Effects of tail length on an escape maneuver of the Red-billed Streamertail

Abstract

Sexually selected, elongated tails in birds may impair or alter naturally selected functions related to flight. The aim of this study was to test whether experimental manipulation of tail length affected the kinematics of a low-speed escape maneuver. The Red-billed Streamertail (Trochilus polytmus) has the longest tail of any hummingbird and is highly sexually dimorphic in tail length. I hypothesized that streamertails with either a long tail or with the tail removed would perform maneuvers with reduced linear and angular accelerations, relative to maneuvers performed with a short tail. Two high-speed video cameras recorded maneuvers from male and female Red-billed Streamertails under three tail length treatments: short tail, long tail, and no tail. The detailed kinematics of the maneuvers were highly variable. Whereas the birds always performed rolls (rotation about the bird’s X-axis) during the maneuver, there was variation in the relative importance of pitching motions (rotation about the Y-axis) or yawing motions (rotation about the Z-axis), with yawing rotations playing a small role in ‘pitch-roll’ turns, and pitching motions playing a small role in ‘yaw/roll’ turns. Birds missing their entire tail exhibited reduced maximum linear accelerations associated with a curving trajectory. By contrast, birds maneuvering with an elongated tail did not exhibit significantly different kinematics from the controls, suggesting that the greatly elongated tail streamers have relatively small effects on the bird’s ability to maneuver at low speeds. Based on these observations, greatly elongated tails may not pose a large cost to low-speed maneuvering flight.