First DDT now the flu: Peregrine falcons in San Francisco Bay area hit hard by H5N1 variant

Highly Pathogenic Avian Influenza (HPAI) featured prominently in the news during 2020 as it took the lives of millions of birds across the globe. While the worst seems to be behind us, population-level effects are just now emerging. A new study published in the Journal of Raptor Research, titled “Rapid Decline of Nesting Peregrine Falcons in the San Francisco Bay Region of California Synchronous with an Outbreak of H5N1 Highly Pathogenic Avian Influenza,” reports an alarming decrease in the number of occupied territories for a population of Peregrine Falcons (Falco peregrinus) that has maintained a longstanding presence in coastal California. As top predators, peregrines show warning signs when the health of their food web is jeopardized. They indicated the threat of DDT to wildlife in the 1960’s, and they are once again sounding the alarm to those who know where to look. 

Highly Pathogenic Avian Influenza (HPAI) is an infectious disease that typically remains at low levels in wild bird populations. However, in 2020, the variant H5N1 emerged and ran amuck in what is now considered one of the most severe disease outbreaks to affect large animal populations ever recorded. Infections were documented in 400 bird species and at least 50 mammal species that year. Waterbirds are particularly susceptible to becoming carriers of HPAI, so predators and scavengers that feed on them are at a higher risk of exposure. Peregrines are master bird hunters and therefore primed to encounter this bird-born illness. However, it can be challenging to verify cause-of-death for raptors because of their low densities and the challenge of finding their carcasses, so raptor biologists must employ other tools to investigate population-level responses.
 

Lead author on the study Zeka Glucs, Director or the Predatory Bird Research Group at the University of California Santa Cruz, worked with a team of researchers to compare the presence of Peregrine Falcons on known territories in the Bay Area before and after the H5N1 outbreak, which was first detected in the state in 2022. They accessed a long-term dataset from 2000-2025 to examine trends in peregrine presence, underscoring the importance of maintaining research that spans decades. 

The team found that prior to the H5N1 outbreak, 47 peregrine territories in the Bay Area were consistently occupied with stable breeding pairs. Following the outbreak, peregrine presence on these same territories dropped by 65% in three years, and the decline continues. 

Concerned residents have contributed to this work out of fondness for the species. “For many, losing these pairs feels like a personal and community loss,” says Glucs, “so we celebrate what we can to keep our hope alive: returning pairs and successful fledges.” To this end, his team is organizing a volunteer nest survey project called the California Peregrine Project to invite more involvement from the public.   

The team posits that the trend in the Bay Area could be indicative of similar declines in other parts of the peregrine’s range. They also emphasize that peregrines can be reliable signals for the presence of HPAI. “We hope our work will help illuminate the severity of the influenza’s impacts on Peregrine Falcons in our region, inform protections for the remaining pairs, and once again show the value of these birds as a highly sensitive sentinel species,” says Glucs. Peregrines have long been a lauded conservation comeback story, following their brush with extinction in the 1960s due to DDT. Keeping them in the skies is priority for many, and studies like this bolster collective knowledge of where the species needs the most support.