Birds Get Knocked Down, But They Get Back Up Again

Blog Author: Professor Adrian Thomas, CSO, Animal Dynamics

Robustness; birds do crash, but they bounce.

Birds seem to be complete masters of the air – they almost never crash. Except when they fly into windows. In the window of my office there is the perfect outline of a pigeon – both wing, the breast feathers, and you can even see where the beak hit – it looks a bit startled. No sign of a body though – it seems to have shook itself off and flown away. That in itself is pretty remarkable, and the two birds in these video do the same…..https://player.vimeo.com/video/569768667?app_id=122963

The rook is being chased by a piratical red kite. It follows the classic instructions to a dog-fighting pilot who wants to disengage and executes a Split-S – rolling inverted into a dive and intending to pull out and fly away in the opposite direction. The ground is, unfortunately, rather too close and instead the rook achieves CFIT – Controlled Flight Into terrain. For most aircraft that would be that – but the rook shakes itself off, looks around, and flies off as if nothing had happened.https://player.vimeo.com/video/568846497?app_id=122963

The Jackdaw follows the same plan – disengage from aerial piracy by executing a Split-S – but this time the ground is so close the jackdaw literally bounces and flies off.

Why do they both do a Split-S? Possibly for the same reason that manouver was so favoured by the Spitfire as a way to start a dive – the Split-S can allow positive G to be maintained throughout, loading the airframe in the same direction as in normal flight. Birds also have massive roll-authority because they can rotate their entire wing to generate roll-forces (as is obvious in both videos).

How do the birds manage to fly away undamaged? Possibly because feathers are extremely robust and also act together to spread impact loads. Recent research on Gannets (which catch fish by diving head first into the sea from altitude) has shown that their overlapping feathers spread the load from impact across a wide area, dissipating kinetic energy into elastic bending spread across multiple feathers and spreading it over a broad area of the body rather than just the point of impact.

This may be why feathers were used as armour, particularly anti-projectile armour by both the Aztecs and by the people of Hawaii. The Aztecs used feathers on their shields, and particularly extending below the shields where they could stop stray projectiles without overly impairing the movements of the warrior. The Hawaiians wove feathers into spectacularly beautiful cloaks – there is a superb example in Oxford’s Pitt-Rivers museum – those cloaks were apparently not just decorative indicators-of-status, they were also used in warfare – draped over the left arm as a sort of flexible shield that could be held out in front of the warrior to fend off projectiles or ensnare spears.

 References

Bhar Kinjal, Chang Brian, Virot Emmanuel, Straker Lorian, Kang Hosung, Paris Romain, Clanet Christophe and Jung Sunghwan 2019. How localized force spreads on elastic contour feathersJ. R. Soc. Interface.162019026720190267 http://doi.org/10.1098/rsif.2019.0267

 Ross Hassig. Aztec Warfare: Imperial Expansion and Political Control. Norman: University of Oklahoma Press, 1988.