Ecological differences between lookalike species of Zosterops in southwestern Australia and southwestern southern Africa
@tonyrebelo @jeremygilmore @ludwig_muller
Only five genera of passerine birds are indigenous to both southwestern Western Australia (https://en.wikipedia.org/wiki/Southwest_Australia) and the climatically and latitudinally similar southwestern Cape of South Africa (https://en.wikipedia.org/wiki/Western_Cape).
These are, in alphabetical order
- Acrocephalus https://www.inaturalist.org/observations?taxon_id=6701
- Anthus https://www.inaturalist.org/observations?taxon_id=13706
- Corvus https://www.inaturalist.org/observations?taxon_id=7998
- Hirundo https://www.inaturalist.org/observations?taxon_id=11880
- Zosterops https://www.inaturalist.org/observations?taxon_id=17439
In this Post I compare, ecologically, the following taxa:
- Zosterops lateralis chloronotus (https://www.inaturalist.org/journal/milewski/91030-extreme-seasonal-incidence-of-the-silvereye-zosterops-lateralis-chloronotus-in-autumn-of-2024-in-the-perth-metropolitan-area-western-australia-in-response-to-heat-and-drought#) of south-western Western Australia, and
- Zosterops virens capensis (https://thebdi.org/2022/03/04/cape-white-eye-zosterops-virens/ and https://www.inaturalist.org/taxa/472770-Zosterops-virens) of the southwestern Cape of South Africa.
The two taxa seem virtually identical in size and shape.
Furthermore, both of them eat fruit-pulp and nectar.
However, the emphasis/reliance seems different.
Zosterops v. capensis differs from its southwestern Australian congener in several ways.
Firstly, it relies on indigenous fleshy fruit-pulp (https://www.projectnoah.org/spottings/10432262), rather than the indigenous nectar with which Z. l. chloronotus is mainly associated (https://library.dbca.wa.gov.au/static/Journals/082168/082168-25.pdf).
Secondly, it is perennially resident, rather than migrating from temperate latitudes to the subtropics each winter.
In western South Africa, the subtropics are perennially occupied by a different species, viz. Zosterops pallidus (https://www.inaturalist.org/taxa/472771-Zosterops-pallidus).
Thirdly, Z. v. capensis seems particularly suited to dispersing and sowing the relatively small, arillate diaspores of Celastraceae, particularly
- Maytenus (https://www.inaturalist.org/photos/62186140) and scroll to photo of Maytenus procumbens in https://www.hughchittenden.co.za/birds/white-eye/index.php?f=Cape%20White-eye), and
- Gymnosporia (https://www.inaturalist.org/observations?place_id=6987&subview=map&taxon_id=185532&view=species).
These plants lack counterparts in the flora of southwestern Australia.
Fourthly, Z. v. capensis lacks a certain mutualistic relationship with plants, as follows.
Zosterops lateralis pollinates various spp. of Acacia, indigenous to Australia, by being attracted to extrafloral nectaries, located on the foliage.
The bird pollinates the plants despite having a short beak. This is by virtue of accidentally rubbing against the yellowish pom-prom inflorescences - which do not produce nectar - as it moves through the shrub from one nectar-bearing leaf or phyllode to another (Sargent O H 1928 Reactions between birds and plants, Emu 27: 185-192 and https://www.researchgate.net/publication/253953794_Birds_as_pollinators_of_Australian_plants).
The above comparison raises two questions about Zosterops in the southwestern Cape of South Afirca, as follows.
- Is any indigenous plant in the Cape Floristic Region (https://en.wikipedia.org/wiki/Cape_Floristic_Region) naturally pollinated by Z. v. capensis? and
- Has Z. v. capensis taken to visiting the extrafloral nectaries of the various spp. of Acacia introduced anthropogenically to South Africa, thus helping to pollinate these plants?
Comments
https://museum.wa.gov.au/sites/default/files/Checklist%20of%20Birds%20of%20WA%20-%20April%202019.pdf
Zosterops virens capensis visits a wide variety of garden and wild plant flowers.
