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Tail Movements in Birds—Current Evidence and New Concepts

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Tail Movements in Birds—Current Evidence and New Concepts Ornithol Sci 15: 1 – 14 (2016) ORIGINAL ARTICLE Tail movements in birds—current evidence and new concepts Christoph RANDLER# Univ Education Heidelberg, Biology, INF 561-2,D-69120 Heidelberg, Germany, 0049622147734 ORNITHOLOGICAL Abstract Birds of a wide range of species show characteristic movements of their tail, often called tail flicking, tail wagging or tail flashing. Tail flicking refers to verti- SCIENCE cal up-and-down movements of the tail, while tail flashing is defined as a horizontal © The Ornithological Society movement, often including tail spreading. Here, I review proposed functions of such of Japan 2016 behaviour. Most relate to communication with conspecifics, predators or prey. Tail flashing may induce movement of the bird’s prey that makes the prey more vulner- able to capture (‘prey-flushing’). Tail movements may signal to a predator that the signaller has detected it (‘perception advertisement’), or that the signaller is particu- larly alert or otherwise difficult to catch (‘quality advertisement’). Further, it may warn conspecifics of predators (‘alarm signal’), or it may advertise quality as a mate, signal social status, or aid in flock cohesion. This behaviour may, possibly, though it seems unlikely, represent a cue rather than a signal in that it benefits the receiver, but not the signaller. For each postulated function, I develop predictions stemming from that function, and interpret the available empirical evidence in the context of these predictions. I finish by synthesising our current state of knowledge and by identify- ing the future empirical studies that would most improve our understanding of this widespread but unjustly neglected avian behaviour. Key words Alarm signal, Ambushing predator, Flock coherence, Tail flashing, Tail flicking, Tail wagging, Vigilance Tail movements are present and obvious in many about tail movements in birds and offer new con- bird species, and often mentioned as traits in field cepts. The term “tail movement” will be used to guides, as in the case of the White Wagtail Motacilla describe any of the various behaviours that might be alba. These movements have been described using described as tail flashing, tail wagging, tail pump- different terms, such as tail flick, tail wag, tail flash, ing, tail movement, or wag display, where the tail is tail pump or tail up display. Although this behav- moved independently from the body in a non-flying iour is widespread, it has received little attention in bird. The direction and speed of movement, and the ornithology. Tail movements have been more often difference in the speed between any upward and investigated in mammals, such as the tail flick/flag downward stroke are not addressed here (see details of ungulates (Caro et al. 2004; Caro 2005). In orni- in Andrew 1956). thology, tail movements are often described during courtship display (Fitzpatrick 1998, 1999). However, Classification of tail movements most bird species that show tail movements do so Tail movements are diverse, and it would be help- throughout the year, not just in the breeding season. ful if workers in this field could agree on how to This kind of tail movements is the focus of the pres- describe these movements and how to name them. ent study. I used a systematic search through differ- I suggest defining tail flicking as a movement of the ent databases (for details see Appendix) to identify tail upwards and downwards on a vertical axis (see relevant papers. Table 1 for an overview). Flicking is different from In this paper, I will review proposed hypotheses side-to-side movements and spreading of the tail. The side-to-side-movements as shown in motmots (Received 14 April 2015; Accepted 17 June 2015) (Murphy 2006), could be described as pendulous tail # Corresponding author, E-mail: [email protected] movement, or for convenience, explicitly stated as 1 C. RANDLER Table 1. Overview over the tail movements and definitions. In bold: term used for the review. Axis Tail Description Examples Possible function Tail flicking, Vertical unspread Up-and-downward wagtails, moorhens, Predator-prey Tail pumping, movement movement of the tail North American phoebes context, social tail wagging function Tail flashing, Horizontal spread horizontal fanning, Hooded Warbler, Prey flushing Tail fanning movement spreading of the Myioborus redstarts, Tail spreading rectrices, display Rhipidura fantails of conspicuous, contrasting plumage patches in the rectrices Pendulous Horizontal unspread Pendulous movement, Motmot Predator-prey movement axis side-to-side wagging communication “side-to-side wagging”. Third, movements on a hori- functions for tail movements, but also discusses the zontal axis should be described as tail flashing which possibility that tail movements may have non-signal- includes tail fanning and tail spreading. Often these ling functions. movements involve display of conspicuous, contrast- ing plumage patches in the rectrices. They are also Functions of tail movements