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6 CHAPTER THREE PRINCIPLES OF AVIAN ECOLOGY AND BIOLOGY BIRD MOVEMENTS AND SPACE USE source-sink models. Finally, Senar et al. (2002) proposed a model based on work with Citril finches (Serinus citronella), The airport environment comprises a relatively small land whereby animals may disperse from low- to high-quality area in the context of bird movements and space use; thus it sites because the high-quality sites act as pools of genetic is likely only a small proportion of areas used for most spe- variability and are sources of higher-quality food. Given the cies. Furthermore, birds likely spend only a small percent- assumptions of each model, predictions may vary greatly age of time in the airport environment foraging, loafing, or regarding dispersal. Further, the response of individual pop- raising young. Also, considering the patchiness of the typi- ulations to human and environmental disturbances, as well cal landscapes in and surrounding airports, bird likely use as land management actions and deterrent techniques, will only portions of airports. There is also temporal variation in depend on which model of dispersal is applicable in a given bird use of areas including airports. For migrant species, use system and circumstances. may be restricted to fall and spring migration periods. Alter- natively, birds may be present only during winter or sum- Flocking Behavior mer to nest and raise young. Finally, resident species may use habitat on airports year-round. The mechanisms driving Birds may form flocks of individuals of single or multiple bird distributions in the context of habitat are important to species. Flock formation is a balance between costs and consider because they can influence the timing of use and benefits to individuals within a flock by reducing the risk of effectiveness of deterrents, hazing, and repellents. predation and enabling cooperative foraging (Emlen 1952; Powell 1974; Caraco et al. 1980a,b; Caraco 1981; Tinber- Dispersal gen 1981; Pulliam et al. 1982; Fernández-Juricic et al. 2004). Unstable and indefensible areas (e.g., food sources or loafing Several models have been proposed to explain how indi- sites) promote flocking behavior (Verbeek 1972; Gill 1995). viduals within groups or subpopulations disperse from one Verbeek (1972) found that corvids abandoned territories location to another. Slatkin (1985) postulated that dispersal and developed flocks when food supplies became less stable may simply be a random walk in space with few, if any, fac- and more widely and unevenly distributed. Flocking behav- tors driving species dispersal. However, it is not likely that ior has also been demonstrated to be a function of breed- most species perceive the environment in this manner, and ing activity in starlings (Davis 1970). Feeding in flocks can this model is not well supported in the ecological literature. increase competition for food, but has been demonstrated The ideal free distribution model (Fretwell and Lucas 1970), to collectively increase foraging efficiency (Caraco 1981; or balanced dispersal model (Doncaster et al. 1997), states Sullivan 1984). Cooperative feeding is common in species that dispersal patterns are contingent on the fitness (e.g., such as pelicans, cormorants, and mergansers (Bartholomew increased survival or reproductive success) of the individual 1942; Emlen 1952). Flock members can also benefit from in a given habitat type, and dispersal is not constrained by prey that is flushed by a flock-mate. For example, Cezilly et population density in the other habitat patches. Source-sink al. (1990) found that forage striking and number of captures dynamics is another type of model used to describe how per minute improved as flock size increased for little egrets variation in habitat quality can influence use and distribu- (Egretta garzetta). tion of animals. In source-sink models, the source is an area of higher quality habitat that on average can support more Individual fitness of a bird can also be increased in flocks individuals and allows populations to increase. In contrast, through reduced predation risk (Charnov and Krebs 1975; a sink is an area of low-quality habitat that cannot sustain Sirot 2006). Predators can be confused by flock movements a population and generally supports low numbers of indi- that make it more difficult to single out one individual (Lan- viduals. For general source-sink models (Holt 1985; Pulliam deau and Terborgh 1986). Page and Whitacre (1975) reported 1988), dispersal is constrained between patches, density- that merlin (Falco columbarius) hunting success varied independent or dependent dispersal are both possible, and according to prey flock size. Kenward (1978) found that habitat quality may vary greatly among patches. Also, as goshawk (Accipiter gentilis) predation was also disparately the name implies, the presence of a sink is assumed under lower when pigeon flock sizes were large. Another possible