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Hummingbirds are one of nature's most energetic fliers and the only birds to hover in the air by relying on their strength alone.

Now scientists have found that it is the ratio of the bird's wing length to its width that makes them so efficient.

The discovery is helping experts compete with 42 million years of natural selection to build helicopters that are increasingly efficient, which could match the performance of the best hummingbird.

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Hummingbirds are arguably nature's most energetic fliers and the only birds to hover in the air by relying on their own strength. And now scientists have found that it's the ratio of the bird's wing length to its width that makes them so efficient

David Lentink, an assistant professor of mechanical engineering at Stanford University in California, tested wings from 12 different species of hummingbirds, which he sourced from museums.

He positioned them on a machine used to test the aerodynamics of helicopter blades - so they spun around like man-made blades.

Together with his team, he used cameras to capture airflow around the wings and measured the drag and the lift force they exerted at different speeds and angles.

Professor Lentink's team used the same machine to test the rotor blades from a ProxDynamics Black Hornet autonomous micro helicopter, which is one of the most efficient on the market and is used by the UK's army in Afghanistan.

They found that the micro-helicopter's blades are as efficient at hovering as the average hummingbird.

But while the micro-copter's blades kept pace with the middle-of-the-pack hummingbird wings, they could not keep up with the most efficient hummingbird's wing.

The wings of Anna's hummingbird - a species common throughout the West Coast of the U.S. – were found to be about 27 per cent more efficient than the man-made micro-copter blades.

While Professor Lentink wasn't surprised about nature's superiority, he said that helicopter blades have come a long way.

'The technology is at the level of an average Joe hummingbird,' he said.

'A helicopter is really the most efficient hovering device that we can build. The best hummingbirds are still better, but I think it's amazing that we're getting closer. It's not easy to match their performance, but if we build better wings with better shapes, we might approximate hummingbirds.'

A new study led by David Lentink, an assistant professor of mechanical engineering at Stanford University in California, has found that the spinning blades of micro-helicopters are about as efficient at hovering as the average hummingbird - but not as good as those of an Anna's Hummingbird (pictured)

Based on the measurements of Anna's hummingbirds, Professor Lentink said there is potential to improve micro-copter rotor power by just over a quarter.

Although the current study, published in the Journal of the Royal Society: Interface, revealed several details of how a hummingbird hovers in one place, the birds still hold many secrets.

For instance, Professor Lentink said that we don't know how hummingbirds maintain their flight in a strong gust, how they navigate through branches or how they change direction so quickly during aerial 'dogfights'.

He thinks that great strides could be made by studying wing aspect ratios - the ratio of wing length to wing width.

Scientists examined the blades from a ProxDynamics Black Hornet autonomous microhelicopter, which is one of the most efficient on the market and is used by the UK's army in Afghanistan and found that it was outperformed by the most efficient of hummingbirds

The aspect ratios of all the hummingbirds' wings remarkably converged around 3.9, but the aspect ratios of most wings used in aviation measure much higher and the Black Hornet's aspect ratio is 4.7.

'I want to understand if aspect ratio is special, and whether the amount of variation has an effect on performance,' he said.

Understanding and replicating these abilities and characteristics could be a boon for robotics and will be the focus of future experiments.

'Those are the things we don't know right now, and they could be incredibly useful,' he said, adding: 'I think it's nice that there are still a few things about hummingbirds that we don't know.'