Spatio-temporal variation in post-recovery dynamics in a large Peregrine Falcon (Falco peregrinus) population in the Jura mountains 2000–2020†

After spectacular population crashes in the 1960 and 1970s, Peregrine Falcon Falco peregrinus populations recovered worldwide, and in the 2000s many study populations were back to normal. However, post-recovery trends have been documented less well. We combined three long-term population studies covering the entire Jura mountain range (16 304 km²) to examine spatio-temporal variation in population dynamics during 2000–2020 in 420 known nesting sites in five regions: Ain, Jura and Doubs in France, and Jura Sud and Jura Nord in Switzerland. Every year about 60% of all sites were surveyed at least once, and this proportion increased over time. We used Bayesian occupancy modelling to investigate site persistence and colonization probabilities and correct status and trend estimates for bias due to non-random site coverage. Based on the average of two models correcting for preferential sampling, we estimated that population size peaked in 2002 (Jura), 2005 (Doubs), 2007 (Ain, Jura Nord) and in 2007 for the Jura as a whole. Thereafter, the population declined significantly in four regions and in the Jura as a whole, with the strongest decline found in the Jura Nord ( –38%, 95% credible interval –34%/–41%) during 2007–2020. Thus, we found considerable spatial heterogeneity in post-recovery trends and widespread post-recovery declines. Reasons for the latter may include Eagle Owl Bubo bubo predation, illegal persecution and human disturbance. Our results illustrate how quickly positive population trends may be reversed, such that continuous long-term monitoring even for species seemingly ‘out of danger’ remains invaluable. Our study also emphasizes the importance of correcting for both coverage bias in general and preferential sampling in particular when assessing population trends in studies where not every territory can be surveyed in all years.