Predator perches: a visual search perspective

1. Predators hunting by sight often search for prey from elevated perches or hovering positions above the prey habitat. Theory suggests that prey visibility depends strongly on predator perch height and distance, but their quantitative effects have not been experimentally tested in natural habitats. 2. We estimate for the first time how prey visibility depends on predator perch height, distance and vegetation height in an open natural habitat, based on visibility measurements of two targets: a mounted bird and a graduated plate, from five perch heights (0.2-8 m) and six distances (5-120 m). 3. For both targets, their proportion visible increases strongly with observer perch height and proximity. From the lowest perch, visibility of the target bird declines to <5% beyond 20 m distance, but 40% of it remains visible from the highest perch even at 120 m. 4. Models of predator search suggest that hunting success and predation rate depend strongly on the prey detection rate, which is expected to decline with distance r approximately as r(-d). However, d, the distance decay parameter, has not previously been empirically estimated in natural predator habitats. For distance - prey visibility relationships similar to those observed here, we find a realistic estimate of d to be 2.1-2.4. 5. The results demonstrate the crucial role of relative perch and vegetation height for prey visibility, which is of relevance for habitat management. The strong increase of prey visibility with predator search height suggests that removal of predator perches can improve the survival of endangered prey populations in open habitats. Conversely, perch preservation or addition can improve habitat suitability for some predator species where perches are rare or lacking.