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8 FIGURE 3 Conceptual model depicting the different modes of repellents and behavior responses to the stimuli. Arrow width represents relative likelihood of response-stimulus association among birds (Source : Werner and Clark 2003). SENSES the extent of olfactory development in birds is comparable to that in mammals (Mason and Clark 2000). Olfactory cues The primary senses of birds targeted by repellent applica- may serve as conditional stimuli to which learned aversions tions include the chemical senses, vision (sight), audition can be formed when paired in the presence of toxicants or (hearing), and touch (e.g., tactile). If the chemical senses are irritants (Waldvogel 1989; Clark and Smeraski 1990; Raguso treated as one, the likelihood that a chemical repellent will and Willis 2002). The most effective avian repellents will fail is high because it will be designed and delivered in a con- likely be those that produce condition aversions (i.e., avoid- textually inappropriate manner. The chemical senses of an ance rather than escape behavior) in the target species (Rogers animal are composed of olfactory (smell), gustatory (taste), 1974; Mason and Clark 2000; Werner et al. 2008). Deterrents and chemesthetic (irritation and pain) systems (Mason and based merely on offensive flavors or altered flavors associ- Clark 2000). In terms of chemical signals, the integrated ated with a familiar food are not likely to be effective in the perception of all three chemosensory inputs is called flavor. absence of aversive, post-consumptive effects such as gastric Unlike hearing and sight, where the signals are distinctly dif- malaise (Provenza 1997). The coupling of novel odors asso- ferent in nature, the chemical senses involve similar stimuli ciated with chemicals such as pyrazine or methypyrazine is mediated through different sensory systems, which in turn more effective in reducing bird use of resources because of provide the context of the message. the intestinal malaise that creates a primary response (Avery and Nelms 1990; Avery and Mason 1997; Nelms and Avery Smell and Taste 1997). Gustation requires a more intimate contact between the source of the chemical signal and the receptors (Mason Birds can taste and smell, but little is known regarding the and Clark 2000). Gustatory receptors are located in taste buds level of specificity of avian tasting and smelling ability located throughout the oral cavity of birds (Berkhoudt 1985; (Strong 1911; Duncan 1960; Wenzel 1967, 2007; Mason and Ganchrow and Ganchrow 1985). Bird taste receptor sensitiv- Clark 2000). However, research indicates that some species ity is similar to that of mammals and is species specific in of birds have a moderate to excellent sense of smell (Strong their response to various chemicals (Moore and Elliott 1946; 1911; Duncan 1960; Waldvogel 1989; Wallraff et al. 1995; Duncan 1960; Berkhoudt 1985; Ganchrow and Ganchrow Roper 1999; Mason and Clark 2000; Wenzel 2007). Thus, 1985; Mastrota and Mench 1995).

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9 Sound Bioacoustics Sound is one form of communication used for territorial The use of bird alarm and distress calls to disperse birds defense, mate choice, navigation, song learning of individu- is based on sound biological principles. Alarm and distress als, and predator avoidance (Gill 1995). In the context of calls warn other birds in the area that danger is present, typi- repelling birds with sound, predator avoidance and territo- cally causing the other birds to flee. Birds are less likely to rial defense are the two mechanisms targeted. However, few habituate to alarm and distress calls than to other sounds empirical data are available regarding conspecific avoidance because they are related to evolutionary signals of danger behavior elicited through sound in wildlife damage research (Thompson et al. 1968; Johnson et al. 1985; Bomford and (Muller et al. 1997). O'Brien 1990). Auditory Reception Avian Vision The auditory capability of animals is important when consid- The primary sensory pathway in birds is vision (Sillman ering acoustic frightening devices. The frequency of sound 1973; Zeigler and Bischof 1993). However, it is evident is measured in Hertz (Hz), and sound pressure (volume) that there are species-specific vision characteristics (Sill- is measured in decibels at sound pressure level (dB SPL). man 1973; Zeigler and Bischof 1993; Blackwell 2002). To Humans can detect sounds from approximately 2020,000 effectively use light in managing bird conflicts with aviation, Hz (Bomford and O'Brien 1990) with an absolute sensitivity an understanding of avian vision is critical. Color and type of 0 dB SPL (Durrant and Lovrinic 1984). Ultrasonic fre- of light used to frighten birds have shown species-specific quencies are those above 20,000 Hz and infrasonic frequen- reactions ranging from indifference to flight (Belton 1976; cies those below 20 Hz. Blackwell 2002; Gorenzel et al. 2002). Many birds discrimi- nate the color of light at wavelengths between 400 and 700 Birds appear to be most receptive to sounds from 1,000 nm, comparable to humans (Pearson 1972). 3,000 Hz, with an absolute sensitivity of -10 to 10 dB SPL (Dooling 1978; Stebbins 1983; Fay and Wilber 1989; Dooling In addition, some species, including pigeons, mallards et al. 2000). However, the range of sounds detected among (Anas platyrhynchos), belted kingfishers (Megaceryle species varies markedly. For example, barn owls (Tyto alba) alcyon), and some passerines (Bowmaker and Martin 1985; hear best at 6,0007,000 Hz with volumes as low as -18 dB Martin 1986; Cuthill et al. 2000) also perceive ultraviolet SPL (Fay 1988). In contrast, pigeons can detect frequencies light (<390 nm). Rock doves (Columba livia) and some as low as 0.05 Hz (i.e., infrasound), but it is unclear how songbirds have also exhibited sensitivity to the plane of the birds use this capability (Fay and Wilber 1989; Fay and polarization of light (Able 1982; Young and Martin 1984), Popper 2000). to which humans have very limited sensitivity. The avian retina, consisting of high cone densities, deep foveae, near- Reception of high frequencies (>10,000 Hz) is very poor ultraviolet receptors, and colored oil droplets, is likely the in birds (Dooling 1978). Nocturnal predatory species (e.g., most capable daylight retina of any animal (Gill 1995). Fur- owls) generally hear better than other bird species, while thermore, because birds can apparently detect color, it could songbirds hear low frequencies better than nonsongbirds be an important consideration during the construction and (Dooling et al. 2000). development of devices used to deter and disperse birds.