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Last month's Natural Mystery called for attention to detail. Many small songbirds have dark flight feathers, making it a challenge to identify these to species. Congratulations to newsletter Gray Wolf patron Mike Holtz for doing just that.
These are the tail feathers of a house sparrow.
The shape distinguishes these as tail feathers. Rather than showing an even curve along their whole length, as is typical for wing feathers, these all show a single bend near the base of the quill. To identify these feathers to species, I'll turn it over to Mike:
"Tail feathers from a house sparrow. These are pretty small feathers, and small birds in a St. Paul urban area could include black-capped chickadee, dark-eyed junco, house finch and house sparrow. The outer tail feathers of a junco would be white and partially white and the inner would be darker. Chickadee tail feathers would also be darker. House finch is getting a little lighter, but I think house sparrow fits best here, and they are definitely common in urban areas."
To be thorough, we might consider a few less common species as well. A search on eBird shows sightings of a number of other small passerines in Ramsey County in February in the past several years that have dark tail feathers. We can compare the tail feathers of these candidates using the scans on the U.S. Fish and Wildlife Service Feather Atlas. This website allows us to browse or search through scans, and also has some helpful feather ID tools.
Consulting the Feather Atlas we can see the traits Mike points out above, and also see that American tree sparrow tail feathers are larger and its outer feathers are less curved; song sparrow tail feathers are a bit larger, and have a slightly different shape; purple and finch tail feathers appear a bit smaller and also have a different shape. Robin and starling feathers are much larger.
To give you a little sample of the Feather Atlas, here are tail feather scans for the four species Mike compared for his answer:
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