Demographic causes of the pesticide crash in the peregrine falcon

Population crashes in many avian predators during the 1950–70s, caused by organochlorine pesticides, belong to the most spectacular cases in the history of conservation and ecotoxicology. Negative effects of DDT on eggshell thickness, leading to egg breakage and declining productivity, are well-documented. In addition, cyclodiene pesticides such as Dieldrin were strongly suspected to contribute to crashes by increasing mortality, but the hypothesis of a contribution of survival to the crash could never be tested directly owing to a lack of early enough data and suitable analyses. We studied the demography of a large population of the peregrine falcon (Falco peregrinus) in the Jura mountains over 60 years (1964–2023), from crash to recovery. We combined in an integrated population model ring-recovery, productivity and population count data, tested for a reduction in survival during crash years and used retrospective analysis to compare the relative importance of survival and productivity for population dynamics. Incidentally, we discovered that for data with unequal sample sizes over time, only an autoregressive time-series formulation properly captured annual trajectories of demographic parameters, while traditional models with unstructured temporal random effects, assuming stationarity, did not. The population crash continued until the early 1970s, and subsequent recovery was not complete until the early 2000s. Productivity was greatly reduced during the crash and increased afterwards. Between 1964 and 1978, adult survival was strongly reduced, recovering from 0.62 (0.46–0.75) to 0.85 (0.77–0.91), while juvenile survival showed a long-term decline. The variance of the population growth rate was primarily explained by adult survival (67%), followed by productivity and juvenile survival (16% each). Hence, our study reveals an overwhelming effect of adult survival rather than productivity in the pesticide crash and recovery in this peregrine population. This is arguably the first time that the hypothesis, that survival was the major cause of the pesticide crash, was tested and corroborated. The success of our detective story illustrates well the key role of intensive long-term monitoring schemes: in combination with modern analytics, they can generate critical demographic knowledge with wide conservation implications and thus serve as invaluable environmental early-warning systems.