Fifty Years of Bird Ringing Reveal Opposing Seasonal Responses of Migration Timing to Temperature and Rainfall on Hilbre Island.

Understanding how populations respond to environmental variability is crucial for interpreting long-term monitoring data, especially for migratory species with complex life cycles that are susceptible to changing weather conditions and may show variation in stopover behaviour. Environmental pressures can influence the timing and duration of migratory stopovers, which may affect local species composition and capture patterns at observatories. This study analyses a 50-year bird ringing dataset from Hilbre Bird Observatory, a long-term monitoring site on England's western coast that offers valuable insights into migratory bird patterns. Findings show that Hilbre Island has become warmer and wetter over time, and that the interaction of temperature and rainfall influences observed migration timing. While warmer conditions at the observatory coincide with earlier arrivals, concurrent increases in rainfall can delay arrivals, illustrating how local weather conditions affect patterns of capture. Seasonal ecological differences in residency were observed: in spring, warmer and wetter conditions result in shorter stopovers, whereas in autumn, the same conditions lead to longer stays. Analyses of abundance revealed species-specific variation, but no significant relationships were detected between local weather variables and overall capture numbers. Overall, the study highlights the complex and sometimes opposing effects of temperature and precipitation on bird migration and residency at Hilbre and emphasises the value of long-term ringing data for detecting these patterns. The findings underscore the importance of considering weather-related sampling limitations when interpreting observatory data and provide a foundation for further studies on how local conditions influence observed migratory behaviour.