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. Some additional sites without location information can be found in the related table. The related table includes nesting data 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, summarized by region, has been made available in BIOS as California Least Tern Monitoring Sites by Region [ds3147]. This is a publicly available dataset.