Red fruits exhibit lower colour diversity than red flowers as perceived by birds

1. Pollination and seed dispersal are crucial processes for plant reproduction, sharing ecological relevance and similarities, yet they have rarely been considered together. Flowers appear to express greater phenotypic diversity than fruits due to multiple confounding factors, which pose challenges for comparative analyses. The colours of flowers and fruits are important visual signal traits in pollination and seed dispersal, evolving under different selective pressures from their respective pollinators and seed dispersers. Birds constitute a unique plant-interacting group that participates in both pollination and seed dispersal events. In this study, we focus on red flowers and red fruits associated with avian mutualists to gain insight into the intrinsic differences between flowers/pollination and fruits/seed dispersal. 2. We conducted comparisons of colouration between 94 red flowers pollinated by birds and 99 red fruits dispersed by birds. The colour diversity was compared in both the spectral space and the avian colour vision spaces. Colour conspicuousness was analysed using avian colour vision models, as well as bee models. Pigeon colour preference was tested by controlled experiments utilizing red stimuli with and without secondary peaks at short wavelengths. 3. Red fruits had lower colour diversity than red flowers, with redder hues and fewer secondary reflectance peaks. Avian colour vision models illustrated that fruits were more conspicuous than flowers achromatically, but not chromatically. Pigeons did not show preference for red with or without a secondary peak. 4. Although both are red, there are significant differences between flowers and fruits in terms of colour diversity, spectral properties and colour perceptions. As we exclusively considered avian mutualists, these differences cannot be attributed to the differences in interacting animal groups or to their colour vision properties. This implies that the differences in evolutionary history between flowers and fruits may deserve further attention to understand the colour evolution.