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Many bird species show tail movements called wagging, flicking or pumping. The adaptive function is not yet fully understood, but the behaviour seems to be common among birds that live in the neighbourhood of running water.
Tail wagging and wing fluttering may be involved in flushing insects (Jablonski 1996) as has been found in mockingbirds, This hypothesis is supported in the willie wagtail because it flashes or flutters its wings during feeding but never when perched (Jackson & Elgar1993). Cramp (1988) also supposed that the white wagtail (hereafter wagtail) wags its tail to flush insects. If so, and if tail wagging is used exclusively for this purpose, juveniles should tail-wag to the same extent as adults (Jablonski1996) and tail wagging should not occur during activities other than feeding, such as preening. Wagging might provide information about the prey’s condition and ability to escape as well as its alertness and could be a pursuit-deterrent signal directed towards predators (Hasson 1991; Spitznagel 1996). Signalling need not be restricted to a direct encounter with an approaching predator (Spitznagel 1996), but might also be directed to an ambushing predator when the prey is not aware of it (Bradbury & Vehrenkamp 1998). To be effective, pursuitdeterrent signals are expected to change the predator’s behaviour (Hasson 1991). However, Cresswell et al. (2003) found that opportunistically hunting predators did not prefer nonvigilant prey in a head-down posture to vigilant prey (head-up posture). Therefore, wagging could be a more conspicuous and reliable signal for predators than simple scanning movements of the head. A higher rate of wagging might show greater vigilance and thus that an attack is unlikely to be successful.Wagging might also indicate that the prey is unprofitable, that is, that wagtails are generally less palatable than other species. However, there is little experimental evidence for the unprofitable prey hypothesis (Go¨tmark 1999). In a choice experiment raptors showed no preference for a taxidermic wagtail or a meadow pipit, Anthus spinoletta (Go¨tmark & Unger 1994), although the stuffed skins of the wagtails did not tail-wag.
Wagging might provide information about the prey’s condition and ability to escape as well as its alertness and could be a pursuit-deterrent signal directed towards predators (Hasson 1991; Spitznagel 1996). Signalling need not be restricted to a direct encounter with an approaching predator (Spitznagel 1996), but might also be directed to an ambushing predator when the prey is not aware of it (Bradbury & Vehrenkamp 1998). To be effective, pursuitdeterrent signals are expected to change the predator’s behaviour (Hasson 1991). However, Cresswell et al. (2003) found that opportunistically hunting predators did not prefer nonvigilant prey in a head-down posture to vigilant prey (head-up posture). Therefore, wagging could be a more conspicuous and reliable signal for predators than simple scanning movements of the head. A higher rate of wagging might show greater vigilance and thus that an attack is unlikely to be successful.Wagging might also indicate that the prey is unprofitable, that is, that wagtails are generally less palatable than other species. However, there is little experimental evidence for the unprofitable prey hypothesis (Go¨tmark 1999). In a choice experiment raptors showed no preference for a taxidermic wagtail or a meadow pipit, Anthus spinoletta (Go¨tmark & Unger 1994), although the stuffed skins of the wagtails did not tail-wag. Furthermore, wagging was not restricted to situations in which the wagtail had seen a predator and needed to inform it that it had been seen or that the bird was vigilant or able to escape (Caro 1986; Alvarez 1993; Cresswell 1994; Brooke 1998; Leal 1999; Caro et al. 2004) because it occurred almost continuously during preening and feeding. In addition, during the videotaping no approach of a raptor was recorded. As wagging does not seem to be restricted to direct encounters between wagtails and predators, it may be directed to predators that ambush their prey. Predators may make absolute judgements about the level of vigilance in a group of prey or compare vigilance levels between individuals, and then approach and attack the less vigilant prey. However, opportunistic predators such as sparrowhawks do not prefer to attack nonvigilant rather than vigilant prey (Cresswell et al. 2003). Cresswell et al. (2003) suggested that if predators show preferences, they might do so on the basis of cues more complex than posture alone. Wagging could be such a cue and might be a better long-distance signal than body posture to a possible predator. To investigate this possibility, it would be worth widening the choice experiments presented by Go¨tmark & Unger (1994) and Cresswell et al. (2003) by using a wagging and a nonwagging wagtail model.
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