Flight lines: Look-alike ducks present difficult, though not impossible, challenge.
There are groups of birds that stubbornly defy easy identification for beginners and experts alike. To some it's part of the appeal of the hobby. A dare, a challenge, from nature itself. Try and unmask the identity of my participants, it seems to demand.
We all know who the confounding culprits are: Those pesky non-descript sparrows, those confusing gulls with their ever changing plumages, those look-alike small shorebirds, those similar terns, and let's not even think about that group of wingbar-sporting flycatchers known euphemistically -- and contemptuously by some -- as "Empids" (short for the genus, Empidonax).
By comparison, there are groups made up of species which make themselves quite discernible from their peers. The spring warblers, particularly the males, would be included in this welcome crowd, as well as most adult raptors, the vireos, the tanagers, the large waders, and most ducks.
I say 'most ducks' because two closely related species present one of the most tricky identification challenges for birders on this continent. The Peterson Field Guide to Advanced Birding by Kenn Kaufman was published in 1990. There it is, ominously, in chapter six, "The Scaup." Leahy's The Birdwatcher's Companion to North American Birdlife says, "The greater and lesser scaup are difficult to distinguish in the field except under the most favorable circumstances." How very true.
Bird guides will suggest that the "default" scaup (pronounced "scawp") throughout the continent is lesser scaup (Aythia affinis), unless you are on the coasts. This might be true for most areas but around here in early spring, greater scaups (Aythia marila) are frequently encountered making for some long hesitant head-scratching in the binoculars.
In general, greater scaup is the saltwater specialist in winter while lessers prefers fresh water. Greaters will nest from Alaska through Hudson Bay into northern Quebec, lessers from Alaska down through the Prairie Provinces and across much of North Dakota.
Both adult males are dark-rumped, white-sided, and dark-fronted, sort of an Oreo cookie thing on the water. The backs are gray, the eyes golden yellow, and the bills blue, thus the nickname "bluebill," given them by hunters. Females are an overall brownish duck with a white arc around the base of the bill.
Several physical traits can separate the two but the differences are quite nuanced. Head shape is one. Greaters typically have a larger robust head that looks very round at times and, in good light, males exhibit an iridescent green. The head of a male lesser is daintier, usually with a high forehead that makes it look haughty, and shines dark purple in good light. Rule of thumb: greater's head is longer than it is high and lesser's is higher than it is long. Keep in mind both head shape and color will change depending on light conditions and how the duck is holding his head, even on the same bird.
Bill shape is another subtle clue with the greater's being larger, deeper, and broader than that of its close cousin. The dark "nail" tip on the upper mandible is telling as well. It's noticeably broader on a greater scaup although seeing this field mark can be very difficult.
Yet another feature mentioned by experts is flank color in males. The Oreo stuffing part of the bird will usually be bright white in greaters and somewhat smudgy in lessers. Thus, in a flock of lessers, a greater scaup drake should stand out.
Arguably the most defining difference between the two species is the length of white feathering at the trailing edge of the wing in box sexes. In greater scaup, the white will extend from the inner secondary feathers all the way out to nearly the wing tip, while the amount of white on a lesser scaup is confined to the inner feathers, giving way to grayish ones farther out. The obvious limitation here is that the birds must be flying or at least flapping their wings.
There's a few more but that's enough to get you started. Keep in mind no one characteristic should ever be relied upon to I.D. a scaup. Instead a studied observation and a collection of clues and field marks are required. I would encourage the reader to approach this identification conundrum fearlessly. If it produces nothing more than a confused shrug that's fine. Beginners and experts alike need to be reminded that it's okay to say, "I don't know," once in awhile.