Steep Population Declines in Insectivorous Passerines, Irrespective of Their Migratory Strategies

This study examines a long-term dataset where 16 passerine species, classified as long-distance migrants, short-distance migrants, and residents were monitored at the Santo André National Ringing Station (Portugal) from 1997 to 2024. Using standardized capture data from 16 passerines species collected during the autumn migration period, we evaluated trends in population abundance over a 27-year time span. Our analyses revealed pronounced and statistically robust declines in all long-distance migratory species, particularly savi’s warbler, grasshopper warbler, and sedge warbler, which are now almost locally extinct. In contrast, short-distance migrants and resident species exhibited more heterogeneous patterns depending on their ecological specialization, yet all strictly insectivorous taxa, except for the chiffchaff, showed marked population declines, particularly the bluethroat and the sardinian warbler. The parallel decline in insectivorous species across migratory strategies points to a widespread trophic effect, likely linked to the global depletion of insect populations, driven by habitat destruction, pesticide use, pollution, and climate change. Collectively, these findings emphasize the urgent need for regionally adapted, long-term monitoring programs to inform effective conservation strategies in the face of accelerating climate and land-use change.