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Nectar robbing can affect both male and female components of plant reproduction. To understand the mechanisms driving those plant-reproductive outcomes, I studied the effects of nectar robbing on pollen deposition and pollinator behavior in the hummingbird-pollinated host plant Ipomopsis aggregata. To determine the effects of robbing on pollen deposition, I conducted a field experiment in which I observed single hummingbird visits to emasculated robbed and unrobbed flowers. Once a flower was visited, I collected the stigma, stained it, and counted pollen. I hypothesized that less pollen would be found on robbed flowers than unrobbed flowers. Consistent with this hypothesis, the amount of pollen deposited on unrobbed flowers was twice as much as robbed flowers, indicating that the quality of visits to robbed flowers was lower than to unrobbed flowers. To determine the effects of nectar robbing on hummingbird behavior, I first investigated liquid as an honest signal. Previous studies had found that hummingbirds avoided robbed plants and flowers without nectar, and I focused on the honest signal that nectar provides. I predicted that liquid itself provided a cue that hummingbirds used to make decisions about which plants to visit. I observed hummingbird visits to flowers with either nectar, water or nothing and calculated the mean visitation rate. Contrary to my hypothesis, flowers with nectar had the highest visitation rate driven by the number of visits per day and the percentage of flowers probed. Then, I tested the hypothesis that hummingbirds hovering over flowers caused them to vibrate at different amplitudes and frequencies depending on the amount of nectar. I set up an experiment in the lab with a fan blowing over flowers with varying amounts of nectar and recorded them with a video camera. Flowers with no nectar had significantly higher vibration amplitudes than plants with any amount of nectar. Plants with no or low amounts of nectar vibrated at significantly lower frequencies than plants with higher amounts of nectar. Taken together, these results suggest that robbing may affect no only pollinator visit quantity but also quality, and that the cues associated with hummingbird-avoidance of robbed plants may be associated with a flowers’ vibration frequency and amplitude.
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