Allen, L.D., Howell, S.L., and Kus, B.E., 2017, Distribution and abundance of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) on the Middle San Luis Rey River, San Diego, southern California—2016 data summary: U.S. Geological Survey Data Series 1065, 11 p., https://doi.org/10.3133/ds1065.