Eastern Phoebes (Sayornis phoebe) persistently pump their tails when perched, and the function of this behavior is unclear. We tested four hypotheses concerning the possible functions of tail pumping, including the (1) balance hypothesis, (2) signal‐aggression‐to‐conspecifics hypothesis, (3) foraging‐enhancement hypothesis, and (4) predator‐deterrent hypothesis. Eastern Phoebes were monitored from 1 April to 1 August 2006 and 1 June to 1 August 2007 in Madison County, Kentucky. Phoebes were captured in mist nets and marked with unique combinations of colored‐leg bands. Each focal phoebe was observed one to two times a week and tail‐pumping rates, wind velocity, and perch characteristics were recorded. Our results provided support only for the predator‐deterrent hypothesis. Phoebes exhibited a significant increase in tail‐pumping rates in the presence of a potential predator (Eastern Screech‐Owl, Megascops asio), suggesting that tail pumping, along with increased rates of calling, informs potential predators that a phoebe is aware of their presence and, therefore, more difficult to capture. Phoebes, however, tail pump regardless of whether a predator has been sighted and such movement could make phoebes easier to detect. If so, and if a detected phoebe provides no additional cues (calling) to indicate awareness of a predator, an attack might actually be more likely. However, tail pumping may be beneficial, regardless of whether a predator has been detected, if it serves as a continuous, honest predator‐deterrent signal. This would be the case if predators learned to associate tail pumping and phoebes, and also learned that phoebes, with their alert posture, small size, and impressive aerial maneuvering abilities, represent low reward, difficult‐to‐capture prey. If so, then both phoebes and predators would benefit, with phoebes less likely to be attacked and predators not initiating attacks that would waste time and energy and alert other potential prey to their presence.