Wildlife in Airport Environments: Preventing Animal–Aircraft Collisions through Science-Based Management

4: Tactile and Auditory Repellents to Reduce Wildlife Hazards to Aircraft

Wildlife within the airport environment are hazards to human safety. Lethal removal of targeted individuals reduces the immediate threat, but other approaches should be integrated into control programs to make them more effective and to help meet legal and ethical considerations (Dolbeer et al. 1995). When negative media attention, special interest groups, or calls for restrictive legislation influence public opinion, the resulting public pressure can preclude effective wildlife management and lead to subsequent population control problems (Torres et al. 1996, Coolahan and Snider 1998, Conover 2001). Nonlethal management activities to reduce wildlife use of airports may include habitat modification, exclusion from roosting and nesting areas, and repelling animals from desired locations. When considering repellents alone, there are many that are untested, temporarily effective, or cost-prohibitive (Dolbeer et al. 1995). Effective nonlethal repellents must affect some aspect of physical receptors or psychological perception of the intended targeted animals. In birds and mammals the primary physical receptors are visual (see Chapter 2), auditory, and tactile (Dooling 1982, Fay 1988, Clark 1998a). As explained in Chapter 3, the sense of smell is also important for birds and mammals. In this chapter we focus on auditory and tactile repellents, particularly the physiological bases for tactile and auditory repellent efficacy. We also examine some behavioral aspects of species that influence the efficacy of repellents. Thomas W. Seamans James A. Martin Jerrold L. Belant Tactile and Auditory Repellents to Reduce Wildlife Hazards to Aircraft 4 Animal Sensory Capabilities One must account for the auditory capability of animals when evaluating acoustic frightening devices. Auditory capabilities are measured in part by sound frequency in Hertz (Hz) and sound pressure level (SPL), the logarithmic measure of the pressure of a sound in decibels (dB) relative to a standard reference pressure in air (dB SPL), typically 20 μPa. Despite physical differences , the ears of mammals and birds work remarkably similarly. One obvious difference between the two groups is that avian ears are not externalized, yet have feather patterns that can focus sound waves into the ear in much the same way as the external mammalian ear. The avian inner ear differs from the mammalian inner ear, with one interior bone instead of three (Gill 2007). Even though the avian ear is structurally simpler than the coiled cochlea of a mammal, with its straight or slightly coiled cochlea (inner ear), the acoustical efficiency of birds is similar to that of mammals (Gill 2007). In both mammals and birds, hair cells in the cochlea serve as auditory sensory receptors. However, some birds, unlike mammals, have the ability to regrow some damaged hair cells (Ryals et al. 1999, Stone and Rubel 2000). In general, birds hear well within a limited frequency range, whereas human hearing spans a wider range. Humans can detect sounds at frequencies from about 0.03 to 18 kHz (Heffner and Heffner 1992), with an absolute sensitivity at 0 dB SPL (Durrant and 38 wildlife management techniques Lovrinic 1984). Birds react most to sounds from 1 to 3 kHz, with an absolute sensitivity from –10 to 10 dB SPL (Dooling 1978, 1982; Stebbins 1983; Dooling et al. 2000). However, the range of sounds detected among species varies markedly. Downy woodpeckers (Picoides pubescens) are most sensitive to sounds from 1.5 to 4.0 kHz (Delaney et al. 2011), whereas barn owls (Tyto alba) are most sensitive from 6.0 to 7.0 kHz and at sound pressure levels as low as –18 dB SPL (Fay 1988). Rock pigeons (Columba livia) can detect low frequencies (0.05 Hz; i.e., in the infrasound range 152 m (Delaney et al. 2011). Factors such as breeding season, availability of natural and anthropogenic food resources, and predation can clearly interact to diminish or enhance repellent effectiveness. An understanding of the context of application is critical in determining the types and necessary integration of repellent methods. Auditory Repellents Biosonic Stimuli Auditory repellents are marketed as either ultrasonic, sonic, or biosonic calls. Human-made sounds are thought to frighten birds and therefore rely on the perception of danger (e.g., risk-disturbance hypothesis for nonlethal threats; Frid and Dill 2002). Loud (i.e., >90 dB SPL) sounds may also cause physical distress. The underlying assumptions of biosonic recordings of bird alarm or distress calls are that (1) birds perceive such calls as natural warnings that danger is present and will subsequently flee (Lima and Dill 1990, Hurd 1996, Goodale and Kotagama 2008) and (2) birds are not as likely...