High importance of migratory birds for plant-frugivore interactions on reservoir islands

Seed dispersal by frugivorous birds is vital for forest regeneration and biodiversity. Migratory birds, with their long-distance dispersal capabilities, may play a key role in this process, yet their contribution to plant-frugivore interactions remains underexplored. Over three years, we recorded 11,635 interactions between fruiting plants and frugivorous birds using camera traps on 13 dam-induced forested islands. These interactions involved 48 bird species, including 15 migratory species. Migratory birds interacted with two-thirds of all plant species, and their species richness increased with island area. However, migratory frugivores constituted a higher proportion of the frugivorous bird community on smaller islands. Interactions involving migrants peaked consistently from October to January. These findings emphasize the importance of migratory frugivores in seed dispersal, particularly in fragmented habitats. Conservation efforts should focus on preserving migratory birds to sustain their critical ecological roles in maintaining plant-frugivore interactions and facilitating seed dispersal in fragmented landscapes.

Methods
We collected plant-frugivore interactions using arboreal camera traps. Specifically, our monitoring began each year in early July, involving exhaustive searches for fruiting plants within the transects at least twice a month, continuing until the end of January of the following year. Upon identifying plants with ripening fruits, we installed infrared cameras (LTL Acorn 6210 MC) at suitable locations (e.g., nearby trees or the fruiting plant itself, avoiding direct sunlight on the lens). The camera lenses were oriented towards fruit-abundant areas, with installation heights ranging from 0.5 m to 8.0 m depending on the location of the concentrated fruits. To reduce oversampling, a minimum interval of 20 meters was maintained between cameras when monitoring the same plant species. When the fruits on a monitored plant were either fully consumed or had nearly all fallen off, we retrieved the cameras and transferred the SD card data to a computer. Experienced personnel manually reviewed all photos and videos in the lab, as detailed in recent literature (https://doi.org/10.1002/rse2.232).