Behavioral Lateralization and Optimal Route Choice in Flying Budgerigars

Birds flying through a cluttered environment require the ability to choose routes that will take them through the environment safely and quickly. We have investigated some of the strategies by which they achieve this. We trained budgerigars to fly through a tunnel in which they encountered a barrier that offered two passages, positioned side by side, at the halfway point. When one of the passages was substantially wider than the other, the birds tended to fly through the wider passage to continue their transit to the end of the tunnel, regardless of whether this passage was on the right or the left. Evidently, the birds were selecting the safest and quickest route. However, when the two passages were of equal or nearly equal width, some individuals consistently preferred the left-hand passage, while others consistently preferred the passage on the right. Thus, the birds displayed idiosyncratic biases when choosing between alternative routes. Surprisingly - and unlike most of the instances in which behavioral lateralization has previously been discovered - the bias was found to vary from individual to individual, in its direction as well as its magnitude. This is very different from handedness in humans, where the majority of humans are right-handed, giving rise to a so-called ‘population’ bias. Our experimental results and mathematical model of this behavior suggest that individually varying lateralization, working in concert with a tendency to choose the wider aperture, can expedite the passage of a flock of birds through a cluttered environment.Author Summary: Birds display a clear mastery of the skill of flying rapidly and safely through complex and cluttered environments. An example of this can be viewed at http://www.youtube.com/watch?v=p-_RHRAzUHM, which shows a bird flying at high speed through a dense forest. Such mastery requires the ability to determine, from moment to moment, which of several possible routes would provide the safest and quickest passage. Our study is one of the first to investigate how birds achieve this. Our experiments reveal that, when flying budgerigars are required to choose between two passages, they tend to favor the wider passage. However, this tendency is superimposed upon a bias that, surprisingly, varies from bird to bird: some individuals show an intrinsic preference for the left-hand passage, and others for the passage on the right. This is very different from handedness in humans, where the majority of humans are right-handed. We develop a mathematical model of the interaction between the birds' individual biases with their tendency to prefer the wider passage. The model reveals that this interplay is actually beneficial – it can expedite the passage of a flock of birds through a complex environment.