Journal archives for February 2019

February 24, 2019

Swan Bill Porportions Debate

On January 2, 2019, I spotted a Tundra Swan in McNary National Wildlife Refuge, Washington, with a significantly larger yellow lore than the other fifteen swans there. It also had the largest yellow patch I’ve ever seen. Viewing this swan for over twenty minutes, I hypothesized that it was a potentially a Whistling X Bewick’s intergrade.
For a little background, there are two main subspecies of the Tundra Swan, some authorities even considering them separate species. The American subspecies is the Whistling, and are identified from the closely related Trumpeter Swan by their smaller size and small yellow lore patch. The Asian subspecies is the Bewick’s, known for their much larger yellow skin patch covering between a fourth to half of the bill. They are common visitors to Alaska and Pacific Northwest.
Earlier, I said I hypothesized my individual being an intergrade so I dove into some research. My first source was to check eBird, the online database for bird sightings, powered by Cornell. However, they had no photos of Whistling X Bewick’s intergrades and the only confirmed sightings was two in Hawaii earlier in 2018. However, the notes about that particular sighting were vague and not helpful in my individual.
I started surfing the web and reading my collection of bird guides and books, looking for answers. I even consulted with many expert birders across the country, looking for guidance or advice. Eventually, my eyes landed about a scientific paper titled, “A Comparative Analysis of the Bill Markings of Whistling and Bewick’s Swans and Out-of-Range Occurrences of the Two Taxa”. I read the entire seven page document nearly three times the next two days. The feature of the paper that caught my eye the most was the statement on how the proportions of the yellow can determine subspecies. They used grids to see how much of the bill was yellow.
That very day, I placed a grid on the lateral of the bill of my swan. The bill of my swan could be separated by fifty equally-sized squares. Out of those squares, eight of them were yellow with three being half and half black and yellow. Doing the math, roughly 19% of the bill was yellow. According the scientific paper I mentioned above, that makes my swan only a few percents below a Bewick’s rank, meaning I very likely had an intergrade and I had this paper and my grid to back it up.
I sent my photo and my research on a social media bird identification group. It was certainly a topic of discussion, however none of them agreed with my swan being an intergrade. They advised me to look at the Sibley guide. After that comment, I went into my collection and pulled out my Sibley guide and I read through his Tundra Swan section. He stated that there is little overlap between the subspecies. However his illustration and notes stated his Whistling coming close to 25% of the bill being yellow, well into the paper’s Bewick’s range.
That means I’m left with contradicting information. Someone’s got their facts wrong and I’m left with probably an unidentifiable Tundra Swan. However, I got this impression that maybe this information can be cleared up. Today, we have eBird collecting data from millions of birders across the world. I will be using their data to get to the bottom of this mystery. I will be putting grids on hundreds of photos in breeding regions of the species so see what I can find.
During summertime months, there’s 975 photographed Whistling Tundra Swans on ebird as of February 2019. There’s an addition 19 Bewick’s Tundra and 8 unspecified subspecies that easily fall into the intergrade range assuming that Evan’s and Sladen’s paper is correct. One individual in the “intergrade” was identified as a Whistling Swan, however the observer described it being “almost Bewick’s”, suggesting intergradation.
Other things we noted that Bewick’s Swans were noticeably more common in West Aleutian County, Alaska, with 13 individuals spotted and photographed in summer months. On the other hand, 17 Whistling Swans were photographed. That means the two subspecies are well mixed in the Aleutian Islands and further supports the hypothesis of intergrades. However that statement can be relatively narrow-minded as observers may be unintentionally photographing more Bewick's than Whistling due to their unusual occurrence.
The last bit of information I found interesting is that Hudson Bay Whistling Swans had a much higher concentration of individuals with larger yellow loral patches. In Alaska, only 8% of the Whistling Swans had yellow patches larger than five percent of entire bill. Canada, excluding Ontario, was 7% and in Ontario it was over doubled with 16%.
And this leads to my conclusion, what is the true identity of this swan? From all 975 photos, I saw little information disproving the comparative analysis research paper. I also found no evidence supporting Whistling Swans having loral patches larger than 15.8%. Though I admire Sibley as one of the greatest birders in the world, I found nothing on Whistling Swans having a fourth of the bill yellow.
Though what if my swan is a Whistling? Some may say that this is a rare migrant from the Hudson Bay breeding zone. That may be very well possible but a purebred Whistling flying nearly across the country in an east or west direction is less believable than an rare intergrade flying south from Alaska.
However, my little study has supported two things. One, bill proportions are everything in identifying Tundra Swan subspecies and two, there is much to learn about this species. With mtDNA testing becoming more common in field identification, I see scientists cracking mysteries or new discoveries that stretches our field of knowledge.

Posted on February 24, 2019 22:46 by birdwhisperer birdwhisperer | 1 observation | 0 comments | Leave a comment