Conquering Visual Challenges While Hunting Across the Air-Water Interface; a Study of Foraging Day Herons

Among several visual challenges of hunting across the air-water interface are glare, which obscures prey below the water surface; and refraction, which causes submerged prey to appear where they are not. These challenges are exacerbated by the fact that prey have visual (and other) adaptations for detecting predators. To forage successfully on underwater prey, predators must compensate. Herons are good model animals for studying how predators compensate for visual challenges at the air-water interface; they are visually-hunting, fish-eating birds. Herons also regularly tilt their long necks (“head-tilt”) while hunting, which I hypothesized could aid in compensating for glare, refraction, and being detected. I also hypothesized that foraging herons would face away from the sun to mitigate glare effects. I made field observations to investigate orientations of foraging herons relative to sun position. I used high-speed videography, 3D digitization of herons’ head-tilts, and sea surface reflectance models to test whether head-tilting was related to glare or refraction. I exposed fish to heron decoys in head-tilted and upright positions, and to a non-predatory control, to test whether head-tilting helped herons to avoid being detected by prey. My results suggest that herons do not behaviorally mitigate glare via body orientation or head-tilting, so the question of how herons compensate for glare remains. I did not find evidence that head-tilting aids in correcting for refraction. Rather, I found some evidence that herons head-tilt to avoid detection by positioning the head/bill at angles which would be difficult for fish to see, due to refraction