Since our early ancestor took those first wet steps out of the water and onto land, one rule has united all their descendants: we are forbidden to have an odd number of limbs. We can lose limbs along the way, but our embryological development constrains us to have paired legs. Rules are of course there to be broken and some birds have indeed found a way to bend them.
If you watch a climbing bird, say a parrot, you'll notice that they will occasionally grasp the inclined surface with their beak as they make their way up. Melody Young, a graduate student at the New York Institute of Technology (NYIT), USA, was also curious about this. Given that birds have swapped their front legs for wings, getting a leg-up from their beaks could be a big help.
Young, together with their NYIT colleagues Edwin Dickinson, Nicholas Flaim and Michael Granatosky, set out to investigate whether parrots truly power their stride using their beaks as a third leg. They tested the climbing prowess of six rosy-faced lovebirds (Agapornis rosiecollis), by observing their head, tail and body movements and by measuring the forces they exerted whilst ascending a climbing frame. The incline of the surface was gradually raised, to see whether the lovebirds’ technique changed with the steepness of the climb.
Initially, the lovebirds were happy to walk along on two legs, without any beak assistance. But, as soon as the climbing frame was raised up to a 45 deg incline, the birds started using their beaks and their tails to assist their climb. The frequency of beak and tail use increased each time that Young made the surface steeper. Once the climbing frame was vertical, the birds used their beak and tail in rhythm with every step. This confirmed the received wisdom that birds climb with help from their beaks and tails, but are they really propelling themselves upwards by using one of these structures as a third limb?
Young and colleagues had the answer. They looked at the force data, to see whether, aside from the feet, any other body part was actively pulling the parrots upwards. During vertical climbing, they found that the beak produced just as much propulsive force as the birds’ legs, showing us that the lovebirds were powering their ascent with their beaks as well as their feet in equal measure. They also used their beaks to keep them attached to the vertical surface, gripping on whilst their feet pushed-off with each step.
However, the lovebirds used their tails differently, producing low forces, much lower than those produced by the feet or the tail. This showed Young and colleagues that lovebirds use their tail more like a balancing aid. Imagine using a crutch or a walking stick as you climb a hill, not necessarily using it to push or pull your body forward, but more as a way of stopping yourself toppling over.
Parrots have bent the rules of our ancestors and use their beak as a third limb, and this is no mean feat; they have super-sized their neck muscles to haul themselves upwards. In fact, these muscles match or even beat comparable forces generated by the arms of human rock-climbers. Considering that parrots also defend themselves and eat with their beaks, this piece of anatomy is truly multi-functional, an evolutionary Swiss army knife.