Throughout Central America, Blue-winged Teal is a common to abundant winter visitor, and even oversummers in very small numbers in the region. Its close relative, the Cinnamon Teal, typically winters further north, in southwestern United States and in Mexico, with only a few reaching Central America each winter. (This species also occurs in South America.) Males in bright plumages are easily distinguished, but the eclipse-plumaged males, as well as the females and juveniles, are quite similar between these two species, and cannot always be separated in the field. Matters are further complicated by a phenomenon called “rust staining”, i.e. a chemical process that turns the feathers of some waterbirds a rusty color: in the case of Blue-winged Teal, the result is a duck suggestive of Cinnamon Teal. Sometimes this time of year, we see reports of “male Cinnamon Teal still with some reddish plumage” in eBird, and these are nearly always rust-stained Blue-winged Teal.
What exactly is a rust-stained Blue-winged Teal? Rusty staining on the plumage of waterbirds has long been known, and about a hundred years ago, ornithologists already wondered what caused it. B. R Bales for example, writing in 1909 in the Wilson Bulletin, figured that iron might be involved, and did a chemical experiment on the specimens he collected to prove it. Nine years later, F. H. Kennard, writing in the Auk, reached the same conclusion, namely “that the ferruginous suffusion is caused in every case by an extraneous deposit of oxide of iron (Fe2O3) on the outside of the tips of the feathers.” The rusty stains are seen on many waterbird species, especially ducks and geese.
Before we look at the various plumages worn by Blue-winged and Cinnamon Teal in our region, it may be useful to review some plumage terminology, and to consider that molt in ducks is a little different from that in most birds. First of all, we might intuitively think of the well-marked male Blue-winged Teal with his white crescent-shaped mark around the bill and the brick-red male Cinnamon Teal to be in “breeding plumage”. After all, for most birds that show two different plumages during the year, the brighter plumage is the one worn during the breeding season and the muted plumage is worn during non-breeding season. Older books sometimes refer to these as “summer plumage” and “winter plumage”, but those are terms that only work in temperate zones and even there not for all birds with two plumages. Modern bird guides often call the brighter plumage “alternate” and the drabber plumage “basic” for those species that have two plumages (and just basic for birds that look the same all year). However, recent studies by Peter Pyle and others have demonstrated that in the case of ducks, the brightly colored plumage worn by the male is in fact the basic plumage. They acquire this plumage between August and November; after that period, all adult males are in bright, easily identified plumage.
Each year, when the migratory ducks reach our region, a small percentage of Blue-winged Teal shows that rusty staining suggestive of an eclipse-plumaged Cinnamon Teal. It is good for Central American observers to be aware of the rust-staining as explained by Mr Bales and Mr Kennard. Cinnamon Teal is a rare bird in most of Central America, and you are far likelier to see a rust-stained Blue-winged Teal than an eclipse-plumaged Cinnamon Teal. The best mark to look for in such a case is the color of the eye: red in male Cinnamon Teal, dark brown in male Blue-winged Teal. However, you may not always be close enough or the light may not be good enough to see such detail.
The females and immatures of these two species are quite similar, and cannot be told apart by eye color, as the eyes are dark brown in all of them. The female Blue-winged Teal usually has a brighter pale spot near the base of the bill, and a stronger dark line through the eyes, while on the female Cinnamon Teal, the head pattern is usually more uniform. Another subtle difference is the color: warmer brown on female Cinnamon, colder brown on female Blue-winged. The extremes are usually readily identified, but many birds show intermediate characters and are best left unidentified to species. Some guides mention differences in bill size and shape, but these are too slight to be useful in the field.
In Britain, where both species are rare, birders have relied on a 1977 article in British Birds, still helpfully available online.