Acoustical localization of concealed prey by the diurnal harrier (Circus cyaneus)

• Main Author: Rice, William Richard
• Other Authors: Wiens, John A., Menge, Bruce A., Zoology, Oregon State University. Graduate School
• Format: Doctoral or Postdoctoral Thesis
• Language: English; unknown
• Published: Oregon State University
• Subjects: Circus cyaneus; Harriers; Ruff
• Online Access: https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2b88qg21z

I experimentally examined the adaptive significance of the anatomical and behavioral convergence between the harrier (Circus cyaneus, also refered to as the marsh hawk) and certain owls that are capable of capturing prey in total darkness. Anatomically, both the harrier and owls have evolved a sound collecting facial ruff (a curved wall of skin and feathers surrounding the ears) analogous to the mammalian pinna. Behaviorally, both the harrier and the owls forage very close to the substrate. This position substantially reduces the field of view but concomitantly increases the intensity and directional resolution of the acoustical cues emitted by concealed prey. There are at least two mutually exclusive evolutionary alternatives for localizing highly concealed prey. A predator relying on high resolution vision can circumvent the reduced visibility of individual prey by foraging from a high perch or flight position. Increased foraging height can increase the field of view and hence the number of prey simultaneously scanned. The likelihood of detecting a vulnerable prey item can thus increase with increasing field of view. A second alternative is to employ sonic cues that can penetrate the vegetational barrier to visual prey detection. This alternative constrains a raptor to forage from a low position due to the restricted range of sonic prey localization compared to visual prey localization. Thus, a raptor detecting concealed prey through sonic cues must forfeit the larger field of view available to raptors relying exclusively on visual cues. In this research I test the hypothesis that the diurnal harrier (Order Falconiformes) has converged with certain nocturnal owls (Order Strigiformes) in the ability to sonically locate concealed prey. I also test the idea that a reliance on auditory cues in part explains the harrier's low foraging position. My thesis research was divided into two parts, a laboratory and a field study. In the laboratory, the auditory localization capacity of the harrier was compared to that of ...