Hummingbird avoidance of nectar-robbed plants: spatial location or visual cues

Broad-tailed and rufous hummingbirds avoid plants and flowers that have recently been visited by nectar-robbing bees. However. the cues the hummingbirds use to make such choices are not known. To determine the proximate cues hummingbirds use to avoid visiting nectar-robbed plants, I conducted multiple field experiments and one aviary study using the nectar-robbed, hummingbird-pollinated plant Ipomopsis aggregata. In the first held experiment, free-flying hummingbirds were presented with plants in which I manipulated nectar volume and the presence of nectar-robber holes. Hummingbirds visited significantly more plants with nectar and probed more available flowers on those plants, regardless of the presence of nectar-robber holes. Thus, I hypothesized that hummingbirds may avoid robbed plants based on their spatial memory of unrewarding plants and/or visual cues that nectar absence provides. In an aviary study, I removed spatial cues by re-randomizing the position of plants after each hummingbird-foraging bout, but hummingbirds still selected plants with nectar. Nectar map provide a visual cue in I. aggregata flowers because corollas are translucent, and nectar is visible through the side of the corolla. To determine if hummingbirds use this visual cue to avoid plants with no nectar, I masked corolla translucence in a field study by painting flowers with acrylic paint. Hummingbirds still visited significantly more plants with nectar and probed more flowers on those plants, whether ol not the corollas were painted. These results suggest that hummingbirds use nectar as a proximate cue to locate and avoid non-rewarding, nectar-robbed plants, even in the absence of spatial cues and simple visual cues.