often associated with the spreading or drooping of Based on the theory of animal signalling, the the wings. Tail flashing might be further separated hypotheses drawn from the studies listed in the into two categories that include only very short peri- Appendix can be grouped in different ways: ods of flashing of less than 0.1 sec, and in contrast, A. First, in an intraspecific signalling context, tail fanning with longer periods. In this review, I will movements may be directed at potential mates or summarise both these movements under tail flashing. used in a social context (dominance/submission) or Also, tail wagging is defined as an independent in terms of flock-cohesion. movement of the tail, and not a movement of the B. Second, as an interspecific signal (in this case body, as for example in the Actitis sandpipers where directed at a predator) or as foraging enhancement. the birds do not flick or wag but rather move (bob) C. Thirdly, tail movements might be a cue rather the hind part of their body along with the tail. It than a signal. is also different from head bobbing (Fujita 2004) or nodding, which has an optokinetic function or is Group A: Intraspecific context controlled by biomechanical constraints. Research- Hypothesis A1: Tail movement is a sexual signal ers on this topic should describe the tail movements addressed at a potential mate. of their study species in detail, whether they fall in Intuitively, this hypothesis does not seem to apply the categories described here (following Table 1), or to year-round tail movement displays. In addition, if whether they involve combined movements, shown the signal is predominantly used in a sexual context, habitually or only in response to external stimuli, and then: should describe how the tail is moved (spreading, • it should be given during the presence of a sexual turning, twisting etc.). partner There are two hypotheses for the evolutionary ori- • there should be differences between males and gin of tail movements, suggesting evolution from females (given that in most birds the female is either vigorous hopping or leaping (Daanje 1950), the choosy sex), or non-ritualised intention movements indicating the • the rate should differ between reproductive intention to fly (Andrew 1956). Both hypotheses (adult) and pre-reproductive (juvenile) individu- assume evolution to perform a signalling function als (Spitznagel 1996), being higher in adults, with a sender and one or more intended receivers, • and there should be variation throughout the year both con- or heterospecific individuals (Searcy & and a high incidence of movements in the pre- Nowicki 2005). This review focuses on signalling breeding and breeding season or during pair for- 2 Tail movements in birds mation. be reliably tested across conflicting dyads, rather than Evidence in favour of this hypothesis can be found each individual’s average behaviour. in the Common Moorhen Gallinula chloropus, in Evidence against this hypothesis comes from which species females flicks their tails faster than Alvarez et al. (2006) who found that individual males (females are more actively engaged in mate moorhens with a high flick rate had a higher social acquisition) (Alvarez et al. 2006). In addition, flick- status. Also, Randler (2006, 2007) found no differ- ing was correlated with better body condition, and ence between adults versus juveniles in tail wag- this signal can be interpreted as being directed at ging rates in wagtails or tail flicking rates in moor- potential mates (Alvarez & Sanchez 2003, Alvarez et hens. In addition, rates of tail movements were al. 2006). Motmots Momotus spp. do not wag their higher in single individuals (Randler 2006, 2007) tails much outside the breeding season, also sug- and increased with increasing nearest neighbour dis- gesting a sexual function (Wagner 1950). However, tance (Randler 2007). In Eastern Phoebes, conspe- no differences in tail movements have been found cific playback did not evoke a higher or lower tail between adult and juvenile White Wagtails or Com- movement rate, which renders a social submission mon Moorhens (Randler 2006, 2007), or between hypothesis unlikely; because tail movement should the sexes in the Eastern Phoebe Sayornis phoebe either increase or decrease during encounters with a (Carder & Ritchison 2009), Turquoise-browed conspecific male (Carder & Ritchison 2009). There Motmot Eumomota superciliosa (Murphy 2006) or seems little evidence for tail movement as a submis- White-throated Dipper Cinclus cinclus (Spitznagel sion signal, although it may have this function in 1996). There is some evidence for sexual signalling species living at high densities and in large groups, in the Common Moorhen, but not in the other species as in the case of the Dusky Moorhen. studied. If tail movements occur among mated pairs that remain together year-round, then this behaviour Hypothesis A3: Tail movements are used to coordi- may fall under hypothesis A3. In this case, a distinc- nate flock behaviour and movement. tion can be drawn in function between mate choice Tail movements make a bird conspicuous and and pair formation versus pair maintenance (see A3).
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