INVESTIGATING THE EFFECTS OF ENVIRONMENTAL STRESS ON REPRODUCTIVE SUCCESS IN MIGRATORY BIRD POPULATIONS

Abstract

This study evaluates how gradients of environmental stress on breeding grounds affect reproductive success in two long-distance migratory songbirds. During the 2024 breeding season, we surveyed six sites across Europe and North America, measuring temperature anomalies (0.5–2.5 °C), habitat disturbance indices (0.30–0.90), and pollutant concentrations (20–70 µg/L). We monitored clutch initiation, clutch size, hatching rate, and fledgling survival at each site, and sampled adult blood for baseline and stress-induced corticosterone assays. Generalized linear mixed-effects models revealed significant negative effects of temperature anomaly (β = –0.12, p < 0.01), habitat disturbance (β = –0.18, p < 0.001), and pollutant concentration (β = –0.10, p < 0.05) on overall reproductive success. High-stress sites exhibited reduced clutch sizes (3.5 vs. 5.0 eggs), lower hatching rates (65 % vs. 90 %), and diminished fledgling success (55 % vs. 82 %), alongside elevated corticosterone (baseline up to 3.2 ng/mL; stress-induced up to 7.1 ng/mL). Breakpoint analysis identified critical thresholds—≥ 2 °C anomaly, ≥ 0.75 disturbance index, and ≥ 50 µg/L pollutants—beyond which reproductive metrics declined sharply. These findings underscore the synergistic impacts of multiple stressors on avian reproduction and highlight the importance of threshold-based management. Conservation efforts should prioritize mitigating habitat degradation, controlling pollutant inputs, and implementing adaptive measures against climate extremes to bolster population resilience under accelerating global change.