You may already know this depending on your level of bird-nerdiness, but in some parts of the United States there are actually two different types of crows! Besides the standard American Crow, its cousin the Fish Crow can often be found around bodies of water, especially the coast. The folks living in the Pacific Northwest have a similar situation with the Northwestern Crow. All nearly identical in size and shape, these crows can sometimes only be told apart in the field by their caws. (See their respective pages on All About Birds to see and hear what I mean.)
I wonder, though–can the molted feathers of each of these extremely similar corvids ever be positively identified to the species level?
This is not a question that I have the resources or knowledge to fully answer, but in my experience with Fish and American Crows, species identifications can sometimes be made with a level of caution based on the location of the finding and the crow species that are seen there.
For example, I recently went camping at the Ochlockonee River State Park in Florida. The only crows I saw and heard were Fish Crows, and I even spotted a flock of over 20 of them. Later, I found this long black primary wing feather, clearly a crow feather, on one of the park’s many trails.
I assumed that it came from a Fish Crow, owing to the fact that Fish Crows were the most common crow species I had seen in the area. It is important to note, though, that while my conclusion may be logical, feathers tend to show up in the strangest places either due to migration, the elements, or human activity. And had I seen just a single American Crow in the area, I would not have felt comfortable ruling it out in any capacity.
In conclusion, unless you live outside the areas in which the Northwestern or Fish Crow occur, it is often difficult to identify molted crow feathers with a high degree of confidence. Only make species-level identifications based on your knowledge of the crows in the area and with the understanding that you may never really know whether you are correct.
Update: As of July 2020, the American Ornithologists’ Union has lumped the Northwestern Crow into the American Crow complex, essentially making them the same species.