What Can Birds Hear?

For bids, hearing is second in importance only to vision for monitoring the world around them. ./\vim hearing is most sensitive to sounds from about 1 to 4 kHz, although they can hear higher and lower frequencies. No species of bird has shown sensitivity to ultrasonic frequencies (>20 kHz). Sensitivity to frequencies below 20 Hz (dasound) has not received much attention; however, pigeons and a few other species have shown behavioral and physiological responses to these low frequencies. In general, frequency discrimination in birds is only about one-half or one-third as good it is for humans within the 1 4 kHz range. A problem that birds suffer that is similar to humans is damage to the auditory receptors (hair cells) from loud noises. The sound intensity that produces damage and the amount of damage produced differs depending on the species. Buds residing in the active areas of airports might be constantly subjected to sound pressure levels that damage their hearing. Thus, to effectively disperse birds using sound, auditory alerts must be at frequencies that can he detected by the damaged auditory receptors. Although some if not all species of birds have the abhty to repair damaged hair cells, continued exposure to loud noises would prevent recovety of their hearing. In this paper I review what is h o w about avian hearing and compare that to the operational charactenstics (frequencies, intensities, duration) of techmques and devices to disperse buds. K E Y WORDS: birds, deterrents, hearing, infiasound, sound, ultrasound INTRODUCTION Birds present a hazard to aviation and depredate many crops. Although lethal control is necessary in many situations, it is oflen more desirable to use nonlethal techniques to disperse or deter birds from selected locations, for a variety of reasons. One category of deterrentldispersal techniques is sound. To maximize their effectiveness, the sounds that are used must: 1. be loud enough to be audible to the birds, 2. be within the frequency m g e the birds' ears can detect, and 3. provide a biologically relevant message such that the birds depart. Given this knowledge, we can compare the operational characteristics of sound dispersal devices that are available on the market and make some predictions about their efficacies. AVIAN HEARING Avian ears and hearing differ from those of humans and other mammals in several ways, some obvious and some not. The first, obvious difference is that birds lack an external ear or pinna. Terrestrial mammals use the pinna and external ear canal to concentrate sound and increase the sensitivity of the ear. The sound travels down the auditory canal to the eardrum (tympanic membrane) where it produces vibrations in the fluid-filled inner ear. Transmission of vibrations from the e a r d m to the inner ear, where sound information becomes encoded in the nervous system, is mediated by the ear ossicles (bony elements). Birds have a single ossicle, the columella, compared to three in mammals. The theoretical amplification for a single element is about 20-fold from the tympanum to the fluid of the inner ear. The inner ear of birds serves two functions: equilibrium and hearing. H d g takes place in the cochlea. Unlike the coiled mammalian cochlea, the avian cochlea is a straight or slightly curved tube whose length differs among species. In pigeons (Columba livia) it is about 5 mm long but over 1 cm in the ham owl (Tyto alba) (Schwartzkopff 1968, Smith 1985). The differences in length, both among avian species and between birds and mammals, probably reflect differences in the m g e of frequencies that the species can detect. Longer cochlea allow for more auditory receptors and better sensitivity to either a wider range of frequencies or better resolution among frequencies. The auditory sensory receptors are the hair cells, which are similar in form and function to those of other vertebrates. These cells are equipped with cilia that are stimulated by the vibrations in the fluid of the cochlea. Because of the differences in cilia lengths and the locations of the cells along the basilar membrane, individual cells are most sensitive to specific frequencies; i.e., they are tuned to a narrow band of frequencies. Consequently, the information sent to the brain contains encoded frequency information. As might be expected, species differ in their sensitivities and range of sensitivities to frequencies of sound (Table 1). Different species of birds have the greatest sensitivity to sounds within a relatively narrow range. For most avian species this is around 1 4 kHz, but some species are sensitive to lower or higher frequencies (Konishi 1970, Hienz et al. 1977). Pigeons are most sensitive to sound between 1 2 kHz, with an absolute upper limit of about 10 kHz (Goerdel-Leich and Schwartzkopff 1984). None of the avian species that have been examined has shown sensitivity to frequencies above 20 kHz (ultrasound) (Schwartzkopff 1973) and generally the upper threshold is about 10 kHz (Hamershock 1992, Necker 2000). Sensitivity to inhsound (less than 20 Hz) has been observed in the pigeon and in some other species but not in all species tested (Yodlowski et al. 1977, Kreithen and Table I. Species-specific sensitivities to frequencies, peak sensitivity, and range of sensitivities.