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6 CHAPTER THREE PRINCIPLES OF AVIAN ECOLOGY AND BIOLOGY BIRD MOVEMENTS AND SPACE USE The airport environment comprises a relatively small land area in the context of bird movements and space use; thus it is likely only a small proportion of areas used for most spe- cies. Furthermore, birds likely spend only a small percent- age of time in the airport environment foraging, loafing, or raising young. Also, considering the patchiness of the typi- cal landscapes in and surrounding airports, bird likely use only portions of airports. There is also temporal variation in bird use of areas including airports. For migrant species, use may be restricted to fall and spring migration periods. Alter- natively, birds may be present only during winter or sum- mer to nest and raise young. Finally, resident species may use habitat on airports year-round. The mechanisms driving bird distributions in the context of habitat are important to consider because they can influence the timing of use and effectiveness of deterrents, hazing, and repellents. Dispersal Several models have been proposed to explain how indi- viduals within groups or subpopulations disperse from one location to another. Slatkin (1985) postulated that dispersal may simply be a random walk in space with few, if any, fac- tors driving species dispersal. However, it is not likely that most species perceive the environment in this manner, and this model is not well supported in the ecological literature. The ideal free distribution model (Fretwell and Lucas 1970), or balanced dispersal model (Doncaster et al. 1997), states that dispersal patterns are contingent on the fitness (e.g., increased survival or reproductive success) of the individual in a given habitat type, and dispersal is not constrained by population density in the other habitat patches. Source-sink dynamics is another type of model used to describe how variation in habitat quality can influence use and distribu- tion of animals. In source-sink models, the source is an area of higher quality habitat that on average can support more individuals and allows populations to increase. In contrast, a sink is an area of low-quality habitat that cannot sustain a population and generally supports low numbers of indi- viduals. For general source-sink models (Holt 1985; Pulliam 1988), dispersal is constrained between patches, density- independent or dependent dispersal are both possible, and habitat quality may vary greatly among patches. Also, as the name implies, the presence of a sink is assumed under source-sink models. Finally, Senar et al. (2002) proposed a model based on work with Citril finches (Serinus citronella), whereby animals may disperse from low- to high-quality sites because the high-quality sites act as pools of genetic variability and are sources of higher-quality food. Given the assumptions of each model, predictions may vary greatly regarding dispersal. Further, the response of individual pop- ulations to human and environmental disturbances, as well as land management actions and deterrent techniques, will depend on which model of dispersal is applicable in a given system and circumstances. Flocking Behavior Birds may form flocks of individuals of single or multiple species. Flock formation is a balance between costs and benefits to individuals within a flock by reducing the risk of predation and enabling cooperative foraging (Emlen 1952; Powell 1974; Caraco et al. 1980a,b; Caraco 1981; Tinber- gen 1981; Pulliam et al. 1982; Fernández-Juricic et al. 2004). Unstable and indefensible areas (e.g., food sources or loafing sites) promote flocking behavior (Verbeek 1972; Gill 1995). Verbeek (1972) found that corvids abandoned territories and developed flocks when food supplies became less stable and more widely and unevenly distributed. Flocking behav- ior has also been demonstrated to be a function of breed- ing activity in starlings (Davis 1970). Feeding in flocks can increase competition for food, but has been demonstrated to collectively increase foraging efficiency (Caraco 1981; Sullivan 1984). Cooperative feeding is common in species such as pelicans, cormorants, and mergansers (Bartholomew 1942; Emlen 1952). Flock members can also benefit from prey that is flushed by a flock-mate. For example, Cezilly et al. (1990) found that forage striking and number of captures per minute improved as flock size increased for little egrets (Egretta garzetta). Individual fitness of a bird can also be increased in flocks through reduced predation risk (Charnov and Krebs 1975; Sirot 2006). Predators can be confused by flock movements that make it more difficult to single out one individual (Lan- deau and Terborgh 1986). Page and Whitacre (1975) reported that merlin (Falco columbarius) hunting success varied according to prey flock size. Kenward (1978) found that goshawk (Accipiter gentilis) predation was also disparately lower when pigeon flock sizes were large. Another possible
