Endozoochory largely outweighs epizoochory in migrating passerines

Fruits and seeds are critical food sources for many European passerines during the autumn migration, which in turn contribute to disperse seeds either internally, i.e. after ingestion (endozoochory), or externally, when seeds adhere to the body surface (epizoochory). Despite the recognized importance of birds as seed dispersers, the vast majority of studies focused on endozoochory while the external transport of seeds is frequently invoked as being potentially important, but remains largely unexplored. This is particularly important during the post-breeding migration of passerines, the most ubiquitous and diverse movement of potential seed carriers across Europe and into Africa, which coincides with the fruiting peak of many plant species (August-October). Our aim was to evaluate the role of migrating birds as potential long-distance seed dispersers, and comparing the prevalence of epizoochory and endozoochory during post-breeding migration. We sampled 926 wild birds in nine locations in Portugal, and retrieved 1833 seeds of 19 plant species dispersed internally and only three seeds externally attached to three birds (Serinus serinus, Locustella naevia and Turdus merula), showing an endozoochory prevalence 85 times higher than that of epizoochory. Migrating and non-migrating passerines dispersed seeds equally. While two of the seeds transported externally had specific adaptations to epizoochory, namely spines (Torilis arvensis) and hooks (Galium aparine), the third is a large seed from a fleshy-fruited plant, Frangula alnus (i.e. typical endozoochorous syndrome). These seeds were found on bird species with different diets, but similar behaviour (ground foragers) and in similar habitats (open agro-ecosystems). Our results highlight the strong role of migrating passerines as potentially long-distance seed dispersers and show that, at least in the autumn, the prevalence of epizoochory is several orders of magnitude lower than that of endozoochory.