(noun) The speculum is a patch of often iridescent color on the secondary feathers of most duck species. It is often seen as a bright patch of color on the rear of the wing when the wing is spread during flight or when the bird is stretching, preening, or landing. The color of the speculum will vary by species, as will its width and any non-iridescent borders.
(rhymes with "heck you rum" and "wreck blue plum")
About the Speculum
The speculum may be just a color patch on the wing feathers, but it is very distinctive. The size and color of the speculum varies between species, and any borders that frame the patch can also vary. For example, some of the most colorful speculums of familiar duck species include:
- Mallard: bright blue-purple speculum with a black border all around and white borders on the upper and lower edges
- Green-Winged Teal: half black and half green speculum, with white bordering the top of the black section
- Spectacled Duck: broad crimson-red speculum with thick black border all around and thin white border on the top and bottom
- Northern Pintail: long, narrow speculum that shows dark green or black-brown, with a buff upper border and broad white trailing border
- Ruddy Shelduck: broad bright green speculum that may appear black in some lights, slight black rear border but lacking other borders
- Northern Shoveler: tapering green speculum with a dark rear border and a broad white triangular border on the leading edge
- Cinnamon Teal: half-green, half-black speculum with thin dark borders and a broader white triangular patch above the leading edge
- Spot-Billed Duck: bright green speculum with broad black and white borders on the upper and lower edges, but no border on the edge closest to the wingtip
The exact color, shape, and size of the speculum can appear to vary based on the angle of viewing, wing position, and the lighting conditions, which can dramatically affect the color of the iridescent feathers. The speculum is typically brighter and larger in males, just as most drakes have flashier overall plumage than female ducks.
In addition to ducks, some parrot species and other birds also have a distinct speculum, though less often iridescent and not as notable as with duck species. Depending on how the bird holds its wings, the speculum may not be visible when perched or at rest, but it is always visible in flight.
What a Speculum Isn't
Because a duck's wings can have several distinct color patches, it is important to understand exactly where the speculum is and what other parts of the wing may also show bold coloration. The speculum is not part of the duck's:
- Primary Feathers: These are the fingertip feathers at the very end of the bird's wing, not closer to the body where the speculum is. Ducks may show a border on these feathers, but it is not usually iridescent or colorful.
- Patagium: The patagium is the leading edge of the bird's wing between the shoulder and the wrist. In some ducks, this can be a colorful area, but it generally lacks the iridescent quality that defines the speculum.
- Scapulars: These are the shoulder feathers of the bird that fill in the interior of the wing behind the patagium but in front of the speculum. In many ducks this can be colorful, but it is not usually iridescent and lacks the distinct, bordered appearance of the speculum.
Identifying Birds With a Speculum
Because this part of a duck's wing can be so distinctive, the speculum is useful for identifying ducks. To use the speculum for bird identification, note:
- The color of the speculum in bright sunlight or color changes in different light qualities
- The color and thickness of any borders to the speculum area
- The length and width of the speculum compared to the bird's overall wing size
- How much of the speculum, if any, is visible when the bird is perched at rest
The speculum will not always be visible, but when it can be seen, it can be a diagnostic field mark for many duck species.
Understanding this unique part of the wing structure can be very helpful for identifying birds in flight when other field marks may be less visible.