Plant traits determine seed retention times in frugivorous birds: Implications for long‐distance seed dispersal

Abstract Seed dispersal by frugivorous animals is a key process for plant populations and communities. When frugivores consume fruits, seeds are temporarily retained inside their guts until seed deposition. Hence, information on retention times is essential to estimate seed‐dispersal distances. Although it is well known that retention times are affected by the body size of frugivores, there is an important knowledge gap on the effects of plant species traits, such as seed size and pulp content. Here, we comprehensively address whether and which plant traits affect seed retention times in frugivorous birds. On the one hand, we conducted a first set of experiments to assess variation in retention times among 31 plant species dispersed by a single bird species, and a second set to assess variation among five plant species dispersed by a guild of five frugivorous passerines. On the other hand, we conducted a literature review retrieving retention times for 231 interactions involving 155 plant species and 55 bird species from nine different avian orders. Seed size had negative effects on retention times at the three levels addressed in this study: the larger the seeds, the shorter the times. The effects of seed size were higher within a single bird species, intermediate for the small assemblage of five passerine species, and lower for all frugivorous birds included in the compilation. Notably, the effects of seed size on retention times were comparable to that of frugivore body size in Passeriformes. We analysed the effects of the pulp content on a single bird species, but these lacked predictive power. Importantly, the effects of seed size were indirect and mediated by the type of seed ejection (digestive processing): birds generally defecate smaller seeds and regurgitate the larger ones. We provide comprehensive evidence that retention times are intrinsically linked to plant traits and to frugivores' response to such traits, particularly to seed size and the type of seed ejection, respectively. Thus, our study unveils a source of interspecific variability in the capacity of plants to disperse at long distances. Moreover, we provide methodological improvements to trait‐based models that estimate mean retention times and seed‐dispersal distances. Read the free Plain Language Summary for this article on the Journal blog.