Predation and the Evolution of Island Bird Plumage Colouration: Experimental Insights from Island and Mainland Environments

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 shown to consistently change in island populations, and predation has been hypothesized to influence these differences. While animals often face a trade-off between signalling to conspecifics and avoiding detection by predators, the role of predation in shaping conspicuousness remains underexplored experimentally. 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 on the islands experienced less hits compared to those on the mainland, while sexual dichromatic models were more likely to be dislodged on the mainland. In addition, for mainland sites, increased chromatic contrast correlated with a higher probability of dislodgment, suggesting that more conspicuous models were more likely to be hit. These results highlight that while predation constrains conspicuousness, other ecological and evolutionary factors likely drive the reduced plumage colouration observed in island birds. Our research offers experimental insights into how predation interacts with conspicuous traits in shaping plumage colouration in birds.