season might reveal a change in the appearance of these bird, consistent with the European findings.
2) Dark variants of Atlantic and/or light variants of Black Brant:
Very little is known about color variation in any of the brant populations. Anyone who has looked closely at a flock of Atlantic Brant for example, will soon notice slight differences in the darkness of the wash across the belly or the shape and brilliance of the white flank patch. It is conceivable that some of the 'intermediate birds' picked out from these large flocks simply represent darker than average birds. In support of this idea is that fact the neck collar of many is not outside the range of male Atlantic Brant and it is therefore difficult to rigorously eliminate this possibility. Black Brant also show variation and in general females tend to be paler bellied (more noticeably contrasting with the neck sock) than males. Some evidence for this variation can be seen in my reference photos of Black Brant taken in Morro Bay in Central California (click link and scroll to bottom of page). [Note that these Californian birds are not purposely selected birds but rather the few that swam close enough to photograph, and do not represent extremes of variation in Black Brant.] As with Atlantic Brant, it is hard to know what the extremes can be.
3) Gray-bellied Brant:
Field identification of the different brant populations is a fascinating but poorly understood topic. At present only three taxa are recognized: Atlantic Brant (also known as Pale-bellied Brent Goose comprising two populations or stocks, High- and Low-Arctic); Black Brant and Dark-bellied Brent Goose. There is also some evidence of a fourth population known variously as Gray-bellied Brant, Western High-Arctic or Melville Island Brant. Some authorities also consider a fifth dark form, known as B. b. orientalis, which winter in Korea, Japan and China. A small study by Shields using molecular polymorphisms in the cytochrome b gene suggests that Gray-bellied Brant are genetically distinct from Black Brant but the relationship to the High and Low-Arctic populations of Atlantic Brant was not rigorously addressed. Shield does not analyze Atlantic Brant DNA himself but instead refers to another limited study. Interest in Gray-bellied Brant has been stimulated by the small size of the population (estimated at 4000-8000 birds), its very limited wintering range (certain bays within the Puget Sound, Washington/British Columbia) and continuing threats from sport hunting.
In two pioneering articles (Garner 1998; Garner and Millington 2001), Martin Garner and Richard Millington have presented the argument that Gray-bellied Brant occur as a regular vagrant to Northern Ireland in the company of Irish-wintering High-Arctic Atlantic Brant. This idea is supported by the proximity of the breeding grounds for the two forms in the Canadian arctic. It is probably fair to say that widespread acceptance of this idea is hampered by a poor understanding of (i) the field identification of Gray-bellied Brant, (ii) the extent of variation in Black Brant and (iii) the appearance (and frequency) of Black x Atlantic Brant hybrids. Future banding and/or molecular studies might shed some light on these issues.
Based on field observations in the Puget Sound, Garner and Millington conclude that Gray-bellied Brant are the most variable of brant, and "vary from some being apparently indistinguishable from [Atlantic Brant] through to darker individuals which approach Black Brant in appearance". With help from Hugh Boyd and other scientists, Garner and Millington have drawn up a list of criteria for identification of Gray-bellied. Here are the major points, but I would urge interested parties to study their thought-provoking article in Birding World, which includes a number of very interesting color photos from Boundary and Padilla Bays.
The Floyd Bennett bird can easily fit within these criteria, but this is hardly surprising when the criteria are so broad and all-encompassing. Studying wintering Black Brant in central California earlier this winter, I saw numerous birds that would also fit comfortably within these rules. It is not hard to see why waterfowl biologists have been reluctant to designate Gray-bellied Brant a full subspecies given the apparent variability of the population (much greater than for any other goose I know) and the overlap in range with other forms. Molecular analysis tied to individual birds of KNOWN appearance may help. Perhaps the wintering flocks are a mix of Gray-bellied and Black Brant? Perhaps Gray-bellied x Black intergrades exist as well?
Putting is simply, I don't think we know enough to say what this bird is! On face value, an intergrade (hybrid) seems the simplest explanation. After all, we know that Black Brant are making their way into the wintering flocks of Atlantic Brant that visit Long Island and we know that waterfowl have a high propensity to form mixed pairs and produce hybrid offspring. Given these facts, it seems hard to imagine that mixed pairing does not occur. These 'intermediate' birds could simply be the products of an expected event. The fact that this almost certainly occurs in Western Europe, where brant receive more careful, and more consistent, scrutiny provides additional support.
The case for Gray-bellied Brant reaching New York strikes me as more of a stretch. Gray-bellied Brant are not numerous as a population and migrate from their High-Arctic nesting grounds (the Prince Patrick Island group) to a very localized wintering area in the Puget Sound. Banding studies show that the brant wintering on Long Island (including bands that I have read myself at Floyd Bennett and elsewhere) are from the Low-Arctic breeding population (specifically from Southampton Island and the southern end of Baffin Island), and are unlikely to overlap with Gray-bellied Brant breeding much further to the north. As Martin Garner has argued, there may be some overlap between High-Arctic Atlantic Brant (wintering in Ireland and western Scotland) and Gray-bellied. Whether there is much mixing between these two Atlantic Brant populations is unknown.
In contrast, the nesting range of the far more numerous Black Brant extends further south and probably does overlap with that of Low-Arctic Atlantic Brant. Mixing on the breeding grounds as well as on the post-breeding molting grounds, may explain the well-documented occurrence of Black Brants in the huge flocks of Low-Arctic Atlantic Brant that winter in New England (MA to NJ, 200,000+ birds). Mixed pairings will produce hybrid (F1) offspring or even backcrossed offspring (F2), which would be expect to show variable characters that are intermediate between those of the parents. I have unable to locate any information from waterfowl collectors on the results of mixed pairing and the collectors I have talked to are careful to keep the different forms separate. However, an apparent mixed pair with at least one odd-looking young in tow, was photographed at Riis Park on 14 March 2002.
Without genetic data it is hard to ruminate on the possibility that these intermediates are simply extremes of variation in Atlantic Brant. Certainly, anyone who has looked at brant will agree that variation exists and that no two birds are quite the same. The darkness of the breast below the neck sock is one of the more variable aspects. Another thing to remember is that the plumage appears darker when it is very wet. After heavy rain, most Atlantic Brant can look like the intermediate bird shown here and this is something to keep in mind if faced with a lone individual.
Gives these many uncertainties, I think it is best to keep an open mind and leave these birds unidentified. To call them Gray-bellied Brant without more concrete evidence could leave us with a misleading impression of the abundance and wintering range of this poorly known population. After all, claims of Gray-bellied Brant from the east are unlikely to help conservation of the Gray-bellied stock which needs urgent protection in the Puget Sound and this would be diluted by inaccurate reports of a larger wintering range. A better understanding of the appearance and measurements of Black x Atlantic intergrades would be very useful. Of course, there is a good chance that these will fall within the current catch-all definitions of Gray-bellied Brant!
Andy Guthrie played a major role in developing the ideas laid out on these web pages and made extensive and very helpful comments on the text. My thanks to him.
More photos of other Intermediate-type Brant from New York
More photos of other Black Brant from New York and elsewhere
[*Note: This last page includes reference photos of Black Brant taken in Morrow Bay, a traditional wintering site for Black Brant in Central California.]