Flight is an incredibly complex but wonderful mode of transport. Perhaps because of this, and definitely because of our inability to fly, humans are fascinated by it. We have been trying to achieve flight for a long time, with many failures, but also some extraordinary success. While many animals evolved the ability to fly long before humans were around, bats are the only mammals capable of powered flight to get from place to place and to hunt. But remaining stationary while hovering in front of a food source is even more impressive. As some bats are capable of hovering, Rivers Ingersoll and colleagues from Stanford University, USA, were interested to see whether there were any similarities in hovering flight between mammals and birds.

The team travelled to the jungles of Costa Rica to capture 20 bat and hummingbird species. Hummingbirds are well known for their hovering ability; their preferred food of choice is the delicious nectar located deep inside flowers, so hovering is a necessity for extracting the precious fluid. While some hovering bat species chiefly consume fruit, many of them are also partial to nectar. However, bats are less dainty when sipping nectar; they just stuff their whole face into the flower. Once the group had captured their test subjects, they trained the hummingbirds to feed from a syringe filled with sugar water, so they could film the birds while they hovered. Unfortunately, the scientists were unable to train the unruly bats to hover at a flower, but they were still able to film the animals hovering as they moved around a flight chamber.

Once all of the movies of the hovering creatures had been captured, Ingersoll and his colleagues examined the footage to decipher similarities and differences between the hovering techniques of the two groups. Interestingly, the amount of power used by the two while hovering was comparable, but this was where most of the similarities ended. Hummingbirds are better at supporting their weight with the upstroke, when the wings beat upwards. They accomplish this by beating their wings backwards and forwards horizontally, which is a very efficient way to produce lift. Bats, in contrast, create most of the lift they need to support their body weight on the downstroke of the wing beat. Furthermore, bats do not beat their wings horizontally, but at more pronounced angles, and their wings are significantly larger, allowing them to achieve the same power output as hummingbirds.

The disparities in how hummingbirds and bats achieve hovering flight largely boils down to differences in their wing design. Hummingbird wings are covered in feathers and are composed solely of their forelimbs, whereas bat wings consist of a membrane stretched over their forelimbs that is also attached to their legs. This huge variance in flapping appendages makes it even more fascinating that these two groups have converged evolutionarily on a way to feed from a stationary object while flying.

Ingersoll and his colleagues hope that achieving a better understanding of how animals hover may help to create aerial robots that are able to hover just as well. And I recommend taking a look at their supplementary videos of hovering bats and hummingbirds, which are truly captivating.

Biomechanics of hover performance in Neotropical hummingbirds versus bats
Science Advances