Pursuing prey through clutter is a complex and risky activity requiring integration of guidance subsystems for obstacle avoidance and target pursuit. The unobstructed pursuit trajectories of Harris' hawks Parabuteo unicinctus are well modelled by a mixed guidance law feeding back target deviation angle and line-of-sight rate. Here we ask how their closed-loop pursuit behavior is modified in response to obstacles, using high-speed motion capture to reconstruct flight trajectories recorded during obstructed pursuit of maneuvering targets. We find that their trajectories are well modelled by the same mixed guidance law identified previously, which produces a tail-chasing behavior that promotes implicit obstacle avoidance when led by a target that is itself avoiding clutter. When presented with obstacles blocking their path, hawks resolve the pursuit-avoidance conflict by applying a bias command that is well modelled as an open-loop steering correction aiming at a clearance of one wing length from an upcoming obstacle.

The flight behaviour of predatory birds is well modelled by a guidance law called proportional navigation, which commands steering in proportion to the angular rate of the line-of-sight from predator to prey. The line-of-sight rate is defined with respect to an inertial frame of reference, so proportional navigation is necessarily implemented using visual-inertial sensor fusion. In Harris' hawks, pursuit of terrestrial targets is even better modelled by assuming that visual-inertial information on the line-of-sight rate is combined with visual information on the deviation angle between the attacker's velocity and the line-of-sight. Here we ask whether a new variant of this mixed guidance law can model Harris' hawk pursuit behaviour successfully using visual information alone. We use high-speed motion capture to record n=228 attack flights from N=4 Harris' hawks, and confirm that proportional navigation and mixed guidance using visual-inertial information both model the trajectory data well. Moreover, the mixed guidance law still models the data closely if visual-inertial information on the line-of-sight rate is replaced with purely visual information on the apparent motion of the target relative to the background. Whilst the original form of the mixed guidance law provides the best model of the data, all three models can model the behavioural data phenomenologically, whilst making different predictions on the physiological pathways involved.

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