Local bird traits match fruit traits of two alien plants in urban fruit-frugivore interactions

Bird-mediated seed dispersal enhances invasions of many alien plant species. Typically, this mutualism is composed of a generalist assemblage of frugivorous birds in which morphological trait matching between bird and plant species is often thought to be poor due to low reciprocal adaptation. Despite this longheld notion, trait matching between local birds and alien plants has not been investigated. Here, we assessed trait matching by comparing functional traits in multidimensional space for local birds (bill, wing and tarsus size) and two alien invasive plants (fruit and seed size, fruit and seed shape) in an urban area. To do this, we measured bird functional traits in museum specimens and plant functional traits in individuals located in the study area. We found significant trait matching between birds and alien invasive plants (i.e., frugivorous birds interacted with morphologically corresponding invasive plant species). In addition, we detected a positive correlation between morphological and functional specialization of bird species so that extreme morphologies played specialized functional roles. Contrary to the idea that alien plant mutualisms are mediated by generalized seed dispersers without a close match between bird and alien plant traits, our results indicate that species with different morphologies forage on distinct plant species. These findings highlight the importance of studying the functional role of local birds in mutualistic interactions with alien invasive plants, and how these functional roles may promote invasion processes.