Graduate Thesis Or Dissertation
 

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

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  • 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 an owl species (barn owl, Tyto alba) with an experimentally established ability to acoustically locate prey in total darkness, to a second owl species (short-eared owl, Asio flammeus) with a behavioral repertoire similar to that of the harrier, and two typical diurnal raptor species (red-tailed hawk, Buteo jamaicensis and the American kestrel, Falco sparverius) lacking apparent anatomical specializations for directional hearing. Two individuals from each species were tested. The directional hearing of the harrier was found to be superior to that of the sample of typical diurnal raptors and similar to that of the sample of owls. Specifically, the horizontal resolution was two degrees for the harriers, eight to twelve degrees for the sample of typical diurnal raptors, and one to two degrees for the sample of nocturnal owls. The range of effective sonic prey detection was estimated to be three to four meters for the harrier compared to seven meters for the barn owl. In the field experiments a subterranean wiring network (test grid) was used to broadcast synthetic vole vocalizations (squeaks) from concealed miniature loudspeakers. The speakers were covered with replaceable plastic membranes. When a wild free-ranging raptor flew over the test grid, the vole vocalization was broadcast at a normal volume. The sound stimulus was terminated when the bird approached to within three to four meters of the speaker. Five harriers were tested for a total of ten trials. In all but one case the harriers responded to the sound stimulus by striking through the overlying vegetation and piercing the speaker membrane with their talons. Neither of two American kestrels that were tested responded to the auditory stimulus even when actual recordings of the resident voles were used. Because no visual or olfactory cues were available, the harriers determined the simulated vole position exclusively through auditory cues. The field experiments also demonstrated that the sonic localization capacity determined under highly idealized conditions in the laboratory is sufficiently elaborate to compensate for the distortions produced by heterogeneities in the more complex natural environment. Thus the harrier, like many nocturnal owls, is capable of capturing concealed prey by locating them through sonic cues. Because the sonic localization capacity of the harrier only has an effective range of three to four meters, this species is constrained to forage relatively close to the substrate while employing sonic prey detection.
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