CLARK, Christopher J*; DUDLEY, Robert; UC Berkeley: Effects of tail length on hummingbird top speed, power curve, and parasite drag

Hummingbirds (Trochlidae) exhibit enormous variation in their tail morphology and this variation is largely driven by sexual selection. One aerodynamic effect of an elongated tail may be to increase parasite (body) drag, by increasing the surface area of the bird that is in contact with air. To test this hypothesis, we measured top speed, metabolic rate, and parasite drag on Anna�s hummingbirds (Calpyte anna) with their natural tail and with a tail elongated by 15 cm. With the elongated tail, top speed decreases by approximately 2.5%, while metabolic rate increases by up to 8% at high speeds. Parasite drag, as measured with a live bird perched on a force transducer, also increases. These independent measurements of body drag all indicate that elongating the tail by 15 cm increases overall drag by about 7% at high speeds. This measurement is almost twice as high as predicted by surface area alone; we hypothesize that the difference between our measures and theory arises from fluttering of the tail feathers caused by their low flexural stiffness.