Abstract

Islands serve as natural laboratories for exploring evolutionary processes, often fostering unique species through their isolation and distinct ecological conditions. These environments present opportunities to study how a range of selective pressures shape biodiversity. Bird plumage colouration is one trait that has been shown to consistently change in island populations, with island species typically exhibiting reduced plumage colouration relative to mainland relatives. Because islands generally harbour fewer predators, it remains unclear whether predation accounts for this pattern. While animals often face a trade-off between signalling to conspecifics and avoiding detection by predators, experimental evidence linking predation to variation in conspicuousness across ecological contexts remains limited. In this study, we asked how predation pressure differs between insular and mainland habitats, and whether predation risk covaries with conspicuousness of male and female birds across environments. In a field experiment, we investigated predation rates using 3D-printed models painted to represent both sexes of 12 bird species from three archipelagos (Madeira, Azores, and Canary Islands) and their closest mainland relatives. These models were deployed in the species’ natural environments to measure hit rates (a proxy for predation risk), accounting for factors that influence prey detectability, such as colour of the models, background contrast, and vegetation. We found that models experienced less hits on islands than on the mainland. On mainland, sexual dichromatic models were more likely to be dislodged and higher chromatic contrast increased the probability of dislodgment. These results suggest that predation constrains conspicuousness on mainland but does not account for the reduced plumage colouration observed in island birds, highlighting that other ecological and evolutionary factors likely drive this pattern. Our research offers experimental insights into how predation interacts with conspicuous traits in shaping plumage colouration in birds.