Flight costs of long, sexually selected tails in hummingbirds

Abstract

The elongated tails adorning many male birds have traditionally been thought to degrade flight performance by increasing body drag. However, aerodynamic interactions between the body and tail can be substantial in some contexts, and a short tail may actually reduce rather than increase overall drag. To test how tail length affects flight performance, we manipulated the tails of Anna's hummingbirds (Calypte anna) by increasing their length with the greatly elongated tail streamers of the red-billed streamertail (Trochilus polytmus) and reducing their length by removing first the rectrices and then the entire tail (i.e. all rectrices and tail covert feathers). Flight performance was measured in a wind tunnel by measuring (i) the maximum forward speed at which the birds could fly and (ii) the metabolic cost of flight while flying at airspeeds from 0 to 14 m s K1. We found a significant interaction effect between tail treatment and airspeed: an elongated tail increased the metabolic cost of flight by up to 11 per cent, and this effect was strongest at higher flight speeds. Maximum flight speed was concomitantly reduced by 3.4 per cent. Also, removing the entire tail decreased maximum flight speed by 2 per cent, suggesting beneficial aerodynamic effects for tails of normal length. The effects of elongation are thus subtle and airspeedspecific, suggesting that diversity in avian tail morphology is associated with only modest flight costs.

Effects of flight speed upon muscle activity in hummingbirds

“…Of course, such conclusions are speculative and require further study. However, recent metabolic measurements of Anna's hummingbirds (Calypte anna, controls with intact tails) exhibit the same V mp at 6ms -1 (Clark and Dudley, 2009). As amplitude of EMG is positively correlated with wingbeat amplitude (Fig.5B), it is also tempting to conclude that the pattern supports the 'mechanical-oscillator' hypothesis -a model for hummingbird wing motion (Greenewalt, 1960) in which it is predicted that muscle force and strain are correlated.…”

“…To accomplish fast forward flight, Hagiwara et al (Hagiwara et al, 1968) report that hummingbirds increase PECT recruitment: the amplitude of EMG increases and the number of spikes per burst increases to 2-5. However, flight velocity was not measured, so it is not possible to interpret this observation in relation to existing models of flight costs (Rayner, 1979) or to empirical measures of metabolic power input (Berger, 1985;Clark and Dudley, 2009).…”

The elaborate plumage in peacocks is not such a drag

“…There are several other examples of sexually selected traits having rather subtle effects on bird locomotor performance. In Anna's hummingbirds (Calypte anna) and scarlettufted malachite sunbirds (Nectarinia johnstoni), artificial manipulations of the tail by the addition of feathers from a different bird species or the removal of the tail feathers demonstrate that elaborate tail ornaments can have relatively small effects on flight performance by reducing maximum speed (by 3%) and increasing metabolic cost (by 11%, but only at the highest speeds, which represent 1-7% of the birds' flight behaviour) (Clark and Dudley, 2009), and by reducing hawking efficiency (Evans and Thomas, 1992). The vertical display flights performed by collared doves (Streptopelia decaocto) have been estimated to incur a relatively low metabolic cost (~5% of basal metabolic rate) (Usherwood, 2008).…”

Elaborate horns in a giant rhinoceros beetle incur negligible aerodynamic costs

“…Previous authors have similarly argued that the diversity of tail ornaments among birds may reflect the fact that tails are hidden by the wake of the body and thereby have minimal aerodynamic effects [42]. We suggest that, as in bird tail ornamentation, the large horns of rhinoceros beetles may effectively 'hide' from selective pressures because of the enormous wake of the body owing to the beetles' slow flight speeds and high body angles.…”

“…Relative horn mass ranged between 0.5 and 2.5 per cent (1.5 + 0.6%, mean + s.d. 42, p , 0.001); the centre of mass was more anterior in large males than small males. However, the horns themselves had a trivial effect on the centre of mass.…”