A wildfire is burning on the north end of the Warm Springs Reservation has quickly grown to 16,800 acres, and two structures are now reported to have burned.  KTVZ in Bend is reporting that one home has been lost in the Nena Springs Fire along with a historical structure.  A fire public information officer is indicating the buildings were in the Schoolie Flats subdivision.  Residents of about 70 homes remained under a Level 2 pre-evacuation notice Thursday morning.  The fire started within the Juniper Flat Rural Fire Protection District on Tuesday and moved on to Warm Springs land, putting out heavy smoke.  The fire has burned through juniper, sage, grass and dense stands of Ponderosa Pine.       Fire behavior has been extreme with spotting, crowning and wind driven runs.  A Type 2 incident management team is taking over fire operations Thursday.  A regional structure-protection task force is protecting homes threatened by the fire in three subdivisions.  Cause of the fire is undetermined.