British scientists have for the first time measured the migration of insects in the skies and found that more than three trillion pass over our heads each year.

Insects are key players in healthy ecosystems. They pollinate crops, eat crop pests and provide food for birds and bats.

Experts say this first-of-its-kind measurement suggests insect migration -- which most of us never see -- is a major event.

"Insect bodies are rich in nutrients and the importance of these movements is underappreciated," said co-author Jason Chapman of the Centre for Ecology and Conservation at the University of Exeter's Penryn Campus in Cornwall.

"If the densities observed over southern UK are extrapolated to the airspace above all continental landmasses, high-altitude insect migration represents the most important annual animal movement in ecosystems on land, comparable to the most significant oceanic migrations."

By weight, this movement would take up 3,200 tons, more than seven times the mass of the 30 million songbirds which leave Britain for Africa each autumn, said the study in the journal Science.

Most of the traveling -- some 70 percent -- is done by day, said scientists at the University of Exeter and Rothamsted Research.

Using special radars pointed at the sky, lead author Gao Hu tracked the migration of insects at a height of 150 meters (yards) for nearly a decade.

The annual average came out to 3.37 trillion insects.

Travel was heaviest on warm days, researchers said.