A puzzling case of the Australian raven (Corvus coronoides) ignoring the shingleback lizard (Tiliqua rugosa)

@davidsando @max_tibby @joshuagsmith @thebeachcomber @jadonald @bmduggan

On 28 April 2024, at about 14h00, I encountered an individual of Tiliqua rugosa rugosa (https://animaldiversity.org/accounts/Tiliqua_rugosa/ and https://www.inaturalist.org/observations?taxon_id=37460) by the side of a road in Karrakatta Cemetery (https://www.mcb.wa.gov.au/our-cemeteries/karrakatta-cemetery/) in the Perth Metropolitan area (https://en.wikipedia.org/wiki/Perth).

The autumn weather was fine, sunny, and warm, with no wind. The ambient temperature was 26 degrees Celsius.

The lizard was fully sunlit, away from any cover. I approached it to within one meter, and it remained stationary and passive, not reacting to my presence. It seemed unperturbed, despite being completely exposed and vulnerable.

Tiliqua rugosa has camouflage-colouration. This individual, which was probably a large juvenile (estimated body mass about 0.5 kilograms) rather than fully-grown, remained inanimate, not even flicking its tongue. However, its presence was obvious because it was out in the open.

The individual was in normal condition apart from its tail, which was noticeably shrivelled and scruffy. This was probably the result of its store of fat having been used up in this dry season - which has been exceptionally warm and rainless (https://business.weatherzone.com.au/climate/second-hottest-summer-days-on-record-in-perth/ and https://www.abc.net.au/news/2024-03-09/wa-summer-weather-wrap-autumn-outlook/103541460), and prolonged past mid-autumn.

After a moment, I walked on.

I paused after 10 meters to see how the lizard was reacting to my departure. Looking back, I noted an adult individual of the Australian raven (Corvus coronoides, https://ro.ecu.edu.au/cgi/viewcontent.cgi?article=1294&context=theses_hons and https://en.wikipedia.org/wiki/Australian_raven and https://australian.museum/learn/animals/birds/australian-raven/), perched about 7 meters high in a eucalypt tree, about 5 metres beyond the lizard. The body mass of the bird was probably about 650 grams.

This individual had been vocalizing, rather softly and querulously, before I noticed it, and it continued to do so as I watched to see what the lizard did once I was no longer close to it. The vocalisation of the bird did not attract any other individuals of its species.

After about 10 seconds the lizard started to crawl towards the road, intermittently flicking its tongue in the typical manner of its species. The lizard had been in plain view of the bird from the start, and was now even more obvious owing to its slow movement.

I was curious to see whether the bird would fly down from its perch to investigate or attack the lizard. Tiliqua rugosa is, after all, something of a 'sitting duck', with its slow movement and modest armour of reinforced scales.

In this suburban neighbourhood, the Australian raven is unpopular owing to its habitual predation on nestling birds (https://westernweb.net/2015/11/17/are-urban-ravens-really-thugs-and-murderers/). The species is known to be capable of killing juveniles (up to 400 g, https://www.alamy.com/stock-photo-juvenile-european-wild-rabbit-oryctolagus-cuniculus-uk-47511065.html) of the feral rabbit (Oryctolagus cuniculus, https://www.tandfonline.com/doi/pdf/10.1071/MU959041).

A congener, namely Corvus ruficollis (https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=8034) is known to be a predator on another large, slow-moving lizard, namely Uromastyx aegyptia (https://www.inaturalist.org/observations?taxon_id=31332).
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8388000/8388073.stm and https://www.researchgate.net/publication/225220613_Cooperative_hunting_in_Brown-Necked_Raven_Corvus_rufficollis_on_Egyptian_Mastigure_Uromastyx_aegyptius and https://cris.bgu.ac.il/en/publications/cooperative-hunting-in-brown-necked-raven-corvus-rufficollis-on-e).

However, this individual of C. coronoides showed no interest in the lizard, simply continuing to vocalise (in a way I have not heard previously) until it flew off after about three minutes. By this time the lizard, locomoting in a leisurely and unconcerned way, had partly crossed the road.

The whole incident left me with the impression that the bird 'knew something I did not' about the unsuitability of T. rugosa as prey.

What could this possibly be? Why did the Australian raven not muster up a group, to collectively attack the lizard?

While walking at this same location several years previously, I had experienced a 'funeral' congregation (https://corvidresearch.blog/tag/crow-funeral/ and https://birdoftheweek.home.blog/2020/09/19/bird-152-australian-raven/ and https://www.scienceabc.com/nature/animals/do-crows-mourn-their-dead.html) of the Australian raven - testimony to the ability of the species to muster a large group by means of vocalisation.

I had encountered as many as 50 individuals - constituting the entire population of the suburb where I live - congregated in and among the trees above, calling excitedly and alternately flying and perching. I had then observed a freshly dead individual of C. coronoides on the ground near the same road.

The attendant group had followed me fairly cohesively as I walked past, over a distance of several tens of meters, as if 'mobbing' me (https://en.wikipedia.org/wiki/Mobbing_(animal_behavior)) in emotional association with the death of their conspecific.

Returning in time to the incident at hand:
If a mere fraction of such congregation were mustered in response to the spotting of potential prey in the form of a vulnerable individual of T. rugosa, the lizard could potentially have been overcome and eaten.

Posted on April 29, 2024 03:34 AM by milewski