Hawk Eyes II: Diurnal Raptors Differ in Head Movement Strategies When Scanning from Perches

Colleen T O'Rourke,Esteban Fernández-Juricic,Todd Pitlik,Melissa Hoover

doi:10.1371/journal.pone.0012169

Abstract

BackgroundRelatively little is known about the degree of inter-specific variability in visual scanning strategies in species with laterally placed eyes (e.g., birds). This is relevant because many species detect prey while perching; therefore, head movement behavior may be an indicator of prey detection rate, a central parameter in foraging models. We studied head movement strategies in three diurnal raptors belonging to the Accipitridae and Falconidae families.Methodology/Principal FindingsWe used behavioral recording of individuals under field and captive conditions to calculate the rate of two types of head movements and the interval between consecutive head movements. Cooper's Hawks had the highest rate of regular head movements, which can facilitate tracking prey items in the visually cluttered environment they inhabit (e.g., forested habitats). On the other hand, Red-tailed Hawks showed long intervals between consecutive head movements, which is consistent with prey searching in less visually obstructed environments (e.g., open habitats) and with detecting prey movement from a distance with their central foveae. Finally, American Kestrels have the highest rates of translational head movements (vertical or frontal displacements of the head keeping the bill in the same direction), which have been associated with depth perception through motion parallax. Higher translational head movement rates may be a strategy to compensate for the reduced degree of eye movement of this species.ConclusionsCooper's Hawks, Red-tailed Hawks, and American Kestrels use both regular and translational head movements, but to different extents. We conclude that these diurnal raptors have species-specific strategies to gather visual information while perching. These strategies may optimize prey search and detection with different visual systems in habitat types with different degrees of visual obstruction.

Highlights

Establishing what animals are looking at has intrigued zoologists [1,2,3,4], because it can help understand the mechanisms of visual information gathering, and the processes behind visual attention [5]
Cooper’s Hawks, Red-tailed Hawks, and American Kestrels use both regular and translational head movements, but to different extents. We conclude that these diurnal raptors have species-specific strategies to gather visual information while perching
We found that regular head movements were used significantly more frequently than translational head movements (Fig. 1a–b): Red-tailed Hawks (F2,14 = 86.77, P,0.001), Cooper’s Hawks (F2,8 = 76.32, P,0.001), American Kestrels (F2,8 = 60.30, P,0.001)

Summary

Introduction

Establishing what animals are looking at has intrigued zoologists [1,2,3,4], because it can help understand the mechanisms of visual information gathering (i.e., detection of color, motion, size, etc.), and the processes behind visual attention (i.e., object recognition, reduction in visual uncertainty, etc.) [5]. Measuring the targets of visual attention can have important methodological applications for the study of animal communication, social interactions, food search, mate choice, and antipredator behavior in multiple taxa [6,7,8]. In species with laterally placed eyes, visual targets can be at both sides of the head simultaneously, rather than at the front [8]. Little is known about the degree of inter-specific variability in visual scanning strategies in species with laterally placed eyes (e.g., birds). This is relevant because many species detect prey while perching; head movement behavior may be an indicator of prey detection rate, a central parameter in foraging models. We studied head movement strategies in three diurnal raptors belonging to the Accipitridae and Falconidae families

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