I did go out and look for it several times in subsequents day, but I found some farmers engaged in field operations in that field. Nice to finally get a resolution of this puzzle, thanks to Hendrik.
-------- Forwarded Message -------- From: Joel Geier <joel.geier...> To: MidValley Birds <birding...> Subject: Raptor mystery solved: leucistic Northern Harrier along Robison Rd. Date: Sat, 13 Jan 2018 18:06:13 -0800
While browsing recent local eBird postings just now, I happened across this one from Hendrik Herlyn on January 9th, which is apparently from the Cold Creek Hunting Club wetlands off of Robison Rd, on the border between Benton County and Polk County:
This finally solves the mystery of the strange raptor that I reported from within a mile of this location though much farther out from the road, a couple of months back.
The hunting club wetlands are private but can be viewed to some extent from Robison Rd. (west of Hwy 99W, north of the landfill). For folks interested in county boundaries, birds close to the road are in Benton County but birds farther out are in Polk County (the road runs at an angle to the county boundary here).