Townsend's Solitaires certainly do nest on the ground; I've had them
flush out from under my feet a number of times while walking over
rocky ground in the Doug-fir zone. The nests were squirreled away in
under small overhanging rocks. I also ran into one at the base of a
Serviceberry bush.
'Birds of the Okanogan Valley' is a good book for us eastsiders (and
while I think it is out of print, I know David at 'Flora and Fauna'
in Seattle bought several thousand of them, 206 623-4727); in it one
can see that the first breeding records for Solitaires (in the
Okanogan) are the first week of May, with intense activity by the
third week. They've been found nesting in this area 'in the
coniferous forests at all elevations', but are most common between
1500 and 3600 feet.
I'll wager a round tuit that your bird (Hans K) will
move on as the snow melts.

Dana Visalli
Methow Valley