Tail flaring during agonism in Hummingbirds

Bird tails in flight generate lift, reduce pressure drag, and stabilize pitch. Hummingbirds can sustain hovering in still air and generate lift in both the up- and downstroke due to their proportionally enormous primary flight muscles and derived wing skeleton. Given their mass-specific wing power, it is possible that tails are less essential to the aerodynamics of hummingbird flight than they are in other birds, freeing them for non-locomotor functions. Hummingbird tails are well known for their morphological elaboration as sexually selected ornaments, including sound generation. From informal observations of agonism, we hypothesized tail flaring may also be a visual signal used by males during fighting. To begin testing this, we measured the frequency and magnitude of tail flaring during fighting. We measured kinematics of tail flaring during male-male fighting of Calliope hummingbirds (Selasphorus calliope, n=10) indoors. Consistent with our hypothesis, captive males exhibited greater angles of tail flare when engaged in a fight (34.6±45.5o, mean±sd) than when performing solitary landing (-14.2±9.12 o) and takeoff (-14.1±7.0 o) maneuvers. When birds won a fight, the angles of their tail spread were greater than when they lost (p=0.01). Moreover, recordings of seven species of hummingbird in the field revealed 95% of inter- and intra- specific contests included tail flaring. We evaluate these results in the context of signaling during animal contests and propose future tests of whether tail flaring is an honest signal of individual quality and Resource Holding Potential.