Black Phoebes (Sayornis nigricans) persistently pump their tails vertically while perched but the functional causes are unknown. I address four hypotheses about the function of this behavior in this species: (1) tail pumping aids in balance, (2) tail pumping enhances foraging, (3) tail pumping is a signal to territorial intruders, and (4) tail pumping is a signal to potential predators. The balance (mean ± SE; unstable substrates: 0.23 ± 0.024 pumps/sec, stable substrates: 0.22 ± 0.019 pumps/sec), foraging (non-foraging individuals: 0.28 ± 0.036 pumps/sec, foraging individuals: 0.20 ± 0.026 pumps/sec) and intruder (pre-playback trial: 0.20 ± 0.025 pumps/sec, House Finch [Carpodacus mexicanus] control trial: 0.26 ± 0.029 pumps/sec, Black Phoebe experimental trial: 0.17 ± 0.036 pumps/sec) hypotheses did not significantly explain tail pumping behavior. Tail pumping rates increased during predator sound playback (pre-playback trial: 0.23 ± 0.009 pumps/sec, House Finch trial: 0.26 ± 0.016 pumps/sec, Cooper's Hawk [Accipiter cooperii] trial: 0.61 ± 0.013 pumps/sec, post-playback trial: 0.35 ± 0.013 pumps/sec) and were accompanied by a high amount of both approaches (3.8 ± 0.8) and calls (6.7 ± 1.63). These results indicate that S. nigricans may be using tail pumping behavior as a pursuit-deterrent signal to advertise awareness to potential predators.
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