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Only a handful of species show the range of geographic variation that one can see in Song and Fox Sparrows. You may have a good handle on the forms that occur close to where you live, but if you travel about the continent you may come face to face with individuals that leave you wondering which species you are looking at. This is particularly true in the Pacific Northwest, where the "coastal" forms of Song Sparrow are slightly larger and significantly darker than those found along the East Coast and across the interior of the U.S. and Canada.
Sooty Fox Sparrows further complicate matters. Not only are they quite different in appearance than the more easterly Red Fox Sparrow, but for much of the year you will find them in the same brush patches with the equally unfamiliar looking Song Sparrows of the Pacific Northwest. This article offers some key field marks to look for if you find yourself struggling to separate these species away from your home patch. The best part is, you don't have to have them side by side to sort them out.
A good starting point is bill color. Unlike Fox Sparrow, the bill color of Song Sparrow doesn't change much throughout the year. Their bills almost always look uniform in color. Both mandibles are a dark and dusky grayish horn color. Only recently-fledged juveniles will show paler fleshy yellow tones on the bill. The bill color in Fox Sparrows is far more variable and changes seasonally. During the breeding season, their bills are generally paler, ranging from pale grayish to dull grayish-pink. Outside the breeding season (October-March), Fox Sparrows will a darker bill color, particularly the upper mandible, with varying amounts of corn yellow on the lower mandible and along the cutting edge of the base of the upper mandible. Birds seen from March-May will show a gradual reduction in the amount of yellow on the bill (Irons 2012). These seasonal transitions in bill color seem to hold true across all four Fox Sparrow groups (Red, Slate-colored, Thick-billed, and Sooty).
The next thing to look at is the face pattern. All subspecies of Song Sparrow have a well-defined brown or reddish-brown post-ocular stripe that is surrounded by gray or buffy-gray and extends from the back of the eye towards the nape, where it generally flares slightly. In the West, there are three groups of Fox Sparrow subspecies. Sooty, Slate-colored, and Thick-billed Fox Sparrows all lack the strong face pattern that we associate with Song Sparrows and Red Fox Sparrows. Although Red Fox Sparrows have a strong gray and russet face pattern, the auricular patch is usually fairly solid reddish-brown, thus there is no clearly defined brown or reddish-brown post-ocular stripe.
With a good view of the bill color and face pattern, you should be able to readily separate Fox and Song Sparrows, regardless of where you are in North America. By focusing on these two aspects, you can side-step the confusion that is introduced by all the geographic variation in color and you won't get waylaid by Song Sparrows that show a darker or more reddish-brown appearance.
There are a couple of secondary field marks that may help you confirm that you've made the correct identification. First, the lower rumps and tails of Fox Sparrows are almost always noticeably more reddish than other aspects of the bird. In looking at the Fox Sparrows in the images above, even though the overall tone of these birds is quite variable, you might notice that the tails and lower rumps are slightly more reddish on all of these birds. If you look at the tails of the Song Sparrows, they tend to match pretty closely to the general color of the upperparts and don't contrast with the rest of the bird. The Song Sparrows are also somewhat streaked on the rump, while the Fox Sparrows show unstreaked rumps.
Finally, the dark markings on the underparts of Fox Sparrows tend to be more crisp with lots of isolated triangles/chevrons and if you look closely at the streaking you can see that the streaks are actually rows of chevrons. Song Sparrows tend to have blurrier streaking, with very few isolated chevrons and the streaks don't appear to be made up of rows of chevrons. Below are sets of images of both Fox and Song Sparrows that illustrate this difference.
I'll conclude this photo essay with a bird (see photo below) that might cause a bit of a problem. It was photographed in Michigan during April by Allen Chartier. It shows a quite a bit of gray in the auriculars, which are bordered by russet above and below. It also has the suggestion of a post-ocular stripe, although it is thinner behind the eye and connects to the auriculars rather than dead-ending at the nape like it would on a Song Sparrow. Other clues include unstreaked rump and the tail and lower rump, which are considerably more rusty reddish than the rest of the bird. Finally, if you look closely at the bill, it is still showing some yellow on the lower mandible. We can't see much of the underparts, but otherwise the sum of the parts of this bird add up to Fox Sparrow.
I will continue to look for pairs of potentially confusing common birds as subject matter for this sort of article. If you have certain birds or groups of species that you wrestle with, feel free to let me know via a comment. We welcome your feedback about these articles and suggestions for future ID articles.