The photo above was recently sent to me by reader Wil Domke. It shows an adult Trumpeter Swan (based on bill size and shape, and on Wil’s identification in the field) with a small yellowish patch on the loral skin. This yellow marking is similar to the yellow spot shown by most Tundra Swans.
According to The Sibley Guide to Birds Trumpeter Swan is “never yellow on lores”, but obviously that is incorrect. Another example of a yellow-lored Trumpeter Swan was photographed in Ontario in early 2011 and discussed in the Outdoor Ontario forum (thanks to David Bell for pointing this out). An even earlier example from Yellowstone National Park, Wyoming, was discussed with a photo in McEneaney (2005). Finally, Steve Mlodinov (Flickr photo) reports that Trumpeters wintering in Washington “can have a dash of pallid yellow” on the lores.
Distinguishing yellow-lored Trumpeters from Tundra Swan
On seeing a swan with a yellow loral patch, many observers understandably jump to the conclusion that it is a Tundra Swan. This is the first chellenge offered by these odd individuals, but they can be distinguished from Tundra Swan by overall size, details of bill shape, and voice. In addition, the bare parts colors of yellow-lored Trumpeter Swans offer two additional clues to ID.
- yellow patch on bill dull, mottled and poorly-defined, and positioned somewhere between eye and nostril on odd Trumpeters (vs. clear yellow, sharply-defined, and positioned immediately in front of eye on Tundra)
- legs on many (all?) Trumpeters with yellow on bill are not black, instead either grayish or drab yellowish (vs always black on Tundra)
Could they be Trumpeter x Tundra Swan hybrids?
Other than the presence of yellow in the loral region there is nothing on these birds that suggests a hybrid origin. Hybrids between these two species have been reported, both in captivity and in the wild, but these hybrids are described as being intermediate in overall size and bill shape, which has not been the case in any of the well-studied birds being discussed here. One presumed hybrid identified in the wild in Dec 2009 in Skagit County, WA, was reportedly intermediate in size and had extensive yellow on the lores. In photos of that bird, the loral color appears bright yellow like Tundra Swan, and is positioned close to the eye with distinct borders there (like Tundra), but blends into drabber yellow farther from the eye (see photo by Steve Mlodinov, Trumpeter x Whistling (Tundra) Swan | Flickr).
The possibility exists that these yellow-lored Trumpeter Swans are backcrosses, or that the yellow lores are the expression of Tundra Swan genes introduced through past hybridization. Among swans, yellow lores are not unique to Tundra, so it is also possible that yellowish lores are an ancestral trait that any Trumpeter Swan can show, with no hybridization required.
Could they be leucistic swans?
Swans, of course, are all white, so the idea of identifying a leucistic swan might seem like a joke, but such birds do exist. The best-known example is the so-called “Polish” morph of Mute Swan, in which downy young are white with pink bills (unlike the typical gray plumage and dark gray bills). These white downy young molt into a nearly pure white juvenal plumage (unlike the gray-brown of typical juveniles), and can still be distinguished from typical birds even as adults by their pale grayish-yellow legs (rather than the normal black color).
A similar white or leucistic morph has long been known in Trumpeter Swan, and up to 13% of the Yellowstone population is reportedly leucistic (Mitchell and Eichholz, 2010). Kraft (1991, as reported in Mitchell and Eichholz, 2010) states that leucistic adult Trumpeter Swans can show yellowish on the lores. ((I haven’t seen the original publication reporting this connection, so I don’t know how strong the evidence is. If anyone has access to it or any further explanation I would be very interested to know about it.))
The color variants reported by McEneaney (2005) – pink, orange, or yellow legs, pink bill – could all be produced simply by a lack of melanin, and he reports that of three known leucistic Trumpeter Swans followed to adulthood in Wyoming, all three retained pale yellow-gray legs and feet, and one showed pale yellowish on the lores. ((Note that many immature Trumpeter Swans have non-black legs and feet, so this feature will only identify leucistic adults, not immatures)) Among Trumpeter Swans with pale lores, at least the Ontario bird and the Yellowstone bird discussed above showed yellowish-gray legs, not as dark as typical adult Trumpeter Swans (McEneaney, 2005), suggesting that there is a connection between leucism and pale lores.
Interestingly, leucistic downy young are reportedly not found in the Alaska breeding population (Mitchell and Eichholz, 2010), only in the Rocky Mountain population. ((I did not find clear information on the origins of the reintroduced populations in the east, but it may be that most of them came from captive stock and not from the wild. Frequency of white morph birds in the east is unknown.)) However, Mlodinov’s report that tiny pale spots on the lores can be seen in the Washington wintering population indicates either that pale lores can occur in the absence of leucism, or that there are some leucistic white morph birds in the Alaska/Washington population.
Trumpeter Swan can show some pale yellowish on the loral skin, and the evidence suggests that many or all of these birds are leucistic (white morph).
Are white morph birds really absent from Alaskan breeding populations?
Are all pale-lored birds also pale-legged?
Is there a direct connection between white morph and pale legs? (i.e. Do all white morph adults have pale legs, and do only white morphs have pale legs?
Banko, W. E. 1960. The Trumpeter Swan: Its history, habits, and population in the United States. N. Amer. Fauna 63.
Kraft, R. H. 1991. Status report of the Lacreek Trumpeter Swan flock. Pages 88-90 in Proc. and Papers of the 12th Trumpeter Swan Society Conf. (Voigt-Englund, J., Ed.) The Trumpeter Swan Society, Maple Plain, MN.
McEneaney, T. 2005. Rare color variants of the Trumpeter Swan. Birding 37:148-154.
Mitchell, Carl D. and Michael W. Eichholz. 2010. Trumpeter Swan (Cygnus buccinator), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/105