Predator perches: a visual search perspective

Abstract

Summary 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.

Nestling diet optimization and condition in relation to prey attributes and breeding patch size in a patch-resident insectivorous passerine: an optimal continuum and habitat constraints

“…Consequently, owing to the lack of the compact layer of vegetation normally occurring in grasslands, the soil on abandoned farmland was drier. Presumably this is beneficial for some soil-dwelling invertebrates, such as ants, some coleopterans and orthopterans (which are abundant in the diet of the nestling Whinchats we examined), which are also easier to catch on bare ground or in lower vegetation (Andersson et al 2009;Schaub et al 2010). In addition, it should be stressed that Whinchats hunt primarily from a perch, so both the denser vegetation (i.e.…”

“…Further, our dietary analysis showed that for Whinchats soil-dwelling invertebrates tended to be of secondary importance as food because the majority of prey items ([70 % in each age class) were those associated with vegetation. This seems to imply indirectly the importance of a mosaic-like structure of the microhabitat with low vegetation or vegetation-free patches and bare ground, where prey are more accessible and the hunting of foraging Whinchats is more efficient (Bastian et al 1994;Bastian and Bastian 1996;Andersson et al 2009). Lastly, we were able to identify some invertebrate prey items only to the level of class/order (such as Araneae or Lepidoptera larvae), which to some degree hampered a more detailed analysis and the drawing of further inferences.…”

“…In addition, it should be stressed that Whinchats hunt primarily from a perch, so both the denser vegetation (i.e. more concealed prey) and the perch height (or even the lack of perches in modern grasslands) are of critical importance for access to prey and hunting efficiency (Andersson 1981;Andersson et al 2009). This concurs with the assumption that the mobility of ground-foraging insectivorous passerines and access to food are of greater importance during the selection of a foraging site than the abundance of food per se, which translates into preferential foraging in short vegetation, even though more invertebrate prey were available in tall vegetation (discussed in van Oosten et al 2014;Schaub et al 2010).…”

Influence of anthropogenic features and traffic disturbance on burrowing owl diurnal roosting behavior

“…Fences and posts in this study were of a similar height (~1.25 m tall) and likely used by the owls as perches to increase their field of vision (Andersson et al 2009) and maximize detection of predators. Increased visual detection should shorten response time (Devereux et al 2006) and therefore increase the probability of successfully reacting to a threat (Kenward 1978, Fitz gibbon 1990, Krause & Godin 1996.…”

“…Though only available to a small portion of our sample of owls, perches > 2 m tall (power lines and tall shrubs) were not used by any individuals. Larger raptors such as hawks prefer taller perches (Bohall & Collopy 1984, Berkelman et al 2002, Leyhe & Ritchison 2004) because of increased prey visibility (Andersson et al 2009). Increased risk from other raptors is the most likely reason why burrowing owls do not use these taller perches.…”