Skead mentions: Aloe, Tecomaria, Poitsettia, Erythrina, Callistemon (Melaleuca), Watsonia, Royena pubescens, Scutia myrtina, Grevillea spp.
iNaturalist has:
https://www.inaturalist.org/observations?place_id=any&taxon_id=793424&verifiable=any&field:Visiting%20a%20flower%20of:%20(Interaction)=
so we can add: Berzelia, Burchellia, Cotyledon, Chasmanthe, Halleria, Melianthus, Protea, Salvia, Stenocarpus, Strelitzia
@tonyrebelo
Many thanks for this useful information.
What emerges is that Berzelia albiflora (https://www.inaturalist.org/taxa/581121-Berzelia-albiflora) may possibly be pollinated more efficiently by Zosterops than by Promerops or Anthobaphes (https://www.inaturalist.org/taxa/145130-Anthobaphes-violacea).
This would be noteworthy, because Berzelia, like all members of Bruniaceae, is a) absent from Australia, b) typical of the Cape Floristic Kingdom, and c) restricted to vegetation that lacks fleshy fruits, including the arillate members of Celastraceae.
None of the other taxa of plants visited by Zosterops for nectar in the southwestern Cape - as documented by the photos in iNaturalist - is likely to be pollinated by this small, short-beaked, non-specialised bird.
This is because their flowers are a) tubular in most cases, or b) embedded in a deep inflorescence in the case of Protea, which means that Zosterops must 'cheat' the plant w.r.t. the pollination mechanism. Zosterops does this by piercing the floral structures at their base. It thus reaches the nectar while avoiding the anthers, and avoiding having any pollen deposited on its feathers.
Correct. But I dont think Zosterops is more efficient than sunbirds and sugarbirds on Berzelia albiflora, because all of them pollinate it with their feet and not their heads. Ditto Berzelia stokoei.
@tonyrebelo
Many thanks for this correction.
@tonyrebelo
Do you think that the diaspores of Kiggelaria africana (https://treesa.org/kiggelaria-africana/walter-sisulu-724/ and https://treesa.org/kiggelaria-africana/walter-sisulu-722/ and https://treesa.org/kiggelaria-africana/ and https://web.stanford.edu/~siegelr/flora/kiggelaria.html and https://cjmgrowers.co.za/kiggelaria-africana/ and https://www.theplantlibrary.co.za/plants/kiggelaria-africana and https://pza.sanbi.org/kiggelaria-africana), with their red arillate wrapping, are small enough to be swallowed by Zosterops virens?
I.e. do you think that Zosterops virens acts, w.r.t. K. africana, as a seed-disperser or as an aril-stealing 'cheat'?
https://www.perplexity.ai/search/Coliidae-are-dispersers-DDMo40GGTM65ntV9_60dGg
https://www.researchgate.net/figure/The-two-white-eye-species-in-Sri-Lanka-A-Distribution-of-Z-palpebrosus-and-Z_fig2_319022670
@tonyrebelo
Kopij (2004, https://www.researchgate.net/publication/249439178_Summer_and_winter_diet_of_the_Cape_white-eye_Zosterops_pallidus_in_South_African_grassland) recorded the following items, relevant to endozoochory, in the stomach of Zosterops:
'berries', 'fruit', 'fruit-pulp', seeds of Faboideae, seeds of 'Acacia', seeds of Portulaca oleracea, and seeds of Searsia lancea.
Zosterops was observed eating the 'fruits' of Nerium oleander, Ligustrum vulgare, and Crataegus sp.
Also found in stomachs were an intact individual, 3.5 cm long, of a praying mantis, and an individual, 3 cm long, of a millipede.
My commentary:
Zosterops swallowed invertebrates about 3 cm long, intact, as well as ostensibly the whole fruits (diameter about 0.5 cm) of Searsia lancea (https://pza.sanbi.org/searsia-lancea and https://plantnet.rbgsyd.nsw.gov.au/cgi-bin/NSWfl.pl?page=nswfl&lvl=sp&name=Searsia~lancea#:~:text=Fruit%20a%20depressed%2Dglobose%20drupe,or%20brown%2C%20with%20fleshy%20pulp.&text=Distribution%20and%20occurrence%3A%20Searsia%20lancea,Zimbabwe%2C%20South%20Africa%20and%20Lesotho..)
