Long-term decline in the nocturnal capture rate of European Storm-petrels Hydrobates pelagicus migrating past southern Portugal

Abstract

Capsule: The hourly capture rate of European Storm-petrels passing southern Portugal during northward migration has shown a significant decline over 27 years. Aims: To examine within- and between-year changes in European Storm-petrel capture rate. Methods: 6,786 European Storm-petrels were trapped in southern Portugal between 1993 and 2019 (mid-May to early July), using sound lures. Temporal variation in hourly capture rate was examined using generalized additive mixed models. Where available (2006–2017), we also included a range of environmental variables to control for their influence. Results: Hourly capture rate declined significantly between 1993 and 2019, falling by 68.9% (95 CI: 49.1–80.9%). There was also significant variation across seasons (rates peaked in mid-June) and within nights (rates peaked between 00:00 and 03:00). When environmental variables were controlled for, the significant decline in capture rate across years was still evident, alongside significant negative associations with moon luminosity, sea surface temperature and wind speed, as well as wind direction. Conclusion: Such drastic declines in capture rate may represent a decline in the number of birds available to recruit into the Atlantic breeding population or, alternatively, changing migratory behaviour, such as a shift in migration routes and/or timings.