Context-dependent plant-bird interactions shape polychory across the antagonism-mutualism continuum

Animals feeding on plants may benefit them through seed dispersal or harm them through seed predation, causing net interaction outcomes along an antagonism-mutualism continuum that remains poorly understood. We systematically monitored 6,012 foraging visits of 25 bird species interacting with 40 plant species over a full annual cycle. Interaction outcomes (seed predation, fruit defleshing, and seed dispersal) and dispersal mechanisms (endozoochory, stomatochory, and epizoochory) were shaped by fruit traits, bird feeding strategies, and fruit-to-bird size ratios. However, nearly all bird species (and 10% of individual foraging visits) combined multiple interaction outcomes, seed predators often acting as effective non-endozoochorous dispersers. This flexibility generated polychory in 50% of plant species, while 47.5% were dispersed by birds solely through stomatochory or epizoochory, mechanisms overlooked by classical dispersal syndromes. Our findings portray birds as dynamic, context-dependent agents of seed fate, highlight the need to integrate trait-based and behavioural perspectives in seed dispersal ecology.