When hawks attack: animal-borne video studies of goshawk pursuit and prey-evasion strategies

Abstract

Video filmed by a camera mounted on the head of a Northern Goshawk (Accipiter gentilis) was used to study how the raptor used visual guidance to pursue prey and land on perches. A combination of novel image analysis methods and numerical simulations of mathematical pursuit models was used to determine the goshawk's pursuit strategy. The goshawk flew to intercept targets by fixing the prey at a constant visual angle, using classical pursuit for stationary prey, lures or perches, and usually using constant absolute target direction (CATD) for moving prey. Visual fixation was better maintained along the horizontal than vertical direction. In some cases, we observed oscillations in the visual fix on the prey, suggesting that the goshawk used finite-feedback steering. Video filmed from the ground gave similar results. In most cases, it showed goshawks intercepting prey using a trajectory consistent with CATD, then turning rapidly to attack by classical pursuit; in a few cases, it showed them using curving non-CATD trajectories. Analysis of the prey's evasive tactics indicated that only sharp sideways turns caused the goshawk to lose visual fixation on the prey, supporting a sensory basis for the surprising frequency and effectiveness of this tactic found by previous studies. The dynamics of the prey's looming image also suggested that the goshawk used a tau-based interception strategy. We interpret these results in the context of a concise review of pursuit-evasion in biology, and conjecture that some prey deimatic 'startle' displays may exploit tau-based interception.KEY WORDS: Pursuit-evasion, Predator-prey, Avian vision, Northern Goshawk, Visual guidance, Sensory ecology, Accipiter gentilis, Looming, Antipredator behavior, Startle effect INTRODUCTIONMany problems in the study of animal behavior require an integrated biomechanical and sensory ecology approach that considers the organism's locomotor and perceptual capabilities, its sensory cues and the dynamics of its behavioral responses (Fernández-Juricic, 2012;Martin, 2012). Animal-borne video methods (Rutz and Troscianko, 2013) now make it possible to measure the visual cues received by organisms in the field, revealing new information about, for example, how they move through their environment (BBC, 2009), interact with conspecifics (Takahashi et al., 2004) and use tools (Rutz et al., 2007). Recent studies have used the stable video recorded by cameras mounted on the heads of birds (headcams) to explore the visual strategies used by falcons chasing prey (Kane and Zamani, 2014) and peafowl detecting model predators (Yorzinski and Platt, 2014). Here, we report using headcam video to explore for the first time pursuit-evasion and landing behavior in the Northern Goshawk, Accipiter gentilis (Linnaeus 1758) (hereafter, goshawk), a large diurnal raptor (Fig. 1). Biologically derived models have inspired robotic algorithms for swarming, following and collision avoidance (Mischiati and Krishnaprasad, 2012;Srinivasan, 2011), and the goshawk is of special inte...

Sneak peek: Raptors search for prey using stochastic head turns

“…In each case, the goshawk visually tracked the motion of the object of interest in a well-defined retinal fixation area, using rapid head saccades to keep the object at the same point in the image. Comparison with results from a previous study showed that this retinal fixation area was consistent with the center of the bird's visual field, determined by finding the center of motion in the optical flow field during level flight (Kane et al 2015). Since the goshawk flies with its head axis aligned with its body axis and forward velocity, this confirmed that the goshawk tracked objects of interest at the center of its visual field, which corresponds to the head and body's forward direction.…”

“…The falconer was licensed and had all necessary permits, and all activities followed all relevant regulations and laws of the Netherlands. A previous study has documented that the goshawk displayed normal flight and other behaviors while wearing the head-camera methods described here (Kane et al 2015). Two types of natural goshawk prey were hunted by the goshawk: Ring-necked Pheasants (Phasianus colchicus; Giudice and Ratti 2001) and European rabbits (Oryctolagus cuniculus; Tislerics 2000), both common, non-endangered species.…”

“…Because head nystagmus ensured that the video frame remained horizontal and stable during maneuvering, no deshake image-stabilization post-processing was used. Image analysis was performed using the Fiji installation of ImageJ (Schindelin et al 2012;accessed March 26, 2015); an in-depth description of the video methods and image analysis is presented in Kane et al (2015). Goshawk head-camera video was filmed during a wide variety of behaviors (see behavior codes in Table 1).…”

“…Red-tailed Hawks hunt ground-dwelling smallto-medium mammals, reptiles, and birds primarily, and are less maneuverable in flight than either of the accipiters studied (Preston and Beane 2009 Rourke et al 2010a). Although it has been noted that measurements of eye motions made when the bird's head is immobilized may overestimate eye motion when the head is free to move (Land 2015), the goshawk's limited range of eye motion has been estimated previously at up to 638 full range, from field video made with the head unrestrained (Kane et al 2015). Goshawks and Red-tailed Hawks have similar retinal visual receptor densities, foveal geometries, and visual fields (Fite and Rosenfield-Wessels 1975) and similar eye axial lengths and corneal diameters (Hall and Ross 2007), which indicates that they should also have similar visual acuities (Land and Nilsson 2012).…”

How moths escape bats: predicting outcomes of predator-prey interactions

“…Moths did not need to exceed the radial acceleration of their predators to escape, as predicted by the turning gambit (Howland, 1974), but they did need to make a sufficiently fast and tight turn to avoid being followed. Bats, as well as dragonflies and goshawks, use a constant absolute target direction strategy (also referred to as motion camouflage) for intercepting evasive prey (Olberg et al, 2000;Kane et al, 2015). It appears that with this predator pursuit strategy, a key component of successful prey evasion is making a rapid and tight turn that will be difficult for the predator to follow.…”