Collapse of an insular bird species driven by a decrease in rainfall.

Arid island environments harbour a unique biota characterised to have adaptive features that enable them to thrive in such harsh habitats. However, our understanding of how anthropogenic climate change compromises the biodiversity and sustainability of these ecosystems is greatly unknown. Here we used fine-grained field data to evaluate the effects of extreme weather on the population size, distribution, and habitat preferences of an endemic bird species inhabiting an arid Atlantic island, across two temporal windows spanning approximately 20 years (2005-2024). Population size declined sharply (63 %-70 %) between periods, according to a distance-based sampling design and a habitat suitability modelling approach, with the number of individuals estimated in 2024 being 4650 (CI 95 %: 3600-5950) and 4150 (CI 95 %: 3600-4800) respectively. The density of this species in 2024 was reduced by approximately three times compared to the previous study period. The results revealed that in 2024 a larger island area (246 km and 514 km) was necessary to encompass 50 % and 75 % of all individuals of this species, respectively, compared to the previous period (195 km and 434 km). Such a population collapse was associated with the decrease in rainfall on the island, which is closely related to the reproductive success in this species. We recorded a pattern of continuous decrease in rainfall since 2005-2006, which included several extremely dry years immediately before the 2024 study. We also found strong evidence for the population decline of other native bird species of the same foraging guild. Overall, our data emphasizes the significance of recurrent and extreme climatic events on arid island bird species in driving population declines and reducing their distribution ranges; and highlights how fragile such unique taxa can be under longer dry periods, with uncertain consequences for their future viability.