Why does the willie wagtail wag its tail?

A small, black-and-white and charismatic bird, the Australian willie wagtail was named as such because of its famous (and seemingly compulsive) side-to-side tail-wagging. The reasons why the willie wagtail wags its tail are not defined: guesswork and a lot of observations have suggested that it may be an exaggerated form of counterbalancing the weight of the upper and front of the body as the bird darts about, otherwise ecological reasoning suggests it may be either a form of signalling of vigilance to predators or a method for flushing out insects from the grass while foraging (which forms a habit that the bird can't control while it's not foraging, like people picking their nose and eating it). However, a direct relationship between tail-wagging and any of the current ecological theories has not been properly examined to date.

Willie wagtail Rhipidura leucophrys strikes a pose for photographer Jeremy Ringma.

Despite its common name, the willie wagtail is not a true wagtail of the genus Motacilla, it is a member of the fantail genus Rhipidura which is closely related to crows and ravens. Australia has a few other species of fantails, however, the willie wagtail stands out in the crowd as being unusually taller and upright in its stance, and its foraging technique and preferred habitat are completely different.

One big happy family 

Other, 'typical' Australian fantails are arboreal in habit, preferring woody habitats. Generally, foraging in fantails is performed by fully fanning the tail, sweeping it vertically and flashing side to side, and the tail is also used as an aid in maneuverability while performing incredible mid-air acrobatics chasing insects. The willie wagtail is more terrestrial in habit, preferring open grasslands and hopping about in the grass. Foraging behaviour in willie wagtails rarely involves full tail-fanning, and wing-flashing has replaced the vertical sweeping of the tail seen in other fantails. The terrestrial habits of the willie wagtail are also evidenced in osteological adaptations; its leg bones are longer and its pelvis longer and narrower to facilitate hopping and horizontal tail-wagging.

Willie wagtails (left) have a more upright stance and longer legs than typical fantails like the rufus fantail (right).

Although related to fantails in Australia, the closest relatives of the willie wagtail are found in South East Asian species R. javanica, R. aureola and R. albicollis, which also forage on or close to the ground and engage in similar wagging behaviour to willie wagtails, however none prefer more open grasslands to forage. This suggests that something evolutionarily interesting is going on: that firstly, the wagging habits of the willie wagtail is most likely inherited from a common ancestor with its South East Asian relatives rather than that of other Australian fantails, and secondly, that the osteological adaptations and open habitat preferences of willie wagtails suggests that they evolved in Australia in a refuge area with a more open habitat.

...and more kangaroos offering free rides

Speciation and adaptation of avifauna to a terrestrial lifestyle is not a new concept in Australia. Many species of parrots and pigeons show a pattern of peripheral isolation and speciation, and given the dry and open nature of a great proportion of Australia, many refuge areas are very likely to have lacked adequate forest conditions that gave rise to terrestrially-adapted species found in otherwise arboreal taxa, such as the terrestrially-adapted ground cuckoo-shrike Coracina maxima.

For Australian avifauna, terrestrial adaptations are common. Ground cuckoo-shrike (left) and arboreal cuckoo-shrike (right).

So why does the willie wagtail wag its tail? To cut a long story short, when willie wagtails are not foraging its an evolutionary quirk, otherwise it seems the most logical solution to foraging on the ground given its evolutionary history.

Literature:

Harrison, C.J.O. (1976). Some aspects of adaptation and evolution in Australian fantailed flycatchers. Emu 76:115-119

Jackson, J. and Elgar, M.A. (1993). The foraging behaviour of the willie wagtail Rhipidura leucophrys: why does it wag its tail? Emu 93:284-286

Nyari, A.S., Benz, B.W., Jonsson, K.A., Fjeldsa, J., Moyle, R.G. (2009). Phylogenetic relationships of fantails (Aves: Rhipiduridae). Zoologica Scripta 38:553-561