The difference between commensalism and habituation, exemplified by Spilopelia chinensis in Australian suburbs

Certain species of animals associate particularly with humans, usually for the food inadvertently supplied by an affluent and wasteful primate.

Commensalism (https://en.wikipedia.org/wiki/Commensalism) implies habituation. This is because the association implies proximity.

To see the house sparrow (https://www.inaturalist.org/taxa/13858-Passer-domesticus), pecking crumbs close to a table in an outdoor cafe, surprises nobody who understands that this species is exclusively commensal, never living in the wild (https://en.wikipedia.org/wiki/House_sparrow).

However, some commensal species remain unhabituated, instead remaining as shy of humans as fully wild animals would be. They live among us - indeed, in some situations exclusively among us - but they never get used to us enough to trust us any more than wild animals do.

And this vigilance, apprehensiveness, and distrust may continue, generation after generation, even where humans pay no attention to the animal in question, do nothing deliberately to harm it, and indeed hold it in kind regard.

Such a species is Spilopelia chinensis (https://www.inaturalist.org/taxa/1455918-Spilopelia-chinensis), at least in the urban/suburban environment of Perth, Western Australia, where I live.

This species is common in suburban streetscapes, where its peaceful cooing (https://www.youtube.com/watch?v=WEHe6W_D4lo and https://xeno-canto.org/species/Spilopelia-chinensis and https://avocet.integrativebiology.natsci.msu.edu/recordings/5987 and https://search.macaulaylibrary.org/catalog?taxonCode=spodov&mediaType=audio&sort=rating_rank_desc#_ga=2.93916062.2047640190.1660101380-673334876.1658548323 and https://ebird.org/species/spodov) is a familiar sound, disliked by nobody, and a reminder of the inoffensiveness associated with doves.

Most persons in Australia, including naturalists, take S. chinensis for granted in a neutral or positive way.

However, when one thinks about it, there is something odd in its relationship with us.

Spilopelia chinensis forages exclusively on the ground. It habitually flees from approach at the same distances that any wild, terrestrially foraging columbid would, if approached in some remote area of grassland or savanna. This behaviour seems stereotyped, instinctive, and unaffected by experience.

Australia has a rich indigenous fauna of Columbidae (https://www.inaturalist.org/observations?place_id=6744&taxon_id=2715&view=species).

However, the most successful spp. of columbids in Australian built-up areas are introduced deliberately from elsewhere.

These are:

• Spilopelia chinensis (https://www.inaturalist.org/observations/25476684),
• Spilopelia senegalensis (https://www.inaturalist.org/taxa/144550-Streptopelia-senegalensis), and
• Columba livia (https://www.inaturalist.org/taxa/3017-Columba-livia).

Playing a minor role in the metropolitan area of Perth are two indigenous species.

Phaps chalcoptera (https://www.youtube.com/watch?v=v8x07ONuZb8 and https://www.inaturalist.org/taxa/3335-Phaps-chalcoptera) was indigenous to what is now the Perth Metropolitan area, before Europeans arrived about 1820.

It has been marginalised by settlement, remaining only in peripheral, densely wooded suburbs. Here it remains shy of humans, and tends to be detected more by its hooting call (https://ebird.org/species/combro1?siteLanguage=en_AU and https://xeno-canto.org/species/Phaps-chalcoptera) than by being actually seen.

Ocyphaps lophotes (https://www.inaturalist.org/taxa/3681-Ocyphaps-lophotes) is similar in body size and diet to S. chinensis. It was indigenous only to the dry interior when Europeans arrived. It has benefited from agriculture, becoming common in the farmed countryside where wheat is grown.

And, like several other ecologically similar indigenous birds, it has tended to expand its range, towards relatively rainy, formerly well-wooded regions, now altered by the human species (https://absa.asn.au/wp-content/uploads/2021/03/Cor-Vol-35-Pg73-81-Created-Pigeons.pdf and https://birdlife.org.au/bird-profile/crested-pigeon).

Ocyphaps lophotes has recently begun to encroach on the Perth metropolitan area, where it seems to be increasing in the southern suburbs. I have lived in this metropolitan area since 1977, but the first time I spotted O. lophotes in the suburbs was in 2019.

However, the indigenous O. lophotes has yet to usurp most of the suburban habitat of the introduced S. chinensis. The former tends to occur in parks, rather than the streetscapes continually exploited by the latter.

And, significantly, O. lophotes already has achieved a level of habituation not seen in S. chinensis.

This is surprising, because S. chinensis

• is indigenous to parts of Asia that have, for centuries, been heavily populated by humans,
• was deliberately introduced to Australia as an 'instant feral', mainly for its aesthetic qualities, and
• has failed to penetrate wild areas in Australia.

The anomaly, then, is that that S. chinensis, a species somewhat co-evolved with humans from the start, actively promoted by humans in the sense of being deliberately introduced, and universally tolerated - if not liked - by the current human population, shows no sign of treating its human benefactors as anything other than potential predators.

How can we explain this anomaly?

In contrast to the failure of S. chinensis to habituate, various birds, indigenous to Australia, have become so habituated that they can be approached closely in suburbs and streetscapes, and even voluntarily approach one to within 1 metre.

Prime examples are Rhipidura leucophrys (https://www.inaturalist.org/taxa/8143-Rhipidura-leucophrys), Gymnorhina tibicen (https://www.inaturalist.org/taxa/8575-Gymnorhina-tibicen), and Grallina cyanoleuca (https://www.inaturalist.org/taxa/8583-Grallina-cyanoleuca).

This lack of shyness is in line with faunas of islands, and is consistent with the relatively small incidence of indigenous predators in the fauna of Australia - the island continent.

So, here is a hypothesis:

Spilopelia chinensis originates in a part of Asia where there is a minimal ethic of conservation. Instead, the cultural norms are such that animals are a resource for consumption by humans. It has long been the custom, in the farmlands of China, to harvest any edible animal, from invertebrates through amphibians and fishes, to birds and rodents.

At the same time, it is the nature of columbids that they are among the less intelligent of birds. They rely not on an ability to learn, but mainly on a combination of fecundity and explosive flight, for their survival.

Because of risks of predation to S. chinensis in its original, anthropogenic habitat, natural selection has 'hardwired' into it an instinctive pattern of wariness and flightiness, in which proximity to people is avoided.

Because S. chinensis has minimal cognitive versatility, the instinctive pattern of anti-predator behaviour has tended to remain unmodified. And this has applied even after generation after generation of actual experience, in a situation free of predation by the human species, should have taught the bird that it can relax around people.

What we thus find, in the Australian suburbs, is a species with an odd combination of characteristics. Spilopelia chinensis is one of the least approachable species of birds encountered in the metropolitan area: unhabituated even where it is the commonest 'feral' species present.

And this suggests that, eventually, we may be left with the following ironic situation in Perth.

If the indigenous, and not particularly shy, O. lophotes continues to adapt to the suburban environment, as it has been doing in recent decades, then it may eventually replace, and marginalise, S. chinensis on the basis of competition for food.

And, to the extent that this happens, it may transpire that S. chinensis will have come and gone from many Australian streets, its remarkable degree of caution proving to be ultimately futile - and perhaps self-sabotaging - in an adaptive sense.

Posted on August 10, 2022 12:16 AM by milewski