According to Google's maps, the county line falls at the bottom of the
dike, on the parking lot side. But, as has been mentioned, eBird has
determined that the dike, where the hotspot has been located, is in King
County, yet eBird uses Google Maps for its overlays. MapQuest, on the
other hand, draws the county line at the water's edge, just north of the
dike. A look at Snohomish County's official maps does indeed place the
dike in King County (http://gis.snoco.org/maps/permits/index.htm).

When I was there on Monday, the sparrow was in King County, according to
the above, but then flushed to the brambles on the west side of wetland
at a location that must clearly be right on the county line!

Scott Ramos
Seattle