Observations on twitchy wastage of energy in a bird, the willy wagtail

(writing in progress)
 
The willy wagtail (Rhipidura leucophrys, https://www.inaturalist.org/taxa/8143-Rhipidura-leucophrys), common in my garden, seems to exemplify the wastage of energy by small birds.

I observed an individual of the willy wagtail in my garden recently. It was alone and foraging, but much of its behaviour seemed pointless and wasteful, as follows.

See https://www.youtube.com/watch?v=AN8MF8daTWg and https://www.youtube.com/watch?v=pLlvomkR5hs and https://www.youtube.com/watch?v=SbIkZoG0mDA and https://www.youtube.com/watch?v=O9E6D98gNiY and https://www.youtube.com/watch?v=qAO_NNE4uBs and https://www.youtube.com/watch?v=s8Y3raDbyBw and https://www.youtube.com/watch?v=ENxKFVpQlEE and https://www.youtube.com/watch?v=izAWQvGx-CI.
 
The bird twitched continually, to a degree not seen in any South African bird I know of (including wagtails and the familiar chat). The willy wagtail exuberantly rotates its body and tail, all the while uttering an unmusical vocalisation which sounds like an alarm call rather than a contact call, but does not seem to function as either.

The willy wagtail is far from being easily alarmed, being one of the most confiding and fearless of small birds around large mammals including humans. Certainly the individual in my garden this morning did not feel threatened by me.

And since this individual was apparently alone (in keeping with the extremely large home range and the fact that the species visits my garden every few days or every few weeks rather than being permanently present, and seldom travels in pairs or groups) I can’t see that it was calling to a conspecific. Furthermore, there was no reply by any conspecific within earshot.
 
So both the exuberant movement of body and tail (from side to side, not up and down as in motacillids) and the energetic calling seemed quite gratuitous. Although one might suspect that these movements and sounds serve to flush out insects (and a published paper on the topic once found some evidence for that), the individual I observed this morning seemed not to be using its exuberance to flush out food.

The invertebrates in my compost-rich garden are not of the kind that can really be flushed out, and the red wattlebird, which forages regularly in the same spot, simply picks up fly larvae and other invertebrates with little fuss until an individual of the domestic cat appears, whereupon it does utter a loud harsh vocalisation in harrassment of the predator (but with no twitching to speak of).

Much of the time, my willy wagtail this morning was perched on the top of an asbestos fence while flicking itself laterally and while calling noisily. It seems obvious that such exuberance would not flush any item of food out in such a position.
 
The willy wagtail is a good example of a bird species that wastes energy just for the hell of it. Could the conspicuous black-and-white colouration have something to do with this?

I wonder if the behaviour so often observed in the willy wagtail (and by me this morning) is analogous with pronking/stotting in gazelles, but performed routinely and not just when a predator threatens. I am thinking particularly of goshawks.

Perhaps the willy wagtail displays its fitness more or less continually, regardless of whether it has seen a predator (particularly a bird-eating raptor) approaching?
 
By the way, the willy wagtail is also one of the best examples of a small bird with conspicuous tonal contrast, its black dorsal and white ventral surfaces being about as conspicuous as they can be in monochrome.
 
(writing in progress)

Publicado el julio 17, 2022 12:03 MAÑANA por milewski