Avian Risk Assessment: Effects of Perching Height and Detectability

Abstract

We studied two components of predator risk assessment in birds. While many species are limited to seeking safety under cover or under ground, some birds can fly away from their predators and escape to trees. If birds in fact ÔfeelÕ safer (e.g. perceive less risk) in trees, we would expect them to tolerate closer approach by a potential terrestrial predator. Another component of safety is at which point the animal detects an approaching threat, which we expected to increase with eye size, assuming eye size is a surrogate for visual acuity. We used the distance birds moved away from an approaching human [flight initiation distance (FID)] as a metric to determine whether birds associated a lower risk of predation by being in trees, and we used the distance at which birds first displayed alert behaviors from an approaching human (alert distance) to determine if birds with larger eyes had higher detection distances. Although some species were affected by tree height, we found no clear pattern that birds assessed themselves to be at a lower risk of predation when they were ‡3 m above the ground compared with being <3 m above ground. In the 10 species for which height had any significant effect on FID, birds ‡3 m off the ground had greater FIDs in six species, but the remaining three species had the opposite response. While we found a significant positive relationship between eye size and alert distance in 23 species, the relationship was not present in a phylogenetic analysis using independent contrasts, which suggests that the apparent relationship was influenced strongly by the association between the studied species. Together, these results suggest that birds do not obviously associate being in a tree with safety, and that variations in visual acuity, per se, cannot be used as a general indicator of differences in alert distances, as previously suggested in the literature.

Frugivory by birds on Miconia albicans (MELASTOMATACEAE), in a fragment of cerrado in São Carlos, southeastern Brazil

“…Only treelet height showed to be a relevant factor for bird attraction. Since safety is not completely associated with perching height (Blumstein et al, 2004), it is the interaction with many other factors that results in the tree choice by the birds: forraging patterns, plant community composition and structure, predation risk (Howe, 1979;Robinson and Holmes, 1982;Levey and Moermond, 1984;Hasuí et al, 2007).…”

The higher the better: sentinel height influences foraging success in a social bird

“…Sentinels positioned higher in a tree may be able to see further and /or have a wider field of view, and thus be more likely to spot predators sooner (Blumstein et al 2004b;Fernández-Juricic et al 2004a). The detection of terrestrial predators in particular could be enhanced by a higher position; height may be less likely to make a difference when scanning for aerial predators (Blumstein et al 2004b).…”

“…External factors, such as the position adopted by the sentinel, may also play an important role. For example, higher perched individuals may spot predators at a greater distance (see Blumstein et al 2004b;Fernández-Juricic et al 2004a) and hence might be perceived as more reliable. Sentinels occupy different positions during different bouts, and they may move between trees or change their position in the same tree within a particular bout (Gaston 1977;Wright et al 2001).…”

To run or to fly: low cost versus low risk escape strategies in blackbirds

“…We used the blackbird (Turdus merula) as the model prey species due to its abundance and previous knowledge on its escape behaviour (Fernández-Juricic & Tellería, 2000;Fernández-Juricic et al, 2002;Blumstein et al, 2004). We used humans as model predators since human disturbance can cause anti-predator responses similar to those elicited by natural predators (Frid & Dill, 2002).…”

“…We used humans as model predators since human disturbance can cause anti-predator responses similar to those elicited by natural predators (Frid & Dill, 2002). Previous studies have corroborated that blackbirds responded to humans as if they were potential predators (e.g., Fernández-Juricic & Tellería, 2000;Fernández-Juricic et al, 2002;Blumstein et al, 2004).…”