A new interpretation of the evolutionary and ecological strategy of whiteyes/silvereyes (Zosterops), part 1

https://www.researchgate.net/publication/274446673_Resource_availability_and_foraging_of_Silvereyes_Zosterops_lateralis_in_urban_trees

Zosterops spp. (whiteyes and silvereyes, https://www.inaturalist.org/observations?place_id=any&subview=map&taxon_id=17439&view=species) are odd in several ways.

They are

• remarkably wide/versatile in diet, for such small-bodied birds (about 10 grams adult body mass in both sexes);
• the only birds with a brush-tipped tongue that generally 'cheat' on pollination syndromes, rather than participating in ornithophily (https://en.wikipedia.org/wiki/Ornithophily) in return for nectar consumed;
• effective dispersers and sowers of many taxa of plants via endozoochory (https://en.wikipedia.org/wiki/Seed_dispersal and https://onlinelibrary.wiley.com/doi/10.1111/j.1744-7429.2009.00503.x), despite having gapes too narrow (< 1 cm) to swallow the fleshy fruits of most bird-dispersed taxa of plants;
• the only genus of small passerines (https://en.wikipedia.org/wiki/Passerine) that has naturally colonised many oceanic islands despite having a 'generalised' niche; and
• equally abundant in nutrient-poor/fire-prone ecosystems (e.g. in Australia, https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1469-185X.2007.00017.x and https://pubmed.ncbi.nlm.nih.gov/17624961/) and nutrient-rich/fire-free ecosystems (e.g. in Africa).

Crucial is the observation that Zosterops oddly combines a brush-tipped tongue with a short beak (https://www.perplexity.ai/search/Please-list-all-P5272Q3URqe3IcoI547GAg).

In this Post, I introduce the idea that the original evolutionary strategy of Zosterops was to be simultaneously

• predacious on soft homopterans, such as aphids (https://en.wikipedia.org/wiki/Aphid), and
• parasitic on the honeydew-based mutualism between ants (https://en.wikipedia.org/wiki/Ant) and plants (https://en.wikipedia.org/wiki/Honeydew_(secretion)).

https://www.perplexity.ai/search/Which-families-of-sCsbqCTrSvePIqWOpBrZLw
Delphacidae https://en.wikipedia.org/wiki/Delphacidae

Most small birds participating in ornithophily have long beaks (https://pubmed.ncbi.nlm.nih.gov/36062802/ and https://evolution.berkeley.edu/convergent-evolution-its-for-the-birds/). These are inefficient for gleaning homopterans.

Furthermore, aphids tend to be scarce

• in the nutrient-poor, fire-prone ecosystems typically associated with Meliphagidae and Promeropidae, and
• on oceanic islands.

In Australia, aphids are generally replaced by psyllids (https://en.wikipedia.org/wiki/Psyllid) and other armoured forms of sap-sucking Hemiptera (https://en.wikipedia.org/wiki/Hemiptera), predated mainly by small passerines with specialised, broad beaks (https://en.wikipedia.org/wiki/Pardalote and https://en.wikipedia.org/wiki/Weebill).

Long-beaked, nectarivorous birds do take remarkably small insects routinely, but mainly by hawking (https://en.wikipedia.org/wiki/Hawking_(birds) and https://researchportal.murdoch.edu.au/esploro/outputs/other/The-possible-ecological-significance-of-hawking/991005541828107891 and https://www.researchgate.net/publication/283793917_Ecology_of_Honeyeaters_Meliphagidae_in_Western_Australian_Eucalypt_Woodlands_I_Resource_Allocation_Among_Species_in_the_Great_Western_Woodland_During_Spring and https://www.sciencedirect.com/science/article/abs/pii/S0003347205802295).

I suggest that Zosterops has evolved essentially as a consumer of, simultaneously,

• the sugary secretions produced by soft homopterans (by means of the short but brush-tipped tongue), and
• the aphids and other soft homopterans themselves (by means of the short, forceps-like beak, https://www.facebook.com/photo/?fbid=2915641135364167&set=pb.100064864024099.-2207520000 and https://www.researchgate.net/figure/photos-of-the-Zosterops-species-studied-The-right-column-shows-the-Pale-bellied_fig3_332268500).

From this origin, Zosterops has been able

• to succeed on those small islands to which the widespread mutualism between homopterans and protective ants does not extend, and
• to adapt to a remarkably wide range of ecosystems, by means of certain 'laterally-stretched' applications of its basic nature.

The various adaptations of Zosterops have been mainly behavioral, not morphological: it has retained small body size and a consistent shape (including the beak), while being versatile in other ways. The genus has applied a single body-form to various niches, in an 'evolutionary radiation' antithetical to the classic example of modification of body-form in Darwin's finches (https://en.wikipedia.org/wiki/Darwin%27s_finches).

Given that its taste for sugars was originally oriented towards honeydew, the taking of fleshy fruit-pulp and -juice by Zosterops can be interpreted as opportunistic rather than central to the evolutionary strategy.

This in turn helps to explain why Zosterops has remained small-bodied relative to most coexisting birds that eat fleshy fruits, forgoing many fruits too large for it to swallow whole.

In particular, Zosterops varies in

• residency (sedentary vs nomadic vs migratory), albeit never a migrant between hemispheres,
• diet, e.g. emancipated from aphids in Australia, where it takes sugary exudates from eucalypts and Acacia (in the strict sense, https://en.wikipedia.org/wiki/Acacia) instead, and
• mutualistic participation with a few clades of plants (e.g. Corymbia, Berzelia, ?Metrosideros) in pollination, and
• mutualistic participation with many clades of plants in seed-dispersal.

In southwestern Western Australia, the subspecies Zosterops lateralis chloronotus (https://www.inaturalist.org/posts/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) naturally had a habitat - throughout the year and regardless of its partial latitudinal migration - virtually devoid of aphids. I refer to the situation before aphids were anthropogenically introduced to Australia, and inadvertently promoted by horticulture, agriculture, and the application of fertilisers.

Here, it has applied its brush-tipped tongue to the extrafloral nectaries of Acacia (https://www.researchgate.net/figure/Extrafloral-nectaries-EFNs-are-universal-on-the-leaves-and-phyllodes-of-SouthWest_fig16_276919754).

In Hawaii, aphids and ants - and Zosterops - were naturally absent from the indigenous biota (https://www.perplexity.ai/search/Which-genera-of-oM7eVb_fTQyunhB_CqpaRw). The extreme success of the anthropogenic introduction of Zosterops to this archipelago has probably been facilitated by the introduction of the insects and plants necessary to establish the mutualism that produces honeydew.

To what degree has Zosterops adapted to the anthropogenic introduction of aphids in Australia, by eating these insects?

https://animaldiversity.org/accounts/Zosterops_pallidus/

https://www.perplexity.ai/search/In-which-ways-hUmRnLf5Ry2CWdzouOK9_A

to be continued in https://www.inaturalist.org/journal/milewski/95204-a-new-interpretation-of-the-evolutionary-and-ecological-strategy-of-whiteyes-and-silvereyes-zosterops-part-2#...

Publicado el mayo 29, 2024 03:53 TARDE por milewski