On the afternoon of the 5th I scattered seed on the ground beneath the bushes in which it was initially seen and returned to the site on the morning of the 6th to find that the local thicket birds had indeed already discovered the seed. Initially present were two Fox Sparrows. Both were standard Eastern fare. Then three. Then four. But nothing out of the ordinary. By this point I was beginning to question the initial sighting, wondering if lighting conditions had something to do with that first impression.
Finally, while watching the seed plot naked-eye, a grayish-brown Fox Sparrow popped into view. Noticing how different this bird appeared even without binoculars was quite telling. A closer look confirmed that this was the sparrow I had seen a few days prior.
I was able to watch and photograph the bird on-and-off for a couple hours, sometimes in direct comparison with the iliaca Reds. I noted the following:
1) Colder/browner/grayer overall (brown was the predominant color impression). Even in sunlight, and comparing apples to apples in the field with iliaca, this bird lacks the same warmth of typical iliaca, both above and below. This includes a reduced creamy wash to the upper breast, which is extremely subtle on this bird. It is just not a very rufous bird overall, even where brightest at the base of the tail, no matter the lighting conditions.
2) The head pattern is grayer and more muted. Iliaca pretty consistently shows rufous auriculars set off by a gray supercilium and nape, and a crown with coarse rufous and gray streaking. This bird's gray bleeds smoothly (not coarsely) into the crown and into the auriculars (on the left side greater than the right, I believe).
3) The back is streaked, though this is muted and cold relative to iliaca.
4) The rear flanks are blurry and have a light brown background color to them. Some iliaca have messy rear flanks, but the background color remains white or off-white.
5) Pale wing covert tips were not very strong, though this is quite variable in iliaca based on wear etc, The photos illustrate just how different this feature can appear from image to image based on angle and lighting.
After researching zaboria (i.e. western/AK-breeding Red Fox Sparrows), they generally differ from iliaca in having more extensive gray to head and back and may be a bit duller rufous overall, but are otherwise quite similar to the iliaca we see here, complete with strongly patterned back, variably pale wing covert tips, and lack of muddy flank background color.
This bird's colder and browner tones combined with the brown background color to the flanks recalls Sooty Fox Sparrow influence and seems outside the range of what is seen in 'pure' zaboria.
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I am not married to this ID, but I do think it fits best. Outside input has varied from Sooty x Red to 'altivagans' (a bit of a mystery taxon, likely Slate-colored x Red) to zaboria Red to 'no idea', highlighting the difficulty of intergrade/oddball Fox Sparrow ID on the wintering grounds.
A consulted FOSP researcher suggested the following: "Yes, this bird looks well within what I'd consider could be produced from intergradation between coastal sinuosa [Sooty] and interior [Red], intergradation that I know occurs in south-central Alaska."
I am reasonably comfortable ruling out zaboria Red for the reasons noted above, but beyond that my confidence wanes. It seems highly likely that this bird is from a contact zone somewhere out west.