This does not necessarily indicate that it would swallow the arillate diaspores (about 0.7cm diameter) of Kiggelaria africana, let alone whole fleshy fruits of 1 cm diameter.
I am surprised that Zosterops ate seeds of Nerium, of pea-plants (non-endozoochorous pods), of 'Acacia' (genus = ?), and of Portulaca (non-zoochorous).
Given that Zosterops has here been recorded swallowing the fruits of a species of Searsia, and given that most spp. of Searsia have fruits of similar (small) size, it seems possible that Zosterops disperses and sows many spp. of Searsia in southern Africa. These fruits are noteworthy because the pulp layer - although technically fleshy - tends, when fully ripe, to dry out, leaving a palatable 'skin' loosely enveloping the seed.
More generally, the fruits of various spp. of Searsia are recognised as being eaten by Zosterops pallidus, according to the 7th edition of Roberts' Birds of South Africa (https://www.robertsbirds.co.za/roberts-7.htm).
https://animaldiversity.org/accounts/Zosterops_pallidus/
https://www.perplexity.ai/search/Is-the-tongue-duWqgSbUSFi151cQDrbRzA
@tonyrebelo
In Australia, Zosterops eats the succulent fruits of Chenopodium/Rhagodia (semi-halophytic scandent shrubs of Amaranthaceae, bearing small fleshy leaves), according to https://www.jstor.org/stable/4090021.
https://www.inaturalist.org/taxa/502259-Chenopodium-parabolicum
This makes sense because these fruits, albeit conspicuously reddish when ripe, are small.
I have tasted the ripe fruits of Chenopodium baccatum (https://www.inaturalist.org/taxa/511299-Chenopodium-baccatum) on the littoral in the Perth Metropolitan area (https://www.cottesloecoastcare.org/learn/local-plants/). I found them to have an aftertaste so unpleasant and persistent that I have no intention of tasting them again.
https://www.perplexity.ai/search/When-Zosterops-disperses-M5FBt5MVRSmqRfRFj1vS0Q
Endozoochory involving Zosterops japonicus:
https://pubmed.ncbi.nlm.nih.gov/21645275/
https://www.researchgate.net/publication/235998123_Frugivory_and_seed_dispersal_by_birds_in_Hong_Kong_shrubland
https://www.sciencedirect.com/science/article/pii/S2351989417300860
In Hong Kong in mid-winter, Zosterops japonicus is known to defecate the seeds of at least 41 spp. of plants, many/most of them endozoochorous.
However, Z. japonicus swallows only the smaller fleshy fruits available in the local flora, because its gape is only 0.8 cm wide - compared to about 1.25 cm in the case of coexisting spp. of Pycnonotus.
https://www.perplexity.ai/search/Are-the-fruits-ER_mlAv2RN2zNzRO3u0v5A
https://library.dbca.wa.gov.au/static/Journals/082168/082168-25.pdf
https://animaldiversity.org/accounts/Zosterops_pallidus/
@tonyrebelo @jeremygilmore
It has just occurred to me that another plant in the Cape Floristic Region that may possibly attract Zosterops virens capensis as a significant pollinator is Metrosideros angustifolia (https://www.inaturalist.org/taxa/428239-Metrosideros-angustifolia).
What triggered this idea, for me, is that Zosterops japonicus in Hawaii (where it was anthropogenically introduced) competes with various indigenous birds for the nectar of Metrosideros polymorpha (https://www.inaturalist.org/taxa/60447-Metrosideros-polymorpha and https://www.srs.fs.usda.gov/pubs/misc/ag_654/volume_2/metrosideros/polymorpha.htm
and https://saveplants.org/plant-profile/?CPCNum=46689).
The stamens of M. polymorpha are long enough to demand long-beaked birds for pollination. Thus, in Hawaii, Zosterops probably 'cheats' on the pollination syndrome.
However, M. angustifolia has short stamens, suggesting that Zosterops may be part of its natural syndrome of mutualistic pollination.
Your thoughts?
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