BIRDING BY HINDSIGHT: A Second Look at Swans

So, what seems to be the problem? What could be easier to identify than that big white bird swimming around in front of you, perhaps begging for bread scraps — in flagrant violation of the Atkins Diet? This is no cryptically colored or furtive bird hidden in the underbrush with hard-to-see, subtle, or highly variable plumage features. It’s just a swan!

Perhaps the problem is you’re still distracted after being roundly criticized last October for leaving your hummingbird feeder out and, in the process, for heartlessly helping hummers have hypothermia. (Why don’t these same critics object to the handouts the Trumpeters are getting in Monticello?) Or maybe your concentration is clouded by listening to those defending the ill-advised introduction of Trumpeters into Eastern states (where there is no good evidence they ever occurred in the first place).

OK, if there’s an ID problem, just get out the field guides and other references. Besides the Sibley and Geographic guides, I could recommend the book Waterfowl: An Identification Guide to the Ducks, Geese and Swans of the World by Madge and Burn. In addition, there have been three widely read articles: “Distinguishing Tundra and Trumpeter Swans” (Birding 20:223–226), “Immature Swans” (Birding 23:88–91), and “Identifying Trumpeter and Tundra Swans” (Birding 26:306–318). This last article, by the way, is the most valuable of the above references. 

Now, having read all these, it’s easy to see why it’s hard. For example: Geographic says the Tundra’s feathering “cuts straight across forehead” where it meets the bill (other references say it’s U-shaped). Sibley suggests the shape of the feathering at the bill’s gape is diagnostic (only one other reference mentions this). Madge and Burn in Waterfowl claim the reddish line at the cutting edges of the bill is a useful distinction (no one else does). And one of those Birding articles stands suspiciously alone in suggesting you consider a swan’s neck shape at take-off, the angle of its body with the ground when standing, where its neck meets the body when swimming, and that a Trumpeter “bobs” its head while a Tundra only “nods.”

At this point, it’s time for Hindsight to step in, take a second look at swans, and clear up the confusion — or perhaps add to it. Before examining individual field marks which allegedly distinguish the swans, one place to start is to consider the season. No matter what month it is (or what county you’re in, for that matter), a Trumpeter Swan is a possibility in Minnesota, since it now locally but regularly breeds and winters – and also migrates – in the state. 

But there are times when the Tundra Swan is unlikely. After mid-May, all Tundras have normally departed for the Arctic, and none are likely to return here until mid-October. So, any Minnesota swan from June through September is probably a Trumpeter, but not certainly so: there are several records of migrating Tundras lingering into June and of non-breeding Tundras summering here. And, while all Tundras have normally left the state before the end of December, practically any species of waterfowl can and does overwinter at times.

Another initial consideration, and one more important than the season, is to make sure you note the swan’s age, since some field marks are valid on adults but not immatures, or vice versa. A swan with all-white plumage is an adult (generally at least one-and-a-half years old), and one with some dusky grayish or brownish feathering is an immature (less than one-and-a-half years).

The field marks listed below are the ones birders have most often used — with varying credibility — to tell a Tundra Swan from a Trumpeter. Almost none of these marks on its own is diagnostic, virtually all involve caveats and qualifications, many may be useful (but, again, not diagnostic), some work on adults but not immatures, while others are misleading or even useless. Correct identification of a swan is generally possible only when the birder uses a combination of several field marks.

The comments below are intentionally brief and not to be considered thorough analyses: be sure to consult the references cited above for more complete information. These field marks are listed more or less by their validity; those near the beginning of the list are generally those with the most merit, while those further down tend to be less credible:

Vocalizations. Adults can be told apart by their calls, but only if the birder is familiar with them and is aware that individuals can vary their pitch and quality. Either listen to them in the field or on recordings — it is not enough to simply read the written descriptions. It is important as well to know that immature swans can sound different than adults of the same species. Thus, if a flock of swans includes both adults and immatures and you can hear two different calls, don’t assume both species are present. One problem is, of course, that swans are mute much of the time — no pun intended.

Overall size. It’s well known that Trumpeter Swans are larger than Tundras overall, but it’s hardly that simple. Even if you have direct comparison with some swans in a flock looking larger than others, the problem is that male swans average larger than females in both length and weight. Additionally, adults might look bigger than some immatures. Therefore, use size only with caution and within context.

Yellow loral spot. Most birders are aware that some — but not all — adult Tundra Swans have a yellow spot on the lores, which Trumpeters never show. So, an adult swan with yellow lores is a Tundra; one with no yellow can be either species. Few birders, however, are aware of this caveat: a few adult Trumpeters might show a pale “wear patch” on or near the lores, though reportedly this spot is never yellow.  

Feathering at the gape. One of the least-known but best distinctions between adult Tundras and Trumpeters was first publicized by Sibley, but it was apparently first mentioned in the third Birding article cited earlier. Look at the shape of the feathering where it meets the bill between the gape and the eye. If this edge is relatively straight or shallowly curved, it should be a Trumpeter; if this edge looks more angled or abruptly curved, it is likely a Tundra. This feature probably needs more field study to test its reliability, and it does not apply to immatures.

Forehead feathering. The shape of the feathering on the forehead where it meets the top of the bill is another good mark on adult swans — be sure to note, however, this feature does not apply to immatures. Note as well that this mark is usually difficult to see (you have to wait until the swan is facing you and looks down), and most ID references are misleading and overly simplify this feature. If this edge is U-shaped, this indicates a Tundra; if it is more of a shallow V-shape, it is a Trumpeter. But this difference is usually subtle, with the tip of the Trumpeter’s “V” often looking somewhat rounded rather than sharply pointed.

