|Slate-colored Fox Sparrow (possibly altivagans). Anza-Borrego Desert, California. January 26, 2014. Greg Gillson.|
A dozen years ago I put together a web page on the identification of the four forms of Fox Sparrows.
Follow this link for a newer page on Fox Sparrows by Don Roberson.
These forms are shown in all the recent field guides. They have been rumored to be ready for an official split into separate species for two decades, but that has not happened yet. One problem is that some altivagans appear to be half way between the Slate-colored group and the Sooty group.
The Slate-colored Fox Sparrow above has gray upperparts strongly contrasting with red wing and tail. The underparts are heavily spotted with reddish-brown, and the lower mandible is yellow. The lower mandible is also smaller than the upper mandible, unlike the Thick-billed forms (should really be called "wide-billed").