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Most birds that fly through dense, leafy forests have a strategy for maneuvering through narrow windows in vegetation: they flex their wings at the wrist or elbow and run through.
But hummingbirds can’t bend their wing bones in flight, so how do they pass through the gaps between leaves and tangled branches?
A study published today in the Journal of Experimental Biology shows that hummingbirds have developed their own unique strategies – two of them, in fact. These strategies have not been reported before, likely because hummers maneuver too quickly for the human eye to see.
For slit-like openings that are too narrow for their wingspan, they crawl sideways through the slit, continuously flapping their wings so as not to lose altitude.
For smaller gaps – or if the birds are already familiar with what awaits them on the other side – they fold their wings and fly through, returning to fluttering once they are clear.
“For us, when we started the experiments, the folding and sliding would have been the standard. How else could they get through?” said Robert Dudley, professor of integrative biology at the University of California, Berkeley, and senior author of the paper. “This concept of lateral movement with a total blending of the wing kinematics is quite amazing – it is a new and unexpected method of gap passage. They change the amplitude of the wing beats so that they do not fall vertically when they do the sideways scooch.”
Using the slower side scooch technique allows birds to better assess approaching obstacles and voids, reducing the chance of collisions.
“Learning more about how animals interact with obstacles and other ‘building blocks’ of the environment, such as wind gusts or turbulent areas, could improve our general understanding of animal locomotion in complex environments,” said first author Marc Badger, who received his PhD. ..D from UC Berkeley in 2016. “We still don’t know much about how clutter flight can be limited by geometric, aerodynamic, sensory, metabolic, or structural processes. Even behavioral limitations can arise from longer-term effects, such as wear and tear to the body, as evidenced by the shift in diaphragmatic negotiation technique we observed in our study.”
Understanding the strategies birds use to maneuver through a cluttered environment could ultimately help engineers design drones that can better navigate complex environments, he noted.
“Today’s remote control quadrotors can outperform most open space birds on most performance measures. So is there a reason to continue learning from nature?” said Badger. “Yes. I think it has to do with the way animals deal with complex environments. If we put a bird’s brain in a quadrotor, would the cyborg bird or a normal bird be better able to navigate through a dense forest in the wind to fly? There can be many sensory and physical benefits from flapping wings in turbulent or cluttered environments.”
To discover how hummingbirds – in this case four local Anna’s hummingbirds (Calypte anna) – slip through small gaps despite being unable to fold their wings, Badger and Dudley teamed up with UC Berkeley students Kathryn McClain, Ashley Smiley and Jessica Ye.
“We set up a two-sided flying arena and wondered how we could train birds to fly through a 16-square-inch gap in the partition between the two sides,” Badger said, noting that the hummingbirds have a wingspan of about 12 centimeters. 4 3/4 inches). “That’s when Kathryn came up with the great idea of using varied rewards.”
That is, the team placed flower-shaped feeders containing a sip of sugar solution on either side of the partition, but only refilled the feeders remotely after the bird had visited the opposite feeder. This encouraged the birds to continually fly through the gap between the two feeders.
The researchers then varied the shape of the opening, from oval to round, varying in height, width and diameter, from 12 cm to 6 cm, and filmed the birds’ maneuvers with high-speed cameras. Badger wrote a computer program to track the position of each bird’s beak and wing tips as it approached and passed through the opening.
They found that when the birds approached the opening, they often paused to assess it before passing through sideways, reaching forward with one wing while beating the second wing back, flapping their wings to to support their weight as they passed through the opening. Then they turned their wings forward to continue their journey.
“The point is that they still have to maintain the weight support, which comes from both wings, and then control the horizontal thrust, which pushes the wing forward. And they do this while the right and left wings do very special things, said Dudley. “Again, this is just another example of how, when thrust into an experimental situation, we can elicit control traits that we don’t see in just a standard hovering hummingbird.”
Alternatively, the birds folded their wings back and pinned them to their bodies, shooting through them – beak first, like a bullet – before flipping the wings forward and fluttering through safely again.
“They seem to be using the faster method, the ballistic buzz-through, as they become more familiar with the system,” Dudley said.
Only when approaching the smallest openings, which were half a wingspan wide, would the birds automatically resort to the fold and glide, even if they were unfamiliar with the arrangement.
The team pointed out that only about 8% of the birds clipped their wings as they passed through the divider, although one experienced a major collision. Even then, the bird quickly recovered before successfully retrying the maneuver and continuing on its way.
“The ability to choose between different obstacle negotiation strategies may allow animals to reliably squeeze through tight gaps and recover from mistakes,” Badger noted.
Dudley hopes to conduct further experiments, perhaps with a range of different openings, to determine how birds navigate multiple obstacles.
The work was funded primarily by a CiBER-IGERT grant from the National Science Foundation (DGE-0903711).
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