27 September 2012 Last updated at 07:48

Hummingbirds' backward flight is efficient

By Jeremy Coles Reporter, BBC Nature

The mechanisms behind the effortless way that hummingbirds fly backwards have been revealed in a recent study.

Although hummingbirds routinely fly backwards, it has never before been scientifically described in detail.

University of California scientists Dr Nir Sapir and Robert Dudley, recorded the birds' flight biomechanics using high-speed cameras and oxygen uptake.

They found that hummingbirds' backward flight uses similar amounts of energy to flying forwards.

The results are published in the Journal of Experimental Biology.

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Backward flight is frequently used by members of the hummingbird family as they reverse from a nectar-bearing flower after feeding.

Dr Sapir noticed this while observing hummingbirds on a feeder.

"I actually saw it happening in a feeder that was positioned in my balcony. Many hummingbirds were using it and they all were flying backwards. It puzzled me that we know almost nothing about this flight style."

To further understand this type of flight Dr Sapir and Robert Dudley devised an experiment using a sucrose-filled syringe disguised as a flower at the end of a wind tunnel.

Five captive Anna's hummingbirds (Calypte anna) fed individually from the syringe while airflow in the wind tunnel was activated and the direction of the syringe was altered. High-speed cameras captured their movements.

The scientists looked at hummingbirds' oxygen uptake, body posture and wing stroke plane as they flew forwards, backwards or hovered.

According to Dr Sapir, the most important finding of the study was that flying backwards uses a similar amount of energy to flying forwards, both of which were more efficient than hovering.

Hummingbirds in backwards flight had a more upright posture than when flying forwards

This was discovered by using a respiratory mask to measure the rate of oxygen consumption during feeding.

"The findings were very exciting because we expected that backward flight will come with a greater metabolic cost," explained Dr Sapir.

He continued, "During backward flight, the bird's body is held in [a] much more upright posture. We were expecting the body will experience a much higher drag and that the bird will need to invest much more work to overcome this drag."

Further investigation using life-sized models determined that drag during backward flight is only slightly higher than when the bird is flying forward.

"[This is] probably because drag forces are relatively negligible at flight in relatively slow airspeeds, as characterising backward flight," he said.