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Floating, with outstretched wings, many birds fly through the air unhindered. However, species that feast on fruit, seeds and nectar must find small holes in the cluttered foliage to ensure a feast. To pass through openings, many birds retract their wings and fold them closer to their bodies. However, some of the most agile pilots, hummingbirds, have lost the ability to fold their wings at the wrists and elbows. “Unless hummingbirds implement specific strategies to travel through narrow openings, they may not be able to enter gaps of less than one wingspan,” says Marc Badger of the University of California, Berkeley (UCB), USA. How can the extremely agile airlines, which can even fly backwards, cope with the cluttered environment they inhabit? Together with Kathryn McClain, Ashley Smiley, Jessica Ye and Robert Dudley (all from UCB), Badger set out to discover how Anna’s hummingbirds (Calypte anna) slip through small openings despite not being able to fold their wings. The team publishes their discovery in the Journal of Experimental Biology that these birds use two unique strategies that allow them to enter holes barely half a wingspan wide.
‘We set up a two-sided flying arena and wondered how we could train birds to fly through a 16cm2 opening in the partition that separates the two sides. That’s when Kathryn came up with the amazing idea of using alternating rewards,” says Badger, explaining that the team only refilled a flower-shaped feeder with a sip of sugar solution once the bird had returned to the opposite feeder, to keep the birds with a wingspan of approximately 12 cm. to flutter back and forth. Badger, Smiley and Ye then replaced the opening with a series of smaller oval and circular openings – ranging in height, width and diameter from 12 to 6 cm – where the birds could negotiate, while filming the birds’ maneuvers with high-speed cameras. . Badger then wrote a computer program to methodically track the position of each bird’s beak as they approached and passed through each opening, while also locating the bird’s wing tips, to calculate their wing positions as they passed through.
Impressively, the hummingbirds used two unique strategies to bridge the gaps. In the first, they approached the opening, often hovering in front of it to assess it first, before passing through sideways, reaching forward with one wing while folding back the second wing – almost in the shape of a cross – while still flapping their wings. to fly through the opening and then turn forward to continue their journey. In the second strategy, they folded their wings back, attached them to their bodies, and first shot like a bullet through the beak, before folding the wings forward and flying safely through the beak again.
When examining the two strategies, the team realized that the birds that traveled sideways tended to fly more carefully and slower than the birds that darted through the beak openings first. And as the birds became more familiar with the openings after several approaches, they seemed to become more confident, approaching quickly and abandoning the more cautious sideways approach in favor of shooting through the beak first. However, the smallest opening – half a wingspan wide – always posed the most problems, with each bird zooming through with its wings backwards – even on the first attempt – to avoid collisions.
Thus, hummingbirds have evolved strategies that allow them to enter small openings less than a wingspan wide, with the sideways option allowing them to take a more cautious approach, switching to beak-first darting as they become bolder. And the team points out that although about 8% of the birds clipped their wings as they passed through the divider, only one experienced a major collision, and even then the bird quickly recovered before successfully attempting the maneuver again and its road continued.
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