A comparative analysis of fruit feeding among Mediterranean passerine birds

1Fleshy fruits underpin a major mutualistic pathway linking plants and birds in Mediterranean scrublands, yet we still lack a mechanistic understanding of how ecomorphological and digestive traits constrain fruit use, foraging behaviour, and ultimately the effectiveness of avian seed dispersal. Here we assemble an integrative dataset for 146 Iberian bird species combining external morphology, digestive anatomy, diet composition, and fine-grained observations of fruit foraging and handling obtained from standardized focal watches and camera traps at fruiting plants. We classify species into five functional feeding groups (seed dispersers, pulp consumers, pulp consumer-dispersers, pulp consumer-seed predators, non-frugivores) and ask how suites of traits map onto these feeding modes and onto quantitative metrics of frugivory and feeding rate. Across species, the proportion of diet volume made up by fleshy fruits increases with gape width and faster food transit, and decreases with larger gizzards and longer intestines, indicating a tight coupling between frugivory and traits that enable rapid processing of dilute, pulp-rich food. A small subset of traits (body mass, gape width, gizzard mass, transit time) explains over half of the interspecific variation in fruit consumption, with ecomorphological and digestive characters contributing roughly equally to explained variance. Per-visit feeding rates and numbers of fruits ingested per visit scale positively with body mass, and canonical discriminant analysis reveals distinct multivariate trait syndromes separating seed dispersers from pulp consumers, seed predators, and non-frugivores. These trait syndromes, and the associated differences in handling mode and feeding speed, provide a mechanistic link between individual-level foraging decisions and the sparsity, asymmetry, and effectiveness of plant-frugivore interaction networks in Mediterranean systems. Our results highlight how trait-based constraints shape not only who interacts with whom, but also how efficiently seeds are removed and dispersed across a diverse frugivore assemblage.