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 is not
pollinated, is a well-known example of cheating and is 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 the trait barrier:
nectar robbing happened mostly when the bill of the bird was shorter than
the flower tube. This suggests that legitimate flower visits are 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.