The puzzling lack of seed-dispersal of mulberries (Morus) by animals in Perth, Western Australia

@deans_beaver @george_seagull @thebeachcomber @johnnybirder

The red mulberry (Morus rubra, https://en.wikipedia.org/wiki/Morus_rubra) is indigenous to eastern North America (https://www.thegardenofeating.org/2015/06/mulberries.html). The black mulberry (Morus nigra, https://www.inaturalist.org/taxa/126741-Morus-nigra), is indigenous to southwestern Asia. The white mulberry (Morus alba, https://en.wikipedia.org/wiki/Morus_alba and Morus alba, https://www.aldgatetreefarm.com.au/shop/weeping-mulberry-morus-alba-pendula) is indigenous to southern and eastern Asia.

In their original habitats, mulberries are dispersed and sown by dozens of species of indigenous birds (https://www.jstor.org/stable/1938870 and https://smithgilbertgardens.com/birds-of-sgg-mulberry-tree/ and https://www.natureblog.org/waxwings-and-mulberries/ and https://georgiawildlife.com/out-my-backdoor-mulberries-get-no-respect-they-should and https://www.thedailynewsonline.com/lifestyles/master-gardener-red-mulberry-trees-are-bird-magnets/article_6997af1c-0bd6-5278-90aa-3ccf554bda37.html).

The black and white mulberries are commonly planted in gardens in Australia (https://www.theguardian.com/food/2020/sep/19/the-sweeter-the-juice-an-entirely-ripened-mulberry-is-the-flavour-of-self-control).

This includes Perth in Western Australia, where I live (https://www.burkesbackyard.com.au/fact-sheets/in-the-garden/flowering-plants-shrubs/mulberry/).

However, what is surprising is that, in 35 years of incidental observation, I have never seen any bird, indigenous or introduced, eating the fruits of the mulberries, whether unripe or ripe, on the plant or fallen on the ground.

The southwestern Australian avifauna lacks families typically associated with frugivory, e.g. Pycnonotidae and Turdidae, whether indigenous or introduced. This is partly because of the isolation/remoteness of this region, and a recent policy of prevention of introductions.

However, there are several indigenous species which might be expected to eat the fruits of the black and white mulberries in Perth and elsewhere in suburban Western Australia.

In and near my small suburban garden, I have recorded at least 20 species of birds.

Among these, perhaps the prime candidates for consumption of the fruits of mulberries are the singing honeyeater (Gavicalis virescens, https://www.inaturalist.org/observations?place_id=19379&taxon_id=370319) and the red wattlebird (Anthochaera carunculata, https://www.inaturalist.org/taxa/12622-Anthochaera-carunculata).

These belongs to a mainly nectarivorous family (https://en.wikipedia.org/wiki/Honeyeater). However, most meliphagids take some ripe fleshy fruit. The singing honeyeater and the red wattlebird have a short beak relative to other meliphagids, and superficially resemble pycnonotids or sturnids.

Despite these favourable indications, I have never seen any evidence that the singing honeyeater (https://backyardbuddies.org.au/backyard-buddies/singing-honeyeater/) or red wattlebird (https://www.youtube.com/watch?v=OtMaB8tNTF4) eats mulberries.

Another candidate is the western silvereye (Zosterops lateralis chloronotus).

This genus, indigenous to several continents, consists of birds too small-bodied to swallow whole the fruits of the red mulberry. However, Zosterops consumes ripe fleshy fruits as a major part of its diet, by pecking them piecemeal. This would be suited to the mulberries, because of the composite structure of this fruit.

However, once again, I have never observed the western silvereye to eat the mulberries.

I have previously suspected that this is partly be explained by the seasonality involved. The black mulberry fruits copiously in spring, but the western silvereye seems has - based on its sibilant, soft but distinctive vocalisation - seemed to me to be absent from Perth and its suburbs at this time of the year. My consistent impression, year after year for many years, has been that this species is present here mainly in autumn.

However, two lines of evidence have led me to question my assumption.

Firstly, I have for many years monitored the loquat (Eriobotrya japonica, https://www.inaturalist.org/observations/63958529), which shares the fruiting season of the black mulberry, and I have often observed the ripe fruits to have been pecked by some small species of bird - probably the western silvereye.

Secondly, of the 24 observations so far posted from Perth in iNaturalist, there are 4 in September-October, which is the fruiting season of the black mulberry. This contradicts my impression of seasonality, based on hearing the vocalisations of the western silvereye.

Instead of eating this fruit, the western silvereye seems to be attracted to the copious nectar of introduced Callistemon (https://www.inaturalist.org/observations/63958529).

The rainbow lorikeet (Trichoglossus moluccanus, https://www.inaturalist.org/taxa/980095-Trichoglossus-moluccanus) is abundant - to pest proportions - in Perth, after being introduced from the eastern states of Australia.

This psittaciform bird eats mainly nectar and pollen, but also various sweet fleshy fruits (https://birdfact.com/articles/what-do-rainbow-lorikeets-eat). I have often observed it eating the ripe fleshy fruits of the common fig (Ficus carica, https://www.inaturalist.org/taxa/60218-Ficus-carica) in suburban gardens in Perth.

I do not recall ever observing the rainbow lorikeet eating the black mulberry, or even perching in the plants of this species. So, I was surprised to find photograph evidence of this on the Web (https://upload.wikimedia.org/wikipedia/commons/b/ba/Rainbow_lorikeet_%28Trichoglossus_moluccanus%29_eating_mulberries%2C_December_2020_01.jpg).

Posted on October 23, 2023 12:21 AM by milewski