It is often said that birds of a feather flock together, but successfully orchestrating clustered flight has more to do with physics than poetry. Although a number of co-ordinated group flight behaviours, such as the V-formation flights observed in geese, are believed to be energy-saving mechanisms, the costs and benefits of other flocking behaviours are less well understood. Pigeons (Columba livia), for example, will often fly in clusters rather than go it alone, but what are the consequences of choosing quantity over quality? In a recently published study, Lucy Taylor and her team of flight biomechanists from the universities of Oxford and Royal Holloway, London, in the UK, recently set out to answer this question by investigating how the wingbeat kinematics, visual stability and navigational skills of pigeons varied between solo and paired flights.
During the first experiment, the team fitted the homing pigeons with GPS loggers and accelerometers that allowed them to track the birds’ geographic locations and to record their wing movement during flight. Next, they released the birds 7 km from their home roost to fly home solo. After the birds had returned home, the team matched some of the birds with a partner of similar size or with different-sized partners and then released them in pairs, before releasing them one final time for their last solo flight. For the second experiment, the birds performed similar homing flights but the team fitted them with a tiny helmet weighing just 1 g, attached to a snuggly fitting backpack carrying a battery and a memory storage chip, to measure their head movements. Finally, the team used aerodynamic calculations to estimate the energy required for the birds to stay airborne.
The team discovered that while the solo pigeons flapped their wings at a frequency of around 5.5 beats s–1, the wingbeat frequency of the pigeons flying in tandem was actually about 1 beat s–1 higher, representing an 18% increase in flaps per second – quite the opposite from the energy-saving formation flights of geese. They realised that the increase in wingbeat frequency was roughly proportional to the proximity of the birds – the closer the birds, the faster they beat their wings – and that the birds went back using a slower wingbeat if they became separated. From their aerodynamic calculations, the authors estimated that paired flight was 2% more energetically costly than solo flight, but because energy is such a precious resource, any additional expenditure is a threat to the birds’ chances of survival. Finally, the second experiment revealed that the birds moved their heads 30% less when flying in a pair, which the team believes plays an important role in keeping their eyes steady and co-ordinating with their fellow flier.
Despite the additional cost of a faster wingbeat, the birds released in pairs consistently chose to fly together, suggesting that the added expense of flying together must somehow be offset by other energetic benefits. Indeed, when the team looked at the routes taken by the birds, they discovered that the paired pigeons reduced their flight distance by 7% and flight duration by 9% compared with solo pigeons, which more than compensated for the extra energy spent on additional wingbeats. When combined with their improved ability to spot predators, thanks to their pooled vigilance, increased head-steadiness and safety in numbers, it becomes much clearer why pigeons prefer to rely on the power of teamwork to avoid getting in a flap.