UV pigments of red bird-pollinated flowers attract birds and deter bees

Traits that shape ecological interactions and confer assortative mating can lead to reproductive barriers and, in some cases, drive speciation.^1,2 Understanding their role in nature is key to explaining the origins and maintenance of biodiversity. Red flowers in hummingbird-pollinated plants shape ecological performance through pollinator interactions and influence mating by conferring reproductive isolation via pollinator shifts.^3,4 The evolution of red coloration in hummingbird-pollinated flowers has been attributed to mechanisms that deter bees or enhance bird attraction.^5,6 However, the ecological and evolutionary significance of UV floral pigmentation-visible to both hummingbirds and bees-remains largely unexplored. We compared red-flowered species from western North America (California) and southern Spain that are primarily pollinated by hummingbirds and bees. We found that most red bird-pollinated species in California exhibited chromatic contrast values below bee discrimination thresholds, consistent with the bee-avoidance hypothesis. Conversely, in the hummingbird visual system, these species showed 1.5 times higher chromatic contrast than red and non-red bee-pollinated species from Spain and California, respectively, supporting the bird-preference hypothesis as well, which would promote pollinator-mediated isolation. Bird-pollinated red flowers contained 5 times more UV-absorbing pigments than red bee-pollinated species from California and Spain, whereas the anthocyanin levels were similar. By simultaneously enhancing pollinator attraction, deterring less efficient visitors, and providing ecophysiological benefits, UV-absorbing pigments may be key targets of selection by these pollinators and non-pollinator agents alike and represent a potential "magic trait" in the evolution of red flowers in bird-pollinated species.