Effects of nectar robbing on pollen deposition and hummingbird-pollinator behavior in Scarlet Gilia, Ipomopsis aggregata
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Nectar robbing can negatively affect plant reproductive fitness. To better understand this phenomenon, I studied its effects on pollen deposition and hummingbird behavior in the hummingbird-pollinated Scarlet Gilia (Ipomopsis aggregata). To determine the effects of robbing on pollen deposition, I conducted a field experiment where I observed single hummingbird visits to robbed and unrobbed flowers. The amount of pollen deposited on unrobbed flowers was twice that of robbed flowers, suggesting the quality of visits to robbed flowers was lower. Previous studies had found that hummingbirds avoided robbed plants without nectar, although the proximate cues remain unknown. To determine the effects of nectar robbing on foraging behavior, I first investigated liquid within flowers as an honest signal of a reward. Data from 2009 suggested that nectar provided an honest signal, while data from all visits in 2008 and pooled naïve visits from both years suggested the presence of other cues such as taste or spatial memory. Next, I explored whether hummingbirds hovering over flowers caused the flowers to vibrate differently depending on the presence of nectar. Using a fan to simulate hummingbird wing beats, I found that flowers with any amount of added nectar had lower vibration amplitude compared to control flowers, and flowers with no or little nectar had lower vibration frequency compared to flowers with larger nectar quantities. Taken together, these results suggest that robbing may affect pollinator visit quality, and that hummingbirds may use multiple cues associated with nectar absence to avoid robbed plants.