Researchers looked at the "roller coaster" migratory flights of geese to gain insight into how the birds fly through thin high-altitude mountain air.

The researchers used custom-designed data loggers to measure the pressure the geese experienced at high altitudes as well as the birds' heart rates during their annual southern migration from their Mongolian breeding grounds to their wintering grounds in South-eastern Tibet or India, according to a news release. 

The new findings contradict past beliefs suggesting the bar-headed geese fly at relatively high altitudes for the duration of the migration, instead showing they perform more of a "roller coaster" ride.

"We have developed two independent models to estimate changes in the energy expenditure of birds during flight," said Robin Spivey (the Research Officer on the project and developer of the data logging equipment). "One based on changes in heart rate and one based on the vertical movements of the bird's body. These indicate that, as even horizontal flapping flight is relatively expensive at higher altitudes, it is generally more efficient to reduce the overall costs of flying by seeking higher-density air at lower altitudes."

The findings suggests the geese gradually increase wingbeat frequency as they reach higher altitudes and was precisely regulated during the flight.  Heartbeat was found to be closely correlated with wingbeat frequency.

"It seems that geese must keep very fine control over their wingbeat cycles. As they flap faster they also move the wing further, i.e. with bigger amplitude. They are designed with a very high gearing linkage between the movement of the wing and the cardiac output or flow of blood from the heart. It is like riding a bike with an increasingly large cog for the pedals as you move faster and a relatively small cog on the back wheel. An increasing effort is required to move the bike pedal (or the bird's wing) at the same frequency, or even slightly faster, through each revolution but the back wheel (or the bird's heart) is rapidly increasing its activity and overall speed is increasing," said study leader Charles Bishop of Bangor University.

The findings suggest by using this roller coaster flight strategy the birds effectively minimize their overall energy costs during the migration.

The findings were published in a recent edition of the journal Science.