THE NORTHERN SAW-WHET OWL (AEGOLIUS ACADICUS) NESTS WIDELY IN NORTH AMERICA AND IS REGULARLY REPORTED DURING THE BREEDING SEASON (ROUGHLY FEBRUARY–MAY) IN ALAMEDA AND CONTRA COSTA COUNTIES IN THE SAN FRANCISCO BAY AREA. DOCUMENTED INSTANCES OF BREEDING ARE SCARCE, HOWEVER: BREEDING BIRD ATLASES FOR THESE COUNTIES REPORT BREEDING OF NORTHERN SAW-WHET OWLS ON ONLY A FEW OCCASIONS, ALL IN THE OAKLAND–BERKELEY HILLS (GLOVER 2009, RICHMOND ET AL. 2011). NONETHELESS, FROM 2004 TO 2015, SEVERAL COLLEAGUES AND I OBSERVED 19 NORTHERN SAW-WHET OWLS NESTING IN TREE-MOUNTED BOXES DESIGNED FOR WOOD DUCKS (AIX SPONSA) ON PROPERTY MANAGED BY THE EAST BAY MUNICIPAL UTILITY DISTRICT (EBMUD) IN CONTRA COSTA COUNTY. THE BOXES ARE DISTRIBUTED OVER A DISTANCE OF ABOUT 1500 M ALONG SAN PABLO CREEK AND THE ADJACENT SHORELINES OF SAN PABLO RESERVOIR, IN A RIPARIAN WOODLAND OF MONTEREY PINE (PINUS RADIATA), COAST LIVE OAK (QUERCUS AGRIFOLIA), CALIFORNIA BAY LAUREL (UMBELLULARIA CALIFORNICA), AND ARROYO WILLOW (SALIX LASIOLEPIS), AT ELEVATIONS OF ABOUT 100 M. THE STUDY SITE IS PART OF AN EXTENSIVE AREA OF MANAGED WATERSHEDS AND REGIONAL PARKS IMMEDIATELY EAST OF THE URBAN CORRIDOR ON THE EAST SHORE OF SAN FRANCISCO BAY.