Falcons pursue prey using visual motion cues: new perspectives from animal-borne cameras

Abstract

This study reports on experiments on falcons wearing miniature videocameras mounted on their backs or heads while pursuing flying prey. Videos of hunts by a gyrfalcon (Falco rusticolus), gyrfalcon (F. rusticolus)/Saker falcon (F. cherrug) hybrids and peregrine falcons (F. peregrinus) were analyzed to determine apparent prey positions on their visual fields during pursuits. These video data were then interpreted using computer simulations of pursuit steering laws observed in insects and mammals. A comparison of the empirical and modeling data indicates that falcons use cues due to the apparent motion of prey on the falcon's visual field to track and capture flying prey via a form of motion camouflage. The falcons also were found to maintain their prey's image at visual angles consistent with using their shallow fovea. These results should prove relevant for understanding the co-evolution of pursuit and evasion, as well as the development of computer models of predation and the integration of sensory and locomotion systems in biomimetic robots.

Keywords: Avian vision; Falcon; Motion camouflage; Predation; Pursuit-evasion.

Fig. 1.

Trajectories resulting from alternative pursuit strategies. (A) Classical pursuit; (B) constant bearing decreasing range (CBDR); and (C) motion camouflage with the baseline held at a constant absolute angle (after Ghose et al., 2006). In B and C, the instantaneous baseline vector, which points from the pursuer to the evader, is indicated by a dashed line. (D) Orientation of the shallow (S) and deep (D) visual fields and head axis (dashed line) in raptors (adapted from Tucker, 2000). The effect of refraction by the cornea is not shown. (E) Logarithmic spiral trajectory resulting from keeping the prey at optimal visual angle. ϕ, bearing angle; α, constant target angle; β, angle between evader velocity and baseline; v_e, prey (evader) speed; v_p, predator (pursuer) speed; t, time. See ‘List of symbols and abbreviations’.

Fig. 2.

Computer-simulated bird-mounted video images of prey during pursuits. (A) Schematic and (B) simulated prey position on the image for classical pursuit (red circles), motion camouflage (blue circles) and optimal visual angle (black squares). In B, black dashed circles enclose regions with different prey velocities, while vertical lines at θ=±23 deg indicate the edges of the video images. Shaded regions indicate the range of peak visual angles of the left deep (red) and shallow (gray) foveae for other raptor species. χ, angle of the apparent prey position along the vertical direction; θ, angle of the apparent prey position along the horizontal direction.

Fig. 3.

Prey tracking data from bird-mounted videos. (A) Tracked prey positions (red circles and lines) superimposed on a video image of a crow (arrow) during pursuit by a hybrid gyrfalcon/Saker falcon. Black circles indicate prey positions in regions of approximately constant bearing angle. (B) Plots of camera angles θ (black squares and lines) and χ (red circles and lines) versus time for the entire chase sequence for which an excerpt is shown in A. The gray shaded region indicates the peak visual angles for the left raptor shallow fovea for other raptor species.

Fig. 4.

Evidence for falcon use of the deep fovea. (A) Head saccades during chases directed the prey's image to angles not visible on the camera, but reconstructed using the motion of background objects: data for θ from on-camera (black squares and lines) and extrapolated prey tracks (red circles and lines) versus time. Shaded regions indicate the peak visual angles of the left deep (red) and shallow (gray) foveae for other raptor species. (B) Back-mounted video image of the head of a peregrine falcon looking downward during foraging for prey at high altitudes (Jason Jones, Teton Raptor Center, Wilson, WY, USA). (C) In one pursuit, the falcon wearing a camera recorded another gyrfalcon/Saker falcon capturing a crow in mid-air. These still images were recorded 67 and 33 ms before the falcon made contact with the crow (Eddy de Mol and Francois Lorrain).

Fig. 5.

Prey angle frequency distributions for all hybrid falcon pursuit data. (A) Horizontal (θ) and (B) vertical (χ) angles for prey distances z≥8 m (black squares and lines) and z<8 m (red circles and lines). Gray shaded regions indicate the peak visual angles of the left shallow fovea (A) and the retinal streak (B) for other raptor species.

Fig. 6.

Bird-mounted camera mounting geometries. (A) Schematic diagram showing the relative orientation of the camera optical axis and body axis in flight on the back-mounted videocameras. (B) Head-mounted videocamera on gyrfalcon/Saker falcon hybrid. (C) Pinhole optics geometry for measuring angles (θ, χ) of the prey from its image, and z, the distance from the falcon to its prey (not to scale). O, prey actual size (m); f, focal length (pixels); I, prey image size (pixels); h, distance above the eyes.