Long-term changes in eggshell thickness in Peregrine Falcons Falco peregrinus in Sweden

THE PEREGRINE FALCON populations across Europe and North America were almost exterminated in the 1970s; in Sweden, only 15 pairs were known by 1975. One of the main causes was reduced productivity due to eggshell thinning attributed to the widespread use of organochlorine pesticides. This study analysed eggshell thickness in samples collected across Sweden between 1964 and 2023, and compared with eggs from 1865-1945. The results indicate very thin shells in the 1960s to 1980s when eggshells were on average -16% of normal thickness, with records of -29%. Since then, the average thickness steadily increased and, alongside, the Peregrine population has recovered. While differences in shell thickness and recovery rates between northern and southern Sweden were observed, they were not statistically significant. In 2020s the average eggshell thickness is still -6% below normal which is within safe levels. However, because new substances with the potential to affect eggshell thickness are increasing in the environment, it is worthwhile to continue to collect and measure eggshells as a simple and low-cost way to monitor one of the effects of environmental contaminants. The Peregrine remains an important indicator for unintended side effects of chemical substances released into the environment.