Increasing bird diversity in a landscape in transition.

Abstract

Rural depopulation across Mediterranean uplands has driven farmland extensification and landscape transitions, yet their medium-term impacts on birds remain poorly understood. We resurveyed 213 selected 2 km × 2 km grid cells in the Côa River basin (western Iberia) in 2023-2024, following protocols comparable to those applied in 1999-2005, ensuring consistent sampling across periods. Median alpha diversity increased by five species, a 28% gain, with total abundance rising as common taxa became more widespread. Gains were predominantly driven by forest-associated and generalist species, while farmland specialists showed stagnation or slight declines. We discuss how landscape transitions contributed to these biodiversity patterns including lower human density, reduced grazing pressure, and altered disturbance regimes. However, significant avian diversity gains occurred within apparently stable forest-cover mosaics, potentially masking broader ecological shifts.