Hi All
The discussion in the AOU, 7th edition, p. 620:

Correspondence of genetic evidence and plumage types suggest that the groups
[ P. iliaca (Red Fox-Sparrow), P. unalaschensis (Sooty), P. schistacea
(Slate-colored) and P. megarhyncha (Thick-billed)] may represent biological
species, but there is limited hybridization among them, especially between
the schistacea and megarhyncha groups. Additional study is needed in areas
of contact of members of the groups. Three of the four groups were treated
as separate species by Rising (1966) who considered the schistacea and
megarhyncha groups to be conspecific.

The Rising reference is to his A Guide to the Identification and Natural
History of the Sparrows Of the United sates and Canada. Rising says that
genetic and behavioral (vocal) evidence suggests as many as 4 separate
species.

BTW, Rising also has the Red Fox-Sparrow wintering along coastal PNW (USA).

Cheers
Gail Spitler
Johnsons Landing, BC
spitler at wkpowerlink.com