Trait matching affects the probability of nectar robbing in plant–pollinator networks

Mutualistic interactions support many ecological functions including pollination. Interactions are, however, vulnerable to cheaters, species that benefit from interactions without providing anything in return. Nectar robbing, where the nectar is depleted but the flower not pollinated, is a well‐known example of cheating and often observed in pollination networks. Further, pollinating birds often switch between legitimate (i.e. mutualistic) and nectar robbing flower visits. In this study, we quantify how widespread nectar robbing is at high elevations in the northern Andes using interactions recorded with time‐lapse camera traps. Additionally, we assess the importance of two trait‐based mechanisms in explaining legitimate versus nectar robbing flower visits by birds: trait complementarity, measured as the continuous difference between bird bill and flower tube lengths, and trait barrier, which is a binary assessment of whether a species pair is physically able to interact based on the length difference between the flower tube and the bird bill. Nectar robbing occurred in 7% of the interactions we sampled, and the specialised flowerpiercers ( Diglossa ; Thraupidae) relied on this technique at higher frequencies than hummingbirds (Trochilidae). We further observed that the use of nectar robbing was strongly driven by trait barrier: nectar robbing happened mostly when the bill of the bird was shorter than the flower tube. This suggests that legitimate flower visits is the favoured foraging strategy for nectarivorous birds, and that robbing is used mostly to feed on otherwise inaccessible resources. These results suggest that nectar robbing is an important, yet overlooked, characteristic of tropical bird pollination networks.