California least tern colonies are monitored during nesting season along the California Coast and data is reported to California Department of Fish and Wildlife. The colonies in this dataset range from the Tijuana river mouth near the Mexico border to Sacramento. Locations and data include both present and historic sites. Locations are approximate and can have variation over time. These approximate locations were overlaid with a statewide hexagon layer (ds675) and then a buffer was created in order to mask exact nesting locations. These were then combined and broken into 4 main regions: S.F. Bay, Central, Ventura, and Southern. Two additional polygons are separated for the inland sites Kings and Sacramento. Some additional sites without location information can be found in the related table. The related table includes nesting totals by year from 1990 - 2019. Its anticipated to update this dataset to include earlier and newer records. Monitoring data includes estimated fledglings, estimated breeding pairs, total number of nests, and predator information when included in site reports. Annual reports 2017 and earlier can also be found by searching in California Department of Fish and Wildlifes document library (nrm.dfg.ca.gov/documents/docViewer.aspx).A version of this data, monitoring sites generalized to a 2.5 square mile hexagon, has been made available in BIOS as California Least Tern Monitoring Sites Generalized - CDFW [ds3146].