Dr Katsufumi Sato of the University of Tokyo carried out an unusual study on the Crozet Islands, in the southern Indian Ocean, to test flying ability among large sea birds.

He attached accelerometers the size of AA batteries to the wings of 28 birds from five large species, including the wandering albatross, the world's biggest flying bird.

Albatrosses fly by riding shifting winds, thanks to wings spanning 3.5 metres whose shape can be varied to exploit each draft.

When there is no wind, or if the wind blows at a constant speed, the bird can only stay aloft by flapping its wings, otherwise it is forced down by gravity and air resistance.

In a month-long experiment, Dr Sato's instruments showed that the seabirds had two flapping speeds - fast for taking off, and slow, for keeping aloft when the wind dies, New Scientist says.

The bird's flapping speed is limited by its muscle strength, and the speed decreases for heavier birds that have longer wings, Dr Sato found.

According to Dr Sato's calculations, animals heavier than 40 kilos would be unable to flap fast enough to fly in zero winds.

A wandering albatross is fine, as it weighs 22 kilograms, but the news is terrible for pterosaurs. Large ones would be unable to stay aloft, by this benchmark.

The largest pterosaur specimen found, Quetzalcoatlus northropi, had a wingspan of 11-12 metres and its weight is estimated to be in the order of 100kg .

Dr Sato presented his results at a Biologging Science Symposium in Stanford University, California last month.

He has run into flak from pterosaur fans who are convinced that their creatures were 'dynamic soarers' like the albatross and could sustain active flight and not just glide.

Differences in anatomy, physiology and environment must be taken into account when comparing the two sets of flyers, they say, according to the New Scientist report. -- AFP