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It remains to be determined whether Fox Sparrow is one highly variable species or up to four closely-related species (Zink and Kessen 1999). Even within the four major subspecies groups the 18 recognized subspecies of "Red, Thick-billed, Slate-colored", and "Sooty" Fox Sparrows present so much individual plumage variation as to cause birders to scratch their heads in befuddlement. Where I live in the Pacific Northwest, one can readily see 20+ Sooty Fox Sparrows during a winter day, with no two looking exactly alike in terms of plumage (Mlodinow et al. 2012).
My experience with Red and Sooty Fox Sparrows led me to believe that their most consistent aspect is bill coloration, which consists of mostly dark horn color on the maxilla and bright corn-yellow along the basal cutting edge of the maxilla and most of the mandible. I've always thought that this bill coloration was universal among Sooty and Red Fox Sparrows, with Slate-colored Fox Sparrows showing lesser amounts of duller yellow on the bill.
Most of the Thick-billed Fox Sparrows in the images that I examined showed no yellow on the bill. It should be noted that all the photos I found were taken during the spring and summer months, when their over-sized bills are a uniform grayish horn color. My experience with Thick-billed Fox Sparrow is limited to seeing birds on their central Oregon breeding grounds (May-July). None of the Thick-billeds that I've seen or photographed have shown yellow on the bill. This article does include one March photo of a Thick-billed (see below) and it shows a restricted area of dull yellow on the lower mandible. Additionally, Kimball Garrett told me of an Allan Brooks painting of a "Yolla Bolly Fox Sparrow" (references P. i. brevicauda) that was painted from a January specimen taken in Los Angeles County. Brooks' illustration shows "bright yellow-orange on the basal two-thirds" of the mandible according to Garrett.
A day spent sorting and captioning Fox Sparrow images for the BirdFellow Social Field Guide has revealed that bill color–at least in Red and Sooty Fox Sparrows–may change seasonally, adding another confounding element to sorting them out.
Robert Royse, Mark Szantyr, and Glenn Bartley have provided BirdFellow with a nice assortment of spring and summer season images, which depict all four subspecies groups. Several of Royse's Sooty and Red Fox Sparrows were photographed on their northerly breeding grounds. It came as a surprise to me that these birds show no yellow on the mandible. I also found several early spring (March-April) images in which it appears that some of the bright yellow–typically seen on winter birds–was absent or already noticeably faded. On birds photographed in early spring, the base of the mandible had in some cases faded to a paler bone or horn color.
Conversely, photos of fall and winter Fox Sparrow taken by Szantyr, Bartley and others, along with those found elsewhere online, generally show birds with bright corn-yellow on the mandible and along the cutting edge of the maxilla. I looked to the Birds of North America Online account for enlightenment on this topic. Weckstein et al. (2002) suggest that the seasonally variability in the bill color of Fox Sparrows needs of further study, a sentiment echoed by Garrett (pers. comm).
If one peruses the more popular illustrated field guides of North American birds (Sibley, National Geographic, and Peterson), it is easy to conclude that Red and Sooty Fox Sparrows have entirely yellow mandibles or even mostly yellow bills. Similarly, the David Beadle plate that accompanies Zink and Kessen (1999) depicts Reds and Sooties with all-yellow mandibles. The Beadle, Sibley, and Peterson illustrations of Slate-colored Fox Sparrows show them to have duller yellow on the mandible, while the 5th edition National Geographic Guide plate includes a Slate-colored with no apparent yellow on the bill.
The Beadle plate and the illustrations of Thick-billed Fox Sparrow in all three of these guides depict them as having no yellow on their bills. They are shown to have more grayish or dusky horn-colored bills. Beadle's plate along with the Peterson and National Geographic guides show this subspecies group to have a somewhat bi-colored bill, with the maxilla being noticeably darker than the mandible. I have some poor quality photos of Thick-billeds with distinctly two-toned bills (darker upper mandibles) that were taken in June 2009 on the east side of the Cascades in Jefferson County, Oregon. Sibley's illustration of Thick-billed Fox Sparrow shows it to have a mostly uniform grayish bill with a darker culmen, closely matching the bill pattern of the bird in the Royse photo above. Zink and Kessen (1999) and Garrett et al. (2000) discuss a cline that exists between the Thick-billed and Slate-colored Fox Sparrows, which frequently hybridize along the the "Great Basin-Sierra Nevada interface." Both articles describe birds in the White Mountains of California that are morphologically intermediate in terms of bill size and plumage.
It is important to point out that artists charged with creating a single example of a particular species or subspecies for a field guide are left to present a general representation that approximates what you or I perceive when we see these taxa in the field. Thus, it is unrealistic to expect their illustrations to be letter perfect, or to capture the full range of variation that we might see. Both Red and Sooty Fox Sparrows are northerly nesters, whose breeding ranges are far-removed from where most of us spend our time birding. Those who buy and use these field guides are mostly observing Red and Sooty Fox Sparrows on their wintering grounds, hence the illustrations in these guides represent what we are likely to see outside of the breeding season. Below is a series of head only crops of various subspecies of Fox Sparrows. My apologies to the photographers for the heavy cropping of these images, which compromises the quality of their otherwise wonderful uncropped images.
Although this small sample seems to suggest that northerly breeding Fox Sparrows (Red and Sooty) have bright yellow mandibles away from breeding season and show more uniform gray to pinkish horn-colored bills on the breeding grounds, there is, to date, no solid science to support this idea. In multiple captions Zink and Kessen (1999) emphasize that bill coloration is not reliable as a field mark when separating Fox Sparrow subspecies or subspecies groups. It would be great to hear from folks with experience handling these birds at banding stations to learn if they notice any seasonal variations in bill color. One thing seems clear, this question warrants further investigation. I will certainly be looking more closely at Sooty Fox Sparrows in my area, particularly the latest departing northbound and earliest arriving southbound individuals.
This article would not have been possible without the fine collection of photos provided by Glenn Bartley, Robert Royse, and Mark Szantyr. Their images were a pleasure to look at and inspired me to delve into this question further. Kimball Garrett offered a prompt and informative response to several questions that I posed to him and confirmed my notions about some of the taxa with which I'm less familiar. Szantyr and Royse both responded promptly to my queries and provided valuable additional information about the circumstances of their respective observations. Royse was also able to supply higher resolution head shots for a couple of his birds.
2000. Call notes and winter distribution in the Fox Sparrow complex. Birding 32:412-417.
Mlodinow, Steven G., Bill Tweit and David Irons. 2012. Photo Essay: The Sooty Fox Sparrows of Washington's Puget Trough. Birding Vol. 44:2 46-52.
Weckstein, Jason D., Donald E. Kroodsma and Robert C. Faucett. 20o2. Fox Sparrow (Passerella iliaca), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds. cornell.edu/bna/species/715doi:10.2173/bna.715
1999. Species limits in the Fox Sparrow. Birding 31:508-517.
Other sources on this topic:
I recommend taking a look at a lively and informative exchange that occurred on the ID Frontier's listserv back in 1997. Joining in this discussion were the likes of Louis Bevier, P. A. Buckley, Jon Dunn, Kimball Garrett, Alvaro Jaramillo, David Sibley and Mark Szantyr. Greg Gillson created a link to this thread, which can be viewed at: http://thebirdguide.com/fox/frontier.htm