China doesn’t yet have a rarities committee. That will come as birding becomes more popular. And so, for the time being, there is no formal way to assess reports of unusual birds. I am therefore inviting readers of Birding Frontiers to informally assess a record of a putative WHISTLING SWAN (Cygnus columbianus columbianus) in central Beijing this winter.
This is the story.
For context, any wild swan in central Beijing (inside the 6th ring road) is notable. So when local birder Shi Jin discovered a swan on 25 November on his local patch – the Wenyu He, between the 5th and 6th ring roads – he was very happy. He was even happier when he noted that the bill showed a very small amount of yellow as it led him to believe that it just might be a Whistling Swan. The significance of this sighting is that, if confirmed as a Whistling Swan, it would be the first time this North American form of Tundra Swan has been seen anywhere in China.
Although originally alone, the bird was soon joined by a standard Bewick’s Swan (the Eurasian race of Tundra Swan commonly encountered in China) and the two birds remained in the area for several days. Shi Jin reported his sighting to Beijing birders on the day of discovery and circulated a photo, pointing out that it appeared to be of the North American race and asking for views.
On paper, separating the races of Tundra Swan – Whistling (columbianus) and Bewick’s (bewickii) – is relatively straightforward. The most reliable feature is said to be the amount of yellow on the bill. In his excellent Advanced Bird ID Handbook, Nils Van Duivendijk says that, in adults, “at most 1/8th of the bill is yellow in columbianus and that a large majority have less than 8% yellow on the bill.” And that “the yellow is often positioned as a ‘teardrop’ on or just below the lores.”
Peter Pyle, in his Identification Guide to North American Birds, says that the yellow on the bill ranges from 0-16% and includes a helpful diagram showing the maximum extent (p.76 of Part II – Anatidae to Alcidae).
And David Sibley in The North American Bird Guide provides a similar diagram (p.73) of bill patterns, including an example of Whistling Swan “with maximum yellow”.
Of course, this is most relevant if the bird in question is an adult… (more about that in a minute).
As is evident, the extent of yellow on the bill of the Beijing bird is much less than one would expect on a standard Bewick’s. The left-side of the bill even shows a classic ‘teardrop’ shape. Initially the right side of the bill was not seen or photographed, although from a head-on view it seemed to exhibit only marginal differences.
In early December Beijing-based birder Jennifer Leung and I went to see the bird and we took a series of photographs (see below), including of the bill pattern and comparing the putative Whistling Swan with the accompanying Bewick’s. These photos show that the bill is asymmetrical with a little more colour on the bird’s right side. I use the word “colour” here as opposed to “yellow” as close examination of the images reveals that part of the coloured area on the right side of the bill is more pink-grey in colour and that the extent of yellow is limited and roughly the same on both sides of the bill.
Now, back to that guidance about identifying adults. As always, an important first step when attempting the identification of a potential vagrant is to age the bird. On the Beijing swan, the general plumage – white body and wing feathers – appears to be adult. However, there are a couple of aberrations that might suggest immaturity (I very much welcome views on these). First, the crown has some dark feathering (viewable on several images over several days, suggesting that it is probably not staining). Second, the area of colour on the right side of the bill is largely pink rather than yellow. Is the latter evidence of an immature bird maturing and slowly developing its adult bill? And, if so, is the amount of colour on the bill likely to reduce over time to leave the small patch of yellow, currently broadly similar in size and shape on both sides of the bill?
Once this question has been answered, the next obvious question is – “Does the amount of colour, and specifically the yellow, on the bill enable a secure identification?”
A cautionary note in British Birds (June 2006, BB 99, p307) on this issue stated:
“The two subspecies of Bewick’s Swan, C. c. columbianus and C. c. bewickii, have been proposed as potential splits because they differ in the amount of yellow on the bill (summarised in Sangster et al. 1997). However, it is not clear whether there is any overlap, and although hybridisation occurs there is virtually no information on the relationships between the two taxa in that part of eastern Siberia where they come in contact.”
And to muddy the waters further, it seems that Bewick’s from eastern Siberia (the most likely to turn up in Beijing) may have less yellow on the bill than standard Bewick’s. Peter Pyle’s Identification Guide to North American Birds states “Populations [of bewickii] of eastern Siberia (“jankowskii”) may have bills averaging larger and with less amount of yellow but differences, if present, are insufficient and variable, and may indicate intergradation with columbianus.”
Aside from the yellow on the bill, other features have been suggested as helpful in separating columbianus from bewickii.
Cleaner white plumage
An article in Birding World (Vol 12, Number 3) reported that a Whistling Swan in Northern Ireland “stood out from its companions by virtue of being snowy white (A. McGeehan, pers. comm.)…”
Now, the Beijing putative Whistler did look, to me, marginally ‘cleaner’ white than the Bewick’s with which it was associating but how far this goes as a supporting feature is unclear.
Size and Structure
On average, Whistling Swans are slightly larger than Bewick’s but there is significant overlap so, for most birds not at the extreme end of the range, size will not be helpful. Certainly with the Beijing bird, there is no significant size differential between it and the accompanying Bewick’s.
Some people have suggested that Whistling Swan exhibits a different head shape. Birding World (Vol 12, Number 3) suggested that “While Bewick’s Swan tends to be rather dome-headed (with a smoothly rounded rear crown), Whistling Swan may show a different profile: a sloping crown ending in a high rear peak, and a steeply angled nape.”
To my eyes there is little, if any, difference in head shape between the Beijing birds.
Finally, the Birding World article referenced above asked the question directly:
“Can an out of range Whistling Swan be positively identified in the field?”
John Kemp, the author of the article, answered:
“Given adequate views, it seems likely that one of the 85% of classic Whistling Swans (ie those with less than 6% yellow) would be sufficiently distinct to be certainly identifiable, especially if it also showed good supportive structural features. Photographs of reasonable quality may enable the percentage of yellow to be assessed. The problem lies with the very small number of extreme individuals which cannot be convincingly identified, bearing in mind the existence of intergrades/hybrids…. ..Possibly only those individuals showing less than maybe 10% yellow on the bill should be regarded as true identifiable Whistling Swans.”
For me, the right hand side of the Beijing bird shows more than 10% colour (with a smaller amount as yellow) but it is not straightforward to assess this accurately. And, if it is more than 10%, does that rule out safe identification as a Whistling Swan? Or should we only assess the amount of yellow?
Neither Shi Jin, Jennifer nor I have any experience with the North American form and we are far from experts on ageing swans, so we would welcome views from anyone with relevant experience. But no pressure – it’s only a potential first for China!