Abstract
Behavioural side-bias occurs in many vertebrates, including birds as a result of hemispheric specialization and can be advantageous by improving response times to sudden stimuli and efficiency in multi-tasking. However, behavioural side-bias can lead to morphological asymmetries resulting in reduced performance for specific activities. For flying animals, wing asymmetry is particularly costly and it is unclear if behavioural side-biases will be expressed in flight; the benefits of quick response time afforded by side-biases must be balanced against the costs of less efficient flight due to the morphological asymmetry side-biases may incur. Thus, competing constraints could lead to context-dependent expression or suppression of side-bias in flight. In repeated flight trials through an outdoor tunnel with obstacles, tree swallows (Tachycineta bicolor) preferred larger openings, but we did not detect either individual or population-level side-biases. Thus, while observed behavioural side-biases during substrate-foraging and copulation are common in birds, we did not see such side-bias expressed in obstacle avoidance behaviour in flight. This finding highlights the importance of behavioural context for investigations of side-bias and hemispheric laterality and suggests both proximate and ultimate trade-offs between species-specific cognitive ecology and flight biomechanics.
Highlights
Hemispheric specialization, the division of neural processing tasks between the left and right hemispheres of the brain, is generally agreed to be responsible for sensoribehavioral side-biases in reptiles, birds, and mammals [1,2,3]
There was no evidence of functional asymmetry at the individual level; the side chosen between trials two and four did not differ significantly for either experiment one (500 permutations; actual statistic = 6; p = 0.634) or experiment two (500 permutations; actual statistic = 7; p = 0.389)
We found no significant directionality at the population-level in either tarsus or wing asymmetries, and the direction of an individual’s side choice in trial 2 was not related to either the direction of that same individual’s asymmetry for their wings or tarsi (4 G-tests, p.0.3 for all)
Summary
Hemispheric specialization, the division of neural processing tasks between the left and right hemispheres of the brain, is generally agreed to be responsible for sensoribehavioral side-biases in reptiles, birds, and mammals [1,2,3]. Brain lateralization is positively correlated with efficient neural processing and multitasking [1,3,13] Selection for such decision-making should lead to quicker response times, and might explain the apparent ubiquity of hemispheric specialization and side-bias in vertebrates [12,13]. Given the potential cost of behavioural side-bias in wing and muscular asymmetry (and reduced overall flight performance), we expected behavioral side-biases might instead be masked. To our knowledge, this is the first study to consider the potential conflict between selection for wing symmetry and selection for side-bias in flying birds
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