Certain groups of birds seem to invite a degree of lumping by the public when it comes to their descriptions. It's not to fault observers necessarily; the birds themselves can be difficult to separate into species, they being so frustratingly similar to their kin.
"Seagulls" is one of my personal favorites. There are many species of gull in North America, even here in North Dakota. None include the word seagull in their name yet the usage of it remains customary. Another commonly used collective noun is "sparrow." There are about 25 sparrow species regularly seen in North Dakota. Yet the one most familiar to casual the observer - house sparrow - is not a true sparrow, it's an Old World weaver finch.
Similarly, there are black birds and there are blackbirds. A quick count of black-colored birds in North Dakota puts the number at about ten if we include common raven and American crow. Blackbirds, however, those members of the family Icteridae with the actual word "blackbird" in their common name, account for only four of this total. Three of these - yellow-headed blackbird, red-winged blackbird, and Brewer's blackbird - are fairly known to locals as these particular species stay and nest here during the breeding season. One, though, is a migrant, only appearing briefly in the spring and fall. It's the rusty blackbird (Euphagus carolinus).
It may surprise some to learn that, among birders, rusty blackbirds remain somewhat mysterious; mostly because it appears so similar to the more common Brewer's blackbird in spring. Both are slightly larger than red-winged blackbirds (but smaller than common grackles), blackish overall with iridescent green and purple highlights, and with pale yellow irises.
Once fall rolls around and migrant blackbirds begin flocking, rustys definitively separate themselves by molting into their namesake plumage. Obvious rusty feather edging to its face, its back, its breast, and its wings, make identifying it in mixed flocks a fairly easy task. In addition to appearing rusty, the bird even sounds like it needs a little oil. Its call consists of a handful of notes often described as resembling a squeaky rusty hinge.
The breeding range for these migrants stretches from Alaska to the Canadian Maritimes where they seek out wet boggy nest sites among coniferous or mixed woodlands. It nests as close to us as the swampy northern edge of Minnesota. So rarely is this species seen away from water, some have suggested a more appropriate name might be swamp blackbird. Even in migration and on its southeastern U.S. wintering grounds, water is an ever present element in this bird's life.
While insects and other invertebrates make up the typical summer diet of rusty blackbirds, in winter the bulk of the bird's calories are vegetative. Curiously, on occasion rustys have been known to attack and eat other birds such as sparrows.
If bird watchers are somewhat unfamiliar with rusty blackbirds, so, too, are scientists. Little is understood regarding the bird's life history so it shouldn't be surprising to learn its overall population is openly debated. It is a commonly held belief that rusty blackbird populations are dropping dramatically. Cornell University's "All About Birds" states, "The population has plunged an estimated 85-99 percent over the past forty years and scientists are completely puzzled as to what is the cause."
On the other hand, a much more measured and cautious assessment comes from a Birds of North America Online (subscription only) article written by Michael Avery. He cites the difficulty in studying this bird on its nearly inaccessible breeding grounds and the unreliability of basing population conclusions on Christmas Bird Count and Breeding Bird Survey data. Muddling the issue even further is "the ease of confusing it with Brewer's Blackbird and Common Grackle," writes Avery (who received his M. S. degree from NDSU). I have not personally noticed changes in the number of rusty blackbirds but my anecdotal sampling is too small to matter.
Just as all gulls or sparrows are not the same, neither are the blackbirds, especially rusty blackbirds. Watch for this relatively little known species in mixed flocks with, yes, other black birds, but always near water. Right now it should readily stand out from its relatives. As we make our way into winter, don't discount seeing this bird at your feeder. Locally it's a rare but nearly annual overwintering bird.