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A Birds’ Eye View of Avian Flight

Antagain/iStockphoto

Some birds naturally fly left and others right. Credit: Antagain/iStockphoto

By Partha Bhagavatula

The “handedness” of birds enables them to navigate a collision-free path through complex environments, with some flying left of obstacles while others prefer to fly right.

Birds have mastery of life in the air, and have evolved effective strategies of flying rapidly and safely through complex and cluttered environments. Such agility requires quick decision-making and the ability to determine which of several possible flight paths would provide a safe, quick and collision-free passage.

How they do this? To our surprise we found that handedness plays a significant role. Some birds naturally fly left and others right.

Our study of bird flight began entirely by chance. As a graduate student at the Australian National University, I was studying the landing behaviour of budgerigars. This iconic Australian bird, which lives in the Australian outback, has a superb visual system and can perceive four colours: red, green, blue and ultraviolet.

One day, while setting up an experiment, a budgerigar named Casper escaped from the experimental room through a narrow gap between the doors. This piqued my curiosity because Casper managed to negotiate this narrow gap and emerge unscathed out on the other side.

As luck would have it, I happened to have a webcam with me that day, so I filmed my budgerigars flying through the gap between the doors several times over. When analysing the film footage I found that the budgerigars, irrespective of their body size, always flew through the centre of the gap with their wings folded and came out the other end of the opening in one piece. This formed the basis for further study into free flight in budgerigars through narrow spaces.

Soon after our research group moved to Brisbane, and one of our first tasks there was to construct a purpose-built flight tunnel. We could manipulate the visual environment by decorating the walls of the tunnel with various patterns and also include obstacles to study different aspects of bird flight with respect to their visual system. The tunnel was equipped with high-speed cameras to record bird flight trajectories (Fig. 1).

One of our first studies using the flight tunnel addressed how budgerigars were able to pass though narrow gaps and passages. We found that the birds were able to negotiate narrow passages safely by steering a course so that their two eyes experience similar rates of image motion or “optic flow”.

Optic flow is the pattern of image motion that is experienced by a bird’s visual system when it flies. For example, a bird experiences strong optic flow cues when it flies past vertical stripes painted along the flight tunnel, because it is moving perpendicular to the stripes and therefore discerns the changing pattern clearly. On the other hand, it experiences weak optic flow cues when it flies past horizontal stripes because it is moving parallel to the stripes and therefore perceives no image motion.

Budgerigars tend to balance the optic flow information perceived by their two laterally placed eyes. As spatial resolution is uniform when the bird flies in a tunnel with horizontal stripes or without any patterns at all, the bird collides with the walls due to lack of orientation clues.

When both walls have vertical stripes we observe that birds fly through the middle of the tunnel. When only one of the walls presents strong optic flow, the birds move away from that wall in an attempt to restore the balance between the flows experienced by the two eyes.

The birds also used optic flow signals to alter their flight speeds. When obstacles were introduced in the flight tunnel, the birds used visual guidance to avoid a collision.

In nature, birds live in far more complex environments where visual guidance cues keep changing very quickly, so birds have to make split-second decisions that can be crucial to their very survival. Although the flight tunnel was a simplistic model of the highly complex natural environment that the birds encounter, it nevertheless allowed us to answer some interesting questions about the nature of bird flight.

For example, we wanted to know how the budgerigars would react if their flight path was obstructed by a cloth barrier with holes in it through which they could escape. The answers were surprising!

We allowed the birds the choice of flying through either of two apertures in the cloth barrier placed across the flight tunnel. When one of the two apertures was much wider than the other, the birds tended to fly through the wider passage, and continued their transit to the end of the tunnel. However, when both the apertures were of similar diameter, some birds consistently preferred the left-hand passage while others almost always preferred the passage on the right.

What was more surprising was when a left-biased bird persistently flew through the left aperture despite the fact that this was much smaller than the right aperture. Only when the left aperture was kept smaller than a certain threshold size would the left-biased bird fly through the wider opening on the right.

The same held true for right-biased birds. Clearly, this indicates not only a “handedness” among individual birds, but also complex decision-making.

Our experimental procedure was relatively simple. A random number generator was used to change both the position of the barrier and the positioning and the size of the openings in the barrier. Each budgerigar was induced to take off by slowly rotating its perch, and it would fly through the tunnel, clear the obstacles and land at the other end. The entire sequence was captured using a pair of high-speed cameras (Fig. 2) and then analysed to generate motion trajectories for some of the flights (Fig. 3).

In Figure 3a, a left-biased bird flies through the left-hand opening even though it is slightly larger than the right-hand opening. In Figure 3b the fight trajectory of a right-biased bird is shown in finer detail. The bird approaches the opening on the right-hand side but, upon finding that the aperture is smaller, it recomputes its flight plan and takes the larger opening on the left-hand side. Similarly Figure 3c shows the flight path of a right-biased bird when the right opening is kept wide open and the left opening is fully closed.

There was some evidence from previous studies that birds use something akin to “handedness” to detect objects. For example, chickens use their right eye to look for food grains while they watch out for predators through their left eye. Parrots coordinate their leg and beak movements to acquire food while suspended from a string (or branch). Similarly New Caledonian crows exhibit a side bias in tool usage, although the advantage of such side bias remains unexplained. Some researchers have observed that birds that exhibit handedness, or lateralisation, are good at multitasking.

We proposed a mathematical model to explain the interaction between the budgerigars’ individual biases in handedness with their tendency to prefer the wider passage. Our model revealed that individual handedness, coupled with the ability to discern aperture width during flight, endows the birds with a survival advantage in highly complex environments because it allows them to quickly fly away from potential danger and avoid collision with obstacles while doing so.

Our understanding of why handedness evolved in birds is speculative. Lateralisation reflects asymmetry in the brain, but what possible selective advantage could the resulting handedness offer?

Maybe handedness in birds allows their brains to manage sensory information efficiently, and thus enable quick motor output in the form of wing movements so that they can make fast decisions in a constantly changing visual and acoustic environment, and thus increase their chances of survival. Moreover, birds are very diverse and have different shapes of their head and eye structure across different species, so there would be variation in the strategies employed with respect to handedness, if any.

A related study in our research group has found that handedness in budgerigars, in tasks like route selection, may be different in another task, such as which leg to land on a perch for the same individual bird. Not only does this show the amazing plasticity in the animal mind, but also the complex nature of animal thought.

Our study was carried out on individual birds, but in nature birds like budgerigars live in flocks. The primary objective of a flock of birds is to maintain a tight formation. Each bird tends to maintain a safe distance from its neighbour and at the same time retain a constant flight speed and direction.

One question we would love to address is what would happen when a flock of birds tries to negotiate several obstacles, such as the branches of a tree? How would the birds behave under such conditions? Would flock dynamics have any role to play in the flight paths? Would the birds show similar side bias? Or would flock dynamics work in concert with side bias to determine decision-making? Does ‘handedness’ have a population bias? If so, what possible selective advantage could it confer?

A lot more needs to be explored to further our understanding about the complicated nature of the animal mind. In addition, the future holds exciting possibilities because of the potential application of the novel algorithms developed in this area of research to the design of advanced flying machines and the design of bird-friendly buildings and wind turbines.

Partha Bhagavatula is a postdoctoral fellow at Harvard University’s Concord Field Station.