Flying in the rain: hovering performance of Anna's hummingbirds under varied precipitation

Abstract

Flight in rain represents a greater challenge for smaller animals because the relative effects of water loading and drop impact are greater at reduced scales given the increased ratios of surface area to mass. Nevertheless, it is well known that small volant taxa such as hummingbirds can continue foraging even in extreme precipitation. Here, we evaluated the effect of four rain intensities (i.e. zero, light, moderate and heavy) on the hovering performance of Anna's hummingbirds (Calypte anna) under laboratory conditions. Light-to-moderate rain had only a marginal effect on flight kinematics; wingbeat frequency of individuals in moderate rain was reduced by 7 per cent relative to control conditions. By contrast, birds hovering in heavy rain adopted more horizontal body and tail positions, and also increased wingbeat frequency substantially, while reducing stroke amplitude when compared with control conditions. The ratio between peak forces produced by single drops on a wing and on a solid surface suggests that feathers can absorb associated impact forces by up to approximately 50 per cent. Remarkably, hummingbirds hovered well even under heavy precipitation (i.e. 270 mm h 21 ) with no apparent loss of control, although mechanical power output assuming perfect and zero storage of elastic energy was estimated to be about 9 and 57 per cent higher, respectively, compared with normal hovering.

Flapping wing aerodynamics: from insects to vertebrates

“…From a biological perspective, a comprehensive framework will also entail further study of environmental effects on animal flight. For example, studies have shown that hummingbirds change their flight kinematics, such as wingbeat frequency, flapping amplitude and body orientation, to compensate for turbulence (OrtegaJimenez et al, 2014) and precipitation (Ortega-Jimenez and Dudley, 2012). Bats, in contrast, appear to avoid rain because of both sensory and energy limitations (Voigt et al, 2011).…”

Into rude air: hummingbird flight performance in variable aerial environments

“…Similarly, wet hummingbirds hovering in artificial rain for only 2 s experience mass increases of up to 4% and, in parallel with wingbeat kinematic changes, expend substantially more induced power [38]. The rate of wetting, not surprisingly, depends on exposure time and intensity of precipitation.…”

“…Impacts of raindrops can generate significant downward forces on an animal's body. For example, millimetre-sized drops falling on hovering hummingbirds (figure 2b) can increase the effective downwards force by up to 2.5% of body weight, with significant effects on induced power expenditure [38]. Aerodynamic performance may also be impaired; aeroplanes flying in heavy rain experience a 50% increase in profile drag on the wings [43].…”

“…These remarkable feats are unstudied from behavioural and biomechanical perspectives. Aerial control during flight in rain is perhaps the most easily studied experimentally, as falling water drops can readily be generated in the laboratory at different sizes and intensities [38]. Given the increasing miniaturization of quadcopters and other flying vehicles, studies of animal flight in rain, snow and turbulent air will also be of technological relevance.…”

“…By contrast, larger animals will experience much higher levels of momentum transfer during drop impacts (figure 3b), albeit distributed much more evenly across the body and wings. For example, hummingbirds hovering in heavy rain exhibit large-scale compensatory responses in wing and body kinematics, but show no sign of perturbation in roll or loss of flight control [38]. Similarly, large seabirds fly placidly in heavy rain (V.M.O.-J.…”