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Hummingbirds shake off downpours

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Rain or shine Hummingbirds can shake off the effects of heavy rain and hover in a heavy downpour thanks to shock-absorbing feathers, a new US study has found.

Going out in heavy rain is particularly challenging for small flying animals because of the relatively larger impact of raindrops and the weight of the water they carry around.

Raindrops can increase drag, affect control and manoeuvrability, and potentially cause structural damage to the wing itself.

Despite this, waiting it out isn't an option for some birds, bats and insects particularly those from humid environments.

Hummingbirds, which are among the smallest flying vertebrates, are found in neotropical cloud forest regions and lowland rainforest where heavy rains are common.

Their small size and high metabolic rate means they need to forage constantly, and they have evolved an insect-like ability to hover as they drink nectar from downwards facing flowers.

Dr Victor Ortega and Robert Dudley from the Animal Flight Lab at University of California Berkeley looked at how different rainfall intensities affect the hovering performance of Anna's hummingbird Calypte anna.

Male hummingbirds were filmed hover-feeding in the lab under a range of simulated rainfall levels. They then studied the film in slow motion to identify any kinematic (postural and movement) changes made by the birds in response to the increasing rainfall.

Their findings appear in the latest edition of the Proceedings of the Royal Society B.

Adjusting motion

"In our experiment, drops of light-to-moderate and heavy precipitation were only 0.002 per cent and 0.04 per cent of the hummingbird's body mass respectively," says Ortega. "However, these tiny drops can produce huge peak forces during the impact: 16 per cent and 38 per cent of body weight for light-to-moderate and heavy precipitation, respectively.

"Despite these enormous forces, we demonstrated that hummingbirds can deal very well in light-to-moderate precipitation, practically without cost," he says.

But when the rain intensity increased, the birds adjusted their motion to compensate for the impact of the raindrops and their wet plumage.

"During heavy rain, a hummingbird orients its body and tail horizontally, increases wing beat frequency and reduces wing beat amplitude," says Ortega.

"Surprisingly, these tiny birds can maintain flight control in heavy rain conditions with increased metabolic costs of only 9 per cent."

"Wing feathers particularly help to reduce drop impact forces because they are flexible, and due to this, part of the kinetic energy of the drop is transferred into bending."

Mosquitoes and crane flies

As yet, it is unknown whether other small animals exhibit a similar strategy for dealing with downpours.

A recent study showed that mosquitoes can survive drop impacts because their low body mass allows them to glance off the much heavier drops.

"However nobody knows how kinematics are affected during drop impacts," says Ortega, who has been recently exploring the takeoff performance of crane flies (Tipulidae) during rainfall.

"Crane flies are ten times bigger than mosquitoes and two orders of magnitude smaller than hummingbirds.

"I am sure that future research will solve these questions."