Birds’ flight techniques are not for the bats, study finds

Bats and birds, the only two vertebrate fliers on Earth, use their wings very differently, according to scientists who observed nectar-feeding bats flying through a wind tunnel.

In the journal Science on Thursday, the researchers described aerodynamic differences between bats and birds. They both fly by flapping their wings but use the upstroke of the flap in different ways. Unlike birds, bats flick their wings upward and backward to gain lift.

“Bats seem to be mostly specialized for agile and maneuverable flight in complex environments,” said Geoffrey Spedding, a USC professor of aerospace and mechanical engineering and one of the study’s authors.

Pallas’ long-tongued bat, which has the scientific name Glossophaga soricina, was the test pilot for the research team from Sweden, Germany and the United States.

The bats flew in a wind tunnel filled with fog from a machine. Scientists tracked the movement of fog particles in the wake left by the flying bats to understand the aerodynamics of each wing beat. Scientists had previously studied birds flying in wind tunnels.

Powered flight has evolved three times among vertebrates. The first to fly were reptiles known as pterosaurs, which appeared about 220 million years ago but died with the dinosaurs. Scientists think birds descended from small, feathered dinosaurs about 150 million years ago.

Bats, which arose about 50 million years ago, are the only mammals to have developed powered flight.

The wing structure of bats and birds also differs. Birds have feathers projecting back from lightweight fused arm and hand bones. Bats have flexible, relatively short wings with membranes stretched between elongated fingers.

Spedding said that whereas birds can open their feathers like a venetian blind to let air pass through on upstrokes, bats have developed a twisting wing path that increases lift during upstrokes.

There are about 1,000 species of bats, accounting for about 20% of mammal species.