Effects of fruit novelty on feeding preference in four globally invasive frugivorous birds

Invasive fruit-eating animals (hereafter frugivores) can form novel mutualisms with fruiting plants and alter seed dispersal. Plant-frugivore interactions are often linked to frugivore preference for certain fruit traits, but for invasive frugivores, it is uncertain if novelty—whether or not a fruit is familiar—impacts foraging decisions. We experimentally tested fruit preferences of the four most abundant and frugivorous bird species on O‘ahu, a Hawaiian Island, all of which are globally invasive songbirds. With captive wild birds, we tested for preference in relation to fruit size, color, nutrients, and bird sex. We also tested how novelty of fruits affected trait-based preferences, with fruits from plant species either established in the wild or novel. Using arrays that offered multiple fruit species, we conducted 252 trials with 111 plant species, 59.5% of which were considered novel. From all fruits tested, three bird species preferred smaller fruits, and color preferences varied among bird species. Novelty of fruits influenced preference in three ways. First, birds preferred fruits from plants that were established in the wild over novel fruits. Second, repeated exposure to novel fruits increased the probability of interactions. Third, preferred novel fruits were smaller and redder than preferred fruits from established plants. Finally, bird sex and fruit nutrient content did not affect preference. Together, these results suggest that preference for fruit traits varies among invasive bird species and can be flexible when fruits are novel to birds, increasing the likelihood of novel mutualisms and making it increasingly difficult to predict invasion impacts.