The documented history of nesting peregrine falcons in California dates to the 1800s when outdoor enthusiasts enjoyed the hobby of egg-collecting.  Hundreds of ‘sets’ or clutches of peregrine eggs wound up becoming integrated into the specimen collections of various academic institutions, most notably the British Museum, the Western Foundation of Vertebrate Zoology, UC Berkeley’s collections, and a few others. 

These early collections provided an incredible repository for comparing pre-DDT (pre-1940s) eggshell thicknesses with post-DDT eggshell thickness.  This effort was discovered and pioneered Dr. Derik Ratcliffe, Chief Scientist for Britain’s Nature Conservancy Council. This work was expanded upon for North America by collaborating with Dr. Joseph Hickey (Univ. of Wisconsin, Madison) and his graduate student at the time, Dr. Daniel Anderson (UC Davis). 

Many papers were published, including in 1967, Ratcliffe’s Decrease in Eggshell Weight in Certain Birds of Prey, in which he developed the Ratcliffe Index, considered the most reliable measure of relative eggshell thickness.   The earliest field collected samples were the result of dedicated hobbyists, yet excellent field naturalists, who were out collecting and trading bird eggs among themselves.  The scientific results derived from these eggs, decades later, became the indisputable evidence needed to prove to the world that the environmental effects of chlorinated hydrocarbon pesticides, namely DDT, was the cause of a devastating global decline and near-extinction of peregrines (and other predatory bird species.   

The shocking finding of ‘endangered status’ for peregrines around the world lead to decades of research over much of the world, population monitoring, and brought in innovative captive breeding and large-scale release by the Peregrine Fund over a wide geographic area of an endangered species that seemingly was sliding toward extinction. 

Innovative management techniques were conceived, tested in the field, and applied on a large scale to stop the trend toward extinction.  Large numbers of captive-bred young were released for over a decade.  Ultimately, the United States banned the use of DDT domestically. These efforts collectively turned out to be one of wildlife conservation’s most celebrated success stories. 

Through research, captive breeding, and releases into the wild, peregrines recovered from their precarious ‘endangered species’ status to one of no longer needing the legal protections that such an ominous designation required.  It took decades of focused efforts, and the dedication of many peregrine experts and their associates to return peregrines populations to sustainable and seemingly stable numbers.  Peregrine falcons were remarkable removed entirely from the US’s infamous ‘endangered species list’ on 25 August 1999.

After two decades ushering in the substantial reestablishing of a robust and self-sustaining population throughout their nearly worldwide range, the 2020s have revealed a new threat to peregrines.  It now appears that Avian Flu may have a devasting effect on peregrines in many areas around the world. 

The CPP is participating in what’s become a global assessment of this latest threat to peregrines.  We are asking for your help in the research effort, at least for California’s contribution to a global effort.  Please read more about how you can help with this important research effort.