Colourful footage, shot with a thermal camera, has revealed how hummingbirds avoid overheating as they beat their wings up to 70 times per second.

The birds have "windows" for heat loss, around their eyes, shoulder joints, feet and legs.

This study, led by George Fox University in Oregon, US, is part of a Nasa-funded project to uncover the effects of climate change on the birds.

The results are published in the journal Royal Society Open Science.

Dissipating heat is complex in birds because feathers are such effective insulators.

Lead researcher Dr Donald Powers said many nest birds formed bare "brood patches" when sitting on eggs, "so that the eggs are not insulated from their warm skin by the feathers".

"We wanted to understand how hummingbirds, and birds in general, get rid of the extra heat."

As bright white patches on the footage reveal, hummingbirds - and, researchers say, probably birds in general - have special areas around their eyes, shoulders, and legs that have low feather density.

These serve as windows to rapidly dissipate heat during flight.

"As flight power requirement increases- it is highest when hummingbirds hover - the amount of heat generated increases," said Dr Powers.

"But these 'windows' are sufficient at moderate temperatures to dissipate all excess heat across the full range of flight speeds in hummingbirds."

The researchers are currently investigating whether these nimble, nectar-feeding birds will be able to avoid overheating as temperatures increase.