7 hypothesis for reduced predation rates in flocks relates to increased vigilance for predator detection. Flock members can warn other birds of the presence of the predator using alarm calls or visual cues (Charnov and Krebs 1975). We use an example to place the aforementioned ecological concepts and flocking behavior into an airport management context. Merlins may use airport properties as a consequence of songbirds foraging in grasslands. These songbirds may pre- fer to forage in taller grass because of more abundant prey; thus under ideal free distribution we would expect higher abundance of songbirds in tall grass areas. Mowing areas of high grass closer to runways for aircraft safety would leave fewer areas of tall grass, further concentrating songbirds. These flocks may reduce foraging efficiency for merlins, thus reducing overall merlin use of airports and associated risk to aircraft. However, the larger flocks of songbirds would pose a greater risk to aircraft. As individual songbirds pose little risk to aircraft, management actions would not likely be directed toward these individuals. However, larger flocks of these birds may well trigger implementation of control measures. As Figure 2 shows, understanding ecological relationships within and between species, and how these species interact with their environment, is critical for maximizing efficiency and effectiveness of control measures. HARASSMENT, REPELLENT, AND DETERRENT TECHNIQUES Harassment, repellents, and deterrents encompass a wide range of techniques and methods used to manipulate behavior of birds to shift use away from an area or resource (Werner and Clark 2003). The use of a method or device must be coupled with an understanding the mode of behavior response; simply stated, the tool must of match the task, as shown in Figure 3. Essentially, two types of repellents existâprimary and sec- ondary (Clark 1998). Primary repellents cause involuntary withdrawal or escape behavior in an animal usually through taste, odor, or irritation (Clark 1998). Secondary repellents induce an undesirable physiological effect for the animal, such as gastric malaise. The goal of airport biologists is to create avoidance behavior such that the animal will discon- tinue occupying an area or to reduce ease of foraging for food in a given patch (Werner and Clark 2003). The periodicity of repellents is also an important determinant of their effective- ness. Devices used to repel, haze, and generally frighten ani- mals can be periodic, random, or motion activated (Gilsdorf et al. 2002). The timing of the stimuli has a direct impact on effectiveness. Random or animal-activated devices may reduce habituation and increase the time of protection over nonrandom (i.e., systematic) devices (Koehler et al. 1990). FIGURE 2 Integrated pest management (Source: Werner and Clark 2003).
8 the extent of olfactory development in birds is comparable to that in mammals (Mason and Clark 2000). Olfactory cues may serve as conditional stimuli to which learned aversions can be formed when paired in the presence of toxicants or irritants (Waldvogel 1989; Clark and Smeraski 1990; Raguso and Willis 2002). The most effective avian repellents will likely be those that produce condition aversions (i.e., avoid- ance rather than escape behavior) in the target species (Rogers 1974; Mason and Clark 2000; Werner et al. 2008). Deterrents based merely on offensive flavors or altered flavors associ- ated with a familiar food are not likely to be effective in the absence of aversive, post-consumptive effects such as gastric malaise (Provenza 1997). The coupling of novel odors asso- ciated with chemicals such as pyrazine or methypyrazine is more effective in reducing bird use of resources because of the intestinal malaise that creates a primary response (Avery and Nelms 1990; Avery and Mason 1997; Nelms and Avery 1997). Gustation requires a more intimate contact between the source of the chemical signal and the receptors (Mason and Clark 2000). Gustatory receptors are located in taste buds located throughout the oral cavity of birds (Berkhoudt 1985; Ganchrow and Ganchrow 1985). Bird taste receptor sensitiv- ity is similar to that of mammals and is species specific in their response to various chemicals (Moore and Elliott 1946; Duncan 1960; Berkhoudt 1985; Ganchrow and Ganchrow 1985; Mastrota and Mench 1995). 