Recent Expansion of the Breeding Range of Say’s Phoebe (<i>Sayornis saya</i>) in California’s Central Valley

Say’s Phoebes (Sayornis saya), nest regularly in the dry grassland fringes of the San Joaquin Valley in California. Historical records of breeding or other summer occurrences in the agricultural and residential flatlands of the Central Valley and adjacent Sacramento-San Joaquin Delta are scarce and widely scattered geographically and across past decades. In summer 2018, observers documented Say’s Phoebes nesting at multiple sites in lowland areas within Yolo, Solano, Sacramento, San Joaquin, Fresno, and Tulare counties. We summarize the records of this 2018 breeding and explore evidence for historical expansion or other factors that may explain the apparent sudden increase of breeding on the valley floor. Databases, archives, and museum specimens spanning several decades provided only a few scattered observations of Say’s Phoebe breeding in the flatlands of the Delta and the Central Valley. eBird records of Say’s Phoebe presence in all of California from mid-May to the end of July showed an apparent increase in the past four years (2015–2018) into the counties that 46 Central Valley Birds, Summer 2019 include parts of the Delta. The number and geographic spread of the 2018 observations of nests, eggs, and feeding nestlings and fledglings in several counties seem to reflect a real and unprecedented surge of Say’s Phoebe breeding on the valley floor. The mechanisms driving this apparent expansion of the Say’s Phoebe breeding range are unclear. The proximity of most recent summer records to relatively new housing development and the location of almost all nests on buildings or other structures suggests that the expansion is correlated with increased housing development in the Central Valley and Delta, coupled with the species’ high capacity for dispersal and nuances of life history, such as the requirement that nests be close to open space and a possible coexistence with mud-nesting swallows.