Avian Selection of the Color-Dimorphic Fruits of Salmonberry, Rubus spectabilis: A Field Experiment

Abstract

Although the mutualism between frugivorous animals and the fruiting plants whose seeds they disperse has been noted for over a century, it remains unclear whether animal selection has had any part in the evolution of fruit characteristics. We conducted field experiments using the color-dimorphic fruits of salmonberry (Rubus spectabilis) to determine whether free-ranging birds choose fruits on the basis of color, and if so, whether these patterns indicated the potential for birds to exert evolutionarily significant selective pressure on this fruit trait. Birds consistently selected red over orange fruits in experimental displays in the field despite wide geographic variation in fruit-color frequencies, fruit-crop densities, and numbers and species composition of avian frugivores in Oregon and Alaska. This is the first field study demonstrating significant and consistent fruit-trait selection by birds at a scale relevant to coevolutionary processes. These results indicate that forces other than animal selective pressure are also shaping the occurrence of fruit color traits in bird-dispersed fruiting plants.