Predation and the evolution of island bird plumage colouration: experimental insights from island and mainland environments

Abstract

Islands serve as natural laboratories for exploring evolutionary processes, often fostering unique species through isolation and distinct ecological conditions. Bird plumage colouration frequently differs in island populations, with island species typically exhibiting reduced plumage colouration relative to mainland relatives. Predation may influence plumage conspicuousness, and this is particularly interesting on islands, where predator diversity is generally lower, yet no experiments have directly compared predation pressure between island and mainland systems. Consequently, empirical evidence linking predation pressure to variation in conspicuousness across ecological contexts remains limited. We investigated how predation pressure differs between insular and mainland habitats, and whether predation risk covaries with conspicuousness of male and female birds. In a field experiment, we used 3D-printed models representing both sexes of 12 bird species from three archipelagos (Madeira, Azores, and Canary Islands) and their closest mainland relatives. Models were deployed in the species' natural environments to measure hit rates (a proxy for predation risk), accounting for model colours, background contrast, and vegetation. We found that models experienced fewer hits on islands than on the mainland. On mainland, dichromatic models were more likely to be dislodged, and higher chromatic contrast increased this likelihood. In contrast, predation risk on islands showed no relationship with colour conspicuousness. These results suggest that predation constrains conspicuousness on mainland but does not explain the reduced colouration observed in island birds, highlighting that other ecological and evolutionary factors drive this pattern. Our research offers experimental insights into how predation interacts with conspicuousness to shape plumage colouration in birds.