Extent of black in front of eye. On an adult Trumpter Swan, there is typically more black between the eye and the bill, making the eye appear less obvious and more a part of the bill. This appearance results from the top edge of this black area forming a line which is tangent to the top of the eye; in some cases, the lower black edge is also tangent to the bottom of the eye. A narrower area of black, with the eye appearing more evident and distinct, is typical of Tundra Swan. There are times when this difference is harder to judge, especially on immatures, but it is usually apparent and useful on adults.     

Bill length. If the length of the swan’s bill, from its gape to the tip, appears to be twice the distance from its eye to the back of its head, it’s probably a Trumpeter; this gape-to-tip distance is typically only 1.5 times longer on a Tundra Swan. There is overlap, though, in this feature, especially when immatures are involved.  

Immature plumage. Here, for a change, is an ID feature which only applies to immatures. By mid-winter, the overall plumage of Tundra Swans is normally whiter than that of young Trumpeters. Indeed, any immature swan that generally appears almost as white overall as an adult in late winter or spring would be a Tundra. The overall plumage of immature Trumpeter Swans at this time is still dusky. There is no real plumage difference between immatures earlier in the winter or in fall, however, nor is there any magic date when this plumage difference becomes reliable.    

Immature bill color.  On the other hand, there is something useful to distinguish immature swans in fall, and perhaps into early winter. Immatures of both species show a mix of pink and black on the bill, with the bills generally becoming all-black by spring. However, young Tundra Swans in fall tend to have more pink on the bill, with this color often meeting the feathering at the base of the bill. A young Trumpeter Swan always shows black at the base of its bill. Accordingly, an immature swan with black on its bill’s base could be either species, but one where the pink reaches the feathering at its base would be a Tundra.

Note: From here on down, the reliability of field marks to safely distinguish between the two swans rapidly deteriorates.

Nostril placement. It is stated in some credible references that the position of the nostril on the bill is a useful field mark: i.e., on a Trumpeter it is about half-way between the eye and the bill tip, while on a Tundra it is closer to the tip. I have to seriously doubt this, though. In every photo I have seen, no consistent difference is apparent, with the nostril on both species closer to the tip than the eye.

Culmen shape. An oft-mentioned difference between Tundra and Trumpeter swans of any age is the shape of the culmen (i.e., the top edge of the upper mandible). Supposedly, it is concave on a Tundra and relatively straight on a Trumpeter. While there is a tendency for this to be true on adults, this feature simply does not work on any immatures — or even on many adults. Photographs clearly show there are some adult Trumpeters with a somewhat concave shape to the culmen, and conversely some adult Tundras with a relatively straight culmen.   

Crown shape. Some references suggest that a flatter or lower crown profile indicates a Trumpeter Swan, while a swan with a rounder or higher profile would be a Tundra. On the average this may be true, but it’s hardly a reliable difference.  

Reddish tomium. For decades, some birders have been under the impression that a swan showing pink or reddish on its tomium (i.e., the cutting edges of the bill) would be a Trumpeter. Though this may be more obvious on some Trumpeters, many Tundras share this feature, and many Trumpeters do not.

Back profile. It is sometimes alleged that the highest point of a Tundra Swan’s back is near the center, while on a Trumpeter it is closer to the rear. On the average, maybe this is true. In reality, it’s unreliable.

Foot size & color. It has been reliably reported that there is no overlap in the foot measurements of swans, with the Trumpeter’s always longer. Perhaps this might be useful information if you carefully measured the tracks of a swan walking through the mud. And foot color would not be useful in most cases, since adult swans of both species have black legs and feet, and the feet/legs on immature swans are variable in color and not consistently different.

Body major axis. This is something I read about on Joe Morlan’s website (https://fog.ccsf.edu/~jmorlan/trswid.htm). Given Joe’s experience and ability, this feature is probably accurate and useful, but its explanation left me somewhat confused. You’ll also find on the same website an equally long and complex analysis of the difference in the head/bill profiles and proportions of the two swans.

Body posture. It is sometimes suggested that a Trumpeter typically stands on land with its body angled up a bit, while a Tundra stands with its body parallel to the ground. However, most references fail to mention this, and I would be surprised if this were a valid distinction.

Neck/chest position. Is it true, as claimed by some references, that a Trumpeter Swan swims with the base of its neck set back from its chest, while a Tundra’s neck goes straight up from its chest? No, this is hardly a consistent difference: both swans can assume either neck position.

Neck shape. Or is it true, as claimed by one reference, that a Trumpeter briefly holds its neck into a “shallow S-curve” upon take-off, while a Tundra never does? If so, why has no one else ever written about this? 

Head bobbing. Or, what about this same reference’s claim that Trumpeters sometimes “bob” their heads, while Tundra Swans do not — that Tundras can only “nod” their heads? Don’t ask!

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Still confused? Good. Then we’re finally getting somewhere if birders come to realize that swan identification is an under-appreciated problem, that we should be skeptical of — or even ignore — some of the field marks promoted in the references, and that an accurate ID of any swan should rely on more than just one characteristic.

So, now that we have this Tundra vs. Trumpeter ID problem under control, perhaps we can devote our energy to what really matters these days. It’s high time to jump on the Atkins low-carb bandwagon, and feed fewer bread scraps to swans and less sugar water to hummingbirds.