Video: This is how a hummingbird flies through a narrow opening using a sideways flight maneuver. © Marc Badger
On the trail of the secrets of feathered flight artists: Researchers have discovered two techniques that hummingbirds use to overcome bottlenecks that are too narrow for their wingspan. In this rather cautious procedure, they fly sideways through the gap with one wing first. If they already know the obstacle, they also use a more daring technique: They rush through the openings with their wings stretched backwards, the recordings show. The new insights shed light on how the little flying acrobats can maneuver so skillfully through dense vegetation, say the scientists.
Hovering from flower to flower: Hummingbirds have created one of the most amazing forms of locomotion in the animal kingdom. Thanks to their extremely high wing beat frequency, the tiny birds can stand in the air like helicopters and maneuver cleverly. This has given them access to a special source of food: floating, they soak up nectar from flowers with their long beaks and whir skillfully through the thicket. So far, however, there has been an open question: How do they fly through gaps in the vegetation that are smaller than their wingspan?
“Normal” birds are known to fold their wings slightly towards the body using the flexible joints. But the hummingbirds have lost these joint functions in their wings as their special flight technology has developed. In order to clarify how they can still navigate through the complex structures of their habitats, the research team led by senior author Robert Dudley from the University of California at Berkeley conducted experiments with the North American Anna’s hummingbird (Calypte anna).
Tricky passages in sight
They built an experimental facility in which the birds were offered two artificial flowers as food sources. However, there was a partition between them that could only be overcome through a window. Initially, this opening had a width that was easily passable for the approximately twelve centimeter wingspan of the hummingbirds. In order to get them to fly frequently through the hole from one flower to the other as effectively as possible, the researchers used a trick: there was only alternating amounts of sugar juice in the artificial flowers. They only refilled after the bird returned from the feeder opposite. The test animals quickly learned this and therefore rushed back and forth eagerly.
The researchers then put the hummingbirds to the test: They placed a series of circular openings in the window with a diameter smaller than the birds’ wingspan. The spectrum ranged from twelve to six centimeters. The scientists recorded how the hummingbirds reacted to the increasing confinement using high-speed recordings. They were then able to break down the flight maneuvers in detail in slow motion. In addition, a special computer program was used to analyze flight behavior.
Two techniques revealed
This showed that hummingbirds can use two different strategies to overcome bottlenecks. In the first method, they float sideways through the gap with one wing first, also adjusting the orientation of their long beak to the direction of movement. Analyzes of the recordings showed that they perform sophisticated wing movements that enable them to perform this maneuver. This allows them to pass through the gaps comparatively carefully and without losing height. The second strategy, however, is more rapid: the birds align their wings backwards so that they rest against their bodies. In a short shot-flight they then rush through the gap, beak first. After the passage, the flapping of the wings starts again to end the maneuver.
Video: shot-flight method at a glance. © Marc Badger
As the observations showed, the birds apparently use the sideways flight technique primarily when they are unsure of what to expect. Because this comparatively slow and controlled process means they can probably react better to possible further obstacles. However, when the birds had crossed the bottleneck a few times, the researchers found that the shooting flight was used more often. “They seem to use the faster method when they are more familiar with the system,” says Dudley. In addition, this process apparently becomes cheaper as the gaps become narrower. The birds then only managed to overcome the smallest test size of six centimeters using the shot flight method, the researchers report.
They now want to further expand on their previous observations: Among other things, the team plans to investigate the behavior with which the sophisticated flying artists react to a sequence of different bottlenecks.
Source: University of California – Berkeley, specialist article: Journal of Experimental Biology, doi: 10.1242/jeb.245643