Mission possible! How hummingbirds squeeze through tight spots

Lots of birds can fly through small gaps by simply folding in their wings closer to their bodies. But not hummingbirds! They’ve lost the ability to bend their wings at the wrists and elbows.

So how do these avian acrobats make it through gaps smaller than their wingspan?

A new study on Anna’s hummingbirds (Calypte anna) in the Journal of Experimental Biology outlines exactly how they can penetrate gaps barely half a wingspan wide.

Like a spy squeezing through a lattice of deadly lasers by contorting their body sideways or making a daring head-first leap, hummingbirds use two unique strategies.

“We set up a two-sided flight arena and wondered how to train birds to fly through a 16cm² gap in the partition separating the two sides,” says co-author Marc Badger, a PhD student from University of California, Berkeley (UCB), in the US.

To encourage the birds to flit to and fro, the team only refilled a flower-shaped feeder with sugar solution once a bird had returned to the feeder on the opposite side.

Slowly but surely, they replaced the gap with a series of smaller oval and circular apertures ranging in height, width, and diameter from 12cm down to 6cm.

Using a high speed camera to film the birds’ manoeuvres, a computer program then tracked the position of each bird’s bill and wing tips, and calculated their wing positions, throughout the entire manoeuvre.

The first, more cautious approach involved hovering in front of the aperture to assess it. Then, the birds travelled through sideways by reaching one wing forward and sweeping the second one back – fluttering all the while. Imagine a bird T-posing.

But in the second strategy, the birds swept their wings all the way back and pinned them to their bodies in order to shoot through beak first – like a bullet. Once safely through the gap they then swept their wings forward and resumed flapping again.

The team also realised that the birds that were travelling sideways tended to fly more cautiously and slowly than the birds that rocketed through the apertures beak first.

But practice makes perfect and as those birds became more familiar with the obstacle, they became bolder. They transitioned to a swifter approach and dropped their initial tactic in favour of darting through beak first. 

However, the smallest aperture – half a wingspan wide – required every bird to zip through facing forward, even on the first attempt, to avoid collisions.