Species interactions are influenced by the trait structure of local multi-trophic communities. However, it remains unclear whether mutualistic interactions, in particular, can drive trait patterns at the global scale, where climatic constraints and biogeographic processes gain importance. Here we evaluate global relationships between traits of frugivorous birds and palms (Arecaceae), and how these relationships are affected, directly or indirectly, by assemblage richness, climate and biogeographic history. We leverage a new and expanded gape size dataset for nearly all avian frugivores, and find a positive relationship between gape size and fruit size, that is, trait matching, which is influenced indirectly by palm richness and climate. We also uncover a latitudinal gradient in trait matching strength, which increases towards the tropics and varies among zoogeographic realms. Taken together, our results suggest trophic interactions have consistent influences on trait structure, but that abiotic, biogeographic and richness effects also play important, though sometimes indirect, roles in shaping the functional biogeography of mutualisms.

Modified from: McFadden et al. Global plant-frugivore trait matching is shaped by climate and biogeographic history. Ecology Letters (in press).

To assemble this dataset of trait measurements for avian frugivores, we compiled a dataset of beak measurements taken from wild-caught and released individuals, as well as specimens accessed in numerous museums and research collections worldwide (Tobias et al. 2021). We defined frugivores as those species with a diet containing 50% or more fruit, and only kept species matching this criterion. We attempted to measure at least two males and two females of each avian frugivore species, with the final dataset containing measurements from 12925 individuals and a total of 1129 species. For beak volume and diet traits we used data from Pigot et al. (2020) and Tobias et al. (2021). To quantify beak volume (n = 1129 spp.), we multiplied beak length, measured from the tip of the beak along the culmen to the base of the skull, with beak width and beak depth, both measured at the anterior edge of the nostrils (units = mm3).

To quantify avian frugivore gape size (previously termed ‘gape width’ or ‘beak width’ in some studies), we used data from Pigot et al. (2016), Bender et al. (2018) and Hanz et al. (2019), and also collected previously unpublished measurements on thousands of additional individuals. We defined gape size as the horizontal width of the beak measured between the points at which the upper and lower mandibles meet. Unlike standard beak measurements, such as those in Tobias et al. (2021), this global dataset of gape size is unique to McFadden et al. (in revison). We include gape size data in two ways. First, as species averages used in our models, and second, by providing the underlying data from all specimens and wild-caught individuals along with metadata on source, collection locality and measurer. 

References:

Bender, I.M.A. et al. 2018. Morphological trait matching shapes plant–frugivore networks across the Andes. Ecography, 41, 1910–1919.

Hanz, D.M. et al. 2019. Functional and phylogenetic diversity of bird assemblages are filtered by different biotic factors on tropical  mountains. Journal of Biogeography, 46, 291–303.

McFadden et al. Global plant-frugivore trait matching is shaped by climate and biogeographic history. Ecology Letters. In revision.

Pigot, A.L. 2016. Quantifying species contributions to ecosystem processes: A global assessment of functional trait and phylogenetic metrics across avian seed-dispersal networks. Proccedings of the Royal Society B, 283, 20161597.

Pigot, A.L. et al. 2020. Macroevolutionary convergence connects morphological form to ecological function in birds. Nature Ecology and Evolution, 4, 230–239.

Tobias, J.A. et al. 2021. AVONET: Morphological, ecological and geographical data for all birds. Ecology Letters.