Flap-running in birds is key to flight evolution
The ungainly sight of a bird furiously flapping its wings as its spindly legs propel it forward could be a peek at evolutionary history.
"Flap-running", researchers say, may have been a key step in the evolution of flight.
Experiments with pigeons have shown that it helps birds ascend slopes and suggests the earliest flightless birds might have used the same technique.
The study was published in the Journal of Experimental Biology.
Brandon Jackson, from the University of Montana, US, who led the study, explained that he and his colleagues wanted to know why birds would flap-run when they were capable of flight.
His co-researcher, Ken Dial, noticed this behaviour when filming a type of partridge known as a chuckar.
As the rotund birds negotiated obstacles, they would run up the objects flapping their wings. When Dr Dial discussed this behaviour with local ranchers and hunters, some reported that adult chukars would flap to run up cliffs, rather than fly.
Dr Jackson and his team decided to find out if the birds might be using the technique to save energy by measuring the amount of power generated by the flight muscles when birds flew and when they were flap-running.
They surgically implanted electrodes into the flight muscles of pigeons - closely related birds that often flap and run even though they are very good fliers.
The electrodes measured muscle activity in the birds as they flapped and ran up ramps of varying inclines, and as they flew parallel to those same ramps.
The team was most surprised by what they saw when they compared the birds' muscle activity on a ramp with a 65 degree incline.
Running up that ramp, explained Dr Jackson, "required about 10% as much power from the flight muscles" as flying.
"The signal was imperceptible at first, and we actually thought we had a problem with the recording equipment. But when we zoomed in, there it was, about a tenth the magnitude that it was during flight," he said.
"The birds seemed to be using hardly any power to flap their wings as they ran up the slopes."
The method, the researchers say, is also an essential learning step for fledging chicks.
"Flap running... lets young birds that cannot yet fly - because of small muscles, small wings, weak feathers, etc - get off the ground and away from some predators," Dr Jackson told BBC Nature.
"And if baby birds can perform these behaviours, benefit from them, and transition gradually to flight in their life-time, we think it's probable that dinosaurs with (similarly small wings) could have performed these behaviours, benefited from them, and transitioned towards flight over evolutionary time."
So watching birds learn to fly could allow us a glimpse of the stages of flight's evolution.
Dr Jackson concluded: "Very small wings powered by small muscles had aerodynamic function and survival benefits when they were flapped.
"No more major steps were required after that, just gradual but beneficial steps. And we can actually observe [those steps] in developing birds today."