Duetting American cliff swallows prioritise agility over efficiency

Named for their natural habitat, American cliff swallows (Petrochelidon pyrrhonota) reside on mountain cliffs in large colonies, flocking home to their tightly packed mud nests in noisy droves at dusk. But the birds are also pragmatic. As human structures have proliferated, they have relocated to bridges and buildings in urban areas. And they are not averse to laying their eggs in the nests of others, leading to aerial dogfights, where one bird pursues another in a tightly choreographed duet, to protect their home from invasion. In 2014, Ryan Shelton from University of North Carolina at Chapel Hill, USA, noticed that the birds flap in synch when duetting. However, most solitary birds adjust their wingbeats and flap less frequently when flying at intermediate speeds, to use the least energy. Knowing this, Ty Hedrick (University of North Carolina at Chapel Hill) wondered recently whether tandem duetting swallows might care less about flying efficiently than other birds, and so beat their wings at similar frequencies regardless of their flight speed.

Back in 2012 and 2013, Hedrick, Shelton, Brandon Jackson and Nick Deluga (also from University of North Carolina at Chapel Hill) had rigged up three cameras beneath a bridge over Jordan Lake (North Carolina), home to a colony of 60 cliff swallows, to capture the birds’ antics as they wheeled and dived individually in pursuit of insects and during tandem duets. Then the researchers painstakingly reconstructed the birds’ manoeuvres in 3D, identifying and analysing 15 dogfights. Returning to the 3D movies several years later, Sophia Chizhikova and Laura Mendez (University of North Carolina at Chapel Hill) analysed the manoeuvres of the solo flyers, to find out how their flight speeds and wing beats compared with those of the tandem duetting pairs.

The team realised that the duetting birds were flying no faster than those flying solo, recording an average speed around ∼6.7 m s⁻¹. However, the tightly manoeuvring tandem fliers flapped their wings more than twice as frequently (12.35 beats s⁻¹ on average) as solo flyers (5.83 beats s⁻¹). And when the researchers plotted the birds’ wing beat frequencies across speeds ranging from 3 to 11 m s⁻¹, they were surprised; the swallows engaged in a duet always flapped their wings fastest when flying at intermediate flight speeds, in contrast to the solo swallows, which flapped their wings slowest when flying at intermediate speeds around 9 m s⁻¹. Instead of taking advantage of flying together to flap at a slower rate, or maintain the same flap rate across all flight speeds, duetting swallows always flapped their wings faster, even at intermediate flight speeds when solo fliers eased off.

So why do duetting swallows flap their wings at a faster rate than individual flyers? One possibility is that competing swallows may be able to track their foe more precisely when using higher flapping rates, as their vision is more stable when their wings beat faster. Alternatively, birds can alter their trajectory to twist and turn each time they beat their wings, so Hedrick suspects that flapping their wings at a faster rate may improve the agility of duetting swallows, to better track or evade their opponent. He explains that this may be analogous to how sports people in competitive teams move: ‘we can't change direction mid-stride when running, so, in situations that might call for rapid changes in direction, we use quicker strides, but we use longer strides when not expecting to manoeuvre and running for efficiency’.

In short, swallows are content to forego efficiency during brief airborne duets to match their adversary's agility as they defy convention and flap their wings at twice the rate of soloists.