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 primary senses of birds targeted by repellent applica- tions include the chemical senses, vision (sight), audition (hearing), and touch (e.g., tactile). If the chemical senses are treated as one, the likelihood that a chemical repellent will fail is high because it will be designed and delivered in a con- textually inappropriate manner. The chemical senses of an animal are composed of olfactory (smell), gustatory (taste), and chemesthetic (irritation and pain) systems (Mason and Clark 2000). In terms of chemical signals, the integrated perception of all three chemosensory inputs is called flavor. Unlike hearing and sight, where the signals are distinctly dif- ferent in nature, the chemical senses involve similar stimuli mediated through different sensory systems, which in turn provide the context of the message. Smell and Taste Birds can taste and smell, but little is known regarding the level of specificity of avian tasting and smelling ability (Strong 1911; Duncan 1960; Wenzel 1967, 2007; Mason and Clark 2000). However, research indicates that some species of birds have a moderate to excellent sense of smell (Strong 1911; Duncan 1960; Waldvogel 1989; Wallraff et al. 1995; Roper 1999; Mason and Clark 2000; Wenzel 2007). Thus,
9 Sound Sound is one form of communication used for territorial defense, mate choice, navigation, song learning of individu- als, and predator avoidance (Gill 1995). In the context of repelling birds with sound, predator avoidance and territo- rial defense are the two mechanisms targeted. However, few empirical data are available regarding conspecific avoidance behavior elicited through sound in wildlife damage research (Muller et al. 1997). Auditory Reception The auditory capability of animals is important when consid- ering acoustic frightening devices. The frequency of sound is measured in Hertz (Hz), and sound pressure (volume) is measured in decibels at sound pressure level (dB SPL). Humans can detect sounds from approximately 20â20,000 Hz (Bomford and OâBrien 1990) with an absolute sensitivity of 0 dB SPL (Durrant and Lovrinic 1984). Ultrasonic fre- quencies are those above 20,000 Hz and infrasonic frequen- cies those below 20 Hz. Birds appear to be most receptive to sounds from 1,000â 3,000 Hz, with an absolute sensitivity of â10 to 10 dB SPL (Dooling 1978; Stebbins 1983; Fay and Wilber 1989; Dooling et al. 2000). However, the range of sounds detected among species varies markedly. For example, barn owls (Tyto alba) hear best at 6,000â7,000 Hz with volumes as low as â18 dB SPL (Fay 1988). In contrast, pigeons can detect frequencies as low as 0.05 Hz (i.e., infrasound), but it is unclear how the birds use this capability (Fay and Wilber 1989; Fay and Popper 2000). Reception of high frequencies (>10,000 Hz) is very poor in birds (Dooling 1978). Nocturnal predatory species (e.g., owls) generally hear better than other bird species, while songbirds hear low frequencies better than nonsongbirds (Dooling et al. 2000). Bioacoustics The use of bird alarm and distress calls to disperse birds is based on sound biological principles. Alarm and distress calls warn other birds in the area that danger is present, typi- cally causing the other birds to flee. Birds are less likely to habituate to alarm and distress calls than to other sounds because they are related to evolutionary signals of danger (Thompson et al. 1968; Johnson et al. 1985; Bomford and OâBrien 1990). Avian Vision The primary sensory pathway in birds is vision (Sillman 1973; Zeigler and Bischof 1993). However, it is evident that there are species-specific vision characteristics (Sill- man 1973; Zeigler and Bischof 1993; Blackwell 2002). To effectively use light in managing bird conflicts with aviation, an understanding of avian vision is critical. Color and type of light used to frighten birds have shown species-specific reactions ranging from indifference to flight (Belton 1976; Blackwell 2002; Gorenzel et al. 2002). Many birds discrimi- nate the color of light at wavelengths between 400 and 700 nm, comparable to humans (Pearson 1972). In addition, some species, including pigeons, mallards (Anas platyrhynchos), belted kingfishers (Megaceryle alcyon), and some passerines (Bowmaker and Martin 1985; Martin 1986; Cuthill et al. 2000) also perceive ultraviolet light (<390 nm). Rock doves (Columba livia) and some songbirds have also exhibited sensitivity to the plane of polarization of light (Able 1982; Young and Martin 1984), to which humans have very limited sensitivity. The avian retina, consisting of high cone densities, deep foveae, near- ultraviolet receptors, and colored oil droplets, is likely the most capable daylight retina of any animal (Gill 1995). Fur- thermore, because birds can apparently detect color, it could be an important consideration during the construction and development of devices used to deter and disperse birds.
