This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: 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 long-standing presence in coastal California. As top predators, peregrines show warning signs when the health of their food web is jeopardized. They signaled the threat of DDT to wildlife in the 1960s, 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 H5N1 variant emerged and ran amok in what is now considered one of the most severe disease outbreaks ever recorded among large animal populations. 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-borne illness. However, it can be challenging to verify the cause of death for raptors because of their low densities and the difficulty of finding their carcasses, so raptor biologists must employ other tools to investigate population-level responses.
Lead author Zeka Glucs, director of 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 in 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 data set from 2000–2025 to examine trends in peregrine presence, underscoring the importance of maintaining research that spans decades. The team found that before the H5N1 outbreak, 47 peregrine territories in the Bay Area were consistently occupied by stable breeding pairs.
Following the outbreak, peregrine presence in 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," Glucs says, "so we celebrate what we can to keep our hope alive: returning pairs and successful fledglings." To this end, his team is organizing a volunteer nest survey project called the California Peregrine Project to invite more public involvement.
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