Citizen science data reveal possible multi-decadal phenological changes in the arrival time of a migratory tropical seabird species at the breeding ground

Abstract

Climate change and weather variability are having global impacts on the lives of organisms, particularly on high-trophic level predators such as pelagic seabirds. In the North Atlantic, migratory seabirds are expected to respond to climate variability by adjusting their seasonal events, including the timing of migration and arrival at the breeding site. The timing of these events may be in uenced by large-scale atmospheric phenomena like the North Atlantic Oscillation (NAO) and the Atlantic Multi-decadal Oscillation (AMO). The White-tailed Tropicbird (Phaethon lepturus) is a wide-spread tropical migratory seabird breeding at its Atlantic northernmost edge of distribution range in Bermuda Islands (32° 17' 58'' N, 64° 47' 25'' W). Using data from eBird, an online database of bird observations where expert and amateur birdwatchers can report their sightings, we explored trends in Tropicbird rst annual observation (proxy for bird arrival time) at the Bermuda breeding ground from 1953 to 2023. Speci cally, we examined the relationship between the arrival time of the Tropicbird at its breeding site and the NAO and AMO. We show that the progressive early arrival at the breeding site (20-25 days in advance) of Tropicbirds over the last 70 years positively correlated with the NAO and AMO Indices. This suggests that this tropic seabird breeding in the North Atlantic may be responding to climate-induced changes affecting the Atlantic Ocean. Our ndings highlight the fundamental contribution of citizen-science data for ecological long-term studies to understand animals' responses to a changing world.LA/P/0069/2020 granted to the Associate Laboratory ARNET.Data availability: All data generated or analysed during this study are included in this published article and its supplementary information les.Compliance with Ethical Standards: We declare we have no con ict of interest.