Peregrine falcon populations on the California Channel Islands have experienced a notable decline in recent years, prompting concern among wildlife biologists. A comprehensive survey conducted in 2022 revealed a significant decrease in active nesting sites for these swift raptors, raising questions about their future.
In 2016, a high count identified 51 individual peregrine nesting sites across all eight California Channel Islands. However, the numbers have since decreased. The last full survey, conducted in 2022, examined 32 of those 51 sites on five of the eight islands. For this effort, active sites were around 38 percent of those surveyed, indicating what Nick Todd, a wildlife biologist with the Center for Environmental Management on Military Lands, described as a “suggested significant decrease in occupied territories.”
This decline comes as the Highly Pathogenic Avian Influenza H5N1 (HPAI) strain appeared in California in 2022. While no direct evidence links avian flu specifically to the decline in peregrine falcons on the islands, Todd noted that the low occupancy rate seen on the islands is similar to observed declines and low occupancy rates in peregrine populations from Monterey Bay northward to San Francisco Bay and into Western Nevada. Todd stated that HPAI is a suspected culprit, adding that peregrines are a "highly vulnerable species" and that most falcons exposed to the virus are assumed to perish within days of exposure.
Peregrine falcons, known for their incredible speed, face high rates of exposure to HPAI because their diet includes waterbirds such as geese, ducks, shorebirds, and seabirds. These migratory species travel great distances and are prime carriers of HPAI, often exposed to the virus by domestic birds like chickens and ducks. Joe Barnes, a wildlife biologist for the U.S. Fish and Wildlife Service, who has worked with peregrine falcons since 2004, explained that while peregrines are not major spreaders of avian flu, they are vulnerable due to preying on species that readily carry the disease.
The vulnerability of peregrine falcons contrasts with the response seen in other bird species. Barnes pointed to bald eagles as an example of a species that has developed immunity against HPAI. Bald eagles and vultures in the eastern U.S. were heavily impacted when H5N1 first circulated but managed to develop antibodies, allowing them to cope and achieve some level of population immunity.
However, peregrines and other species in the Falco genus, such as kestrels and gyrfalcons, have not shown the ability to develop antibodies to deal with H5N1. This makes them highly susceptible, and peregrines often die within one to three days of exposure or from the first showing of symptoms. Barnes emphasized that until or unless falcons’ immune systems develop antibodies in response to H5N1 exposure, their best hope is for the strain to mutate enough to be less lethal or for it to be replaced by other less lethal and contagious strains.
This current threat is a new challenge for peregrine falcons, a species that previously recovered from the devastating effects of the pesticide DDT. DDT forced raptors like bald eagles and peregrines, along with seabirds like California brown pelicans, to lay thin-shelled eggs that cracked during incubation. While those species successfully recovered from that man-made debacle, they now face an uncertain future with the emergence of HPAI.
