Rapid Decline Of Nesting Peregrine Falcons In The San Francisco Bay Region Of California Synchronous With An H5N1 Outbreak

ABSTRACT After rebounding from near extirpation during the organochlorine era, breeding Peregrine Falcons ( Falco peregrinus ) in California are again facing adversity, this time consistent with an outbreak of a highly pathogenic avian influenza. Following the first detection of the H5N1 variant clade 2.3.4.4b virus in California wild birds in July 2022, we assembled data from long-term monitoring (2000–2025) of peregrine breeding territory occupancy in the broad vicinity of San Francisco Bay to examine possible impacts on falcon populations. Prior to the outbreak, 47 focal breeding territories had shown nearly complete occupancy by pairs (98.5% of 390 site-years), with very few vacancies, single birds in attendance, or subadult pair members. Within 8 mo of the outbreak, occupancy had dropped to 65.1%, and 2 yrs later (2025), only 36.2% of sites remained occupied. An uptick in site-occupancy by single birds also occurred after the outbreak, but it is unclear whether these were survivors or floaters attempting to fill vacant territories where both pair members had perished. The high vacancy rates signaled an impact upon floaters (nonbreeding adults) that normally buffer breeding site-occupancy in healthy peregrine populations. From October 2022 through November 2025, 17 peregrine fatalities were diagnosed with H5N1 within the counties comprising our study area. Evidence that H5N1 caused these territory vacancies includes, (1) the striking temporal coincidence of occupancy loss with the outbreak, and (2) the lethality of the virus to peregrines and its confirmed presence in peregrine prey in our study area. Our study reaffirms the value of long-term territory occupancy monitoring in this sentinel species.