Behavioural lateralisation of swans in response to anthropogenic disturbance differs according to the locomotion type

Abstract

The European population of Cygnus columbianus bewickii has a declining trend in number of individuals. Anthropogenic disturbance could be one of the reasons for this decline. Disturbance influences animal behaviour, including the manifestation of behavioural lateralisation. Therefore, investigating the impact of anthropogenic disturbance on behavioural lateralisation is essential for biodiversity conservation. Behavioural lateralisation manifests itself in a preference to use one of two paired organs (limbs or sensory organs) and a preference to avoid obstacles from a certain side. Earlier studies of behavioural lateralisation did not consider the locomotion type as an independent variable factor in the analysis, although it could affect the manifestation of behavioural lateralisation. We studied the influence of anthropogenic disturbance on behavioural lateralisation of swans, depending on the type of locomotion (swimming or flying). We have analysed 492 photos from aerial counts of two swan species (Cygnus columbianus bewickii, C. cygnus) in Yamal Peninsula and Gydan Peninsula. The photos were taken from a plane, while the birds were escaping from it as a source of anthropogenic disturbance. Pairs without and with chicks alone or in flocks were encountered swimming or flying. We found that swimming swans had a strong right-sided bias and right-eye bias for avoidance and observing the source of anthropogenic disturbance, and flying swans had a left bias. Swimming C. c. bewickii and C. cygnus exhibited similar behavioural lateralisation. These results were the same for following and leading birds. The presence of chicks did not change the direction of behavioural lateralisation but strengthened it for the following partners. The differences in behavioural lateralisation could be caused by the fact that swans in flight experience greater fear of a present aircraft than when they are on water. We conclude that the locomotion types influence behavioural lateralisation in response to anthropogenic disturbance. We recommend paying attention to accompanying factors when comparing the results of lateralisation studies. As the left side bias of flying birds in our study indicates that flying birds are more stressed than swimming ones, we recommend not forcing birds to fly during observations to reduce their stress.