Failure of evolutionary convergence in 'nectarivorous' birds between Australia (Meliphagidae) and southern Africa (Nectariniidae), part 2

...continued from https://www.inaturalist.org/journal/milewski/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae-part-1#

INTERCONTINENTAL SIMILARITY IN LACK OF LERP/MANNA/EXTRAFLORAL NECTAR IN DIETS

Meliphagids tend to eat honeydew/lerp/manna/extrafloral nectar as well as floral nectar (https://www.publish.csiro.au/mu/pdf/mu9800213#:~:text=Manna%2C%20honeydew%20and%20lerp%20have,of%20these%20resources%20and%20nectar. and https://www.tandfonline.com/doi/abs/10.1080/01584197.1980.11799277).

By contrast, nectariniids have not been recorded eating these alternative sugary exudates. The only exception of which I am aware is the ostensible eating of the sap of Elaeis guineensis (https://en.wikipedia.org/wiki/Elaeis_guineensis) by Cinnyris coccinigastrus (https://www.inaturalist.org/taxa/145182-Cinnyris-coccinigastrus), which may be anthropogenic.

I have not found any records of the eating of lerp/manna/extrafloral nectar in the meliphagids examined in this Post. This, as far as it goes, supports the notion of evolutionary convergence.

DISCUSSION

The above information shows a pattern of 'so near and yet so far'. There seems, at first sight, to be evolutionary convergence between certain meliphagids in Australia and certain nectariniids in southern Africa. However, closer scrutiny shows significant disparities in form and function.

There are two main ways of interpreting these disparities. Either the environments on the two landmasses are different enough that the adaptive outcomes (by means of natural selection) are also different, or convergence has been constrained by shortcomings in the ancestors.

In turn, the shortcomings were owing either to phylogenetic constraints or to geographical barriers to recruitment of suitable ancestors.

It is tempting to invoke the longstanding isolation of Australia, in explanation of the fact that a single family, viz. Meliphagidae, has undergone major adaptive radiation to a degree unrivalled by any avian family in southern Africa.

PHYLOGENETIC CONSTRAINTS?

The intercontinental differences pointed out here cannot be explained by means of phylogenetic constraints and the geographical isolation of Australia. This is because meliphagids show much adaptive versatility in relevant ways, including

• iridescence,
• form of the tongue, and
• form of the nest.

Iridescence is known in at least one species of meliphagid (https://en.wikipedia.org/wiki/T%C5%AB%C4%AB and https://www.inaturalist.org/taxa/12580-Prosthemadera-novaeseelandiae). Therefore, the failure of all the members of this family examined in this Post to develop iridescence is unlikely to be an accident of evolutionary history. It is instead likely to be adaptive in some poorly-understood way.

By a similar token, a brush-tipped, not tubular, tongue is known in at least one species of nectariniid (https://sora.unm.edu/sites/default/files/journals/condor/v073n04/p0485-p0486.pdf and https://www.inaturalist.org/taxa/979744-Kurochkinegramma-hypogrammicum).

One species of meliphagid (https://www.inaturalist.org/taxa/12364-Ramsayornis-modestus) does actually build a nest similar to that of nectariniids (https://www.perplexity.ai/search/does-the-brown-backed-honeyeat-Owk0Fg1ySme.S4UJwWfsEA). This is particularly significant because the other species in the same genus builds a cup-shaped nest, typical of meliphagids.

Again: the failure of all the members of this family examined in this Post to build dome nests is unlikely to be an accident of evolutionary history. It is instead likely to be adaptive, and the relative paucity of certain predators on the 'island continent' is a possible factor.

Also relevant to the question of phylogenetic constraints is the fact that nectariniids have in fact reached Australia. Indeed, several genera may have reached what is now the Australian mainland, when there was a broad land-bridge from what is now New Guinea. The failure of nectariniids to undergo evolutionary radiation across Australia indicates ecological unsuitability rather than an accident of history/phylogenetic constraints.

Further investigation is warranted of the possible difference between meliphagids and nectariniids in predation on relatively large and venomous spiders. My impression from the literature is that nectariniids are the family more resembling 'arachnophages', in the sense that they use their long beaks not only to probe flowers, but also to kill spiders safely (https://www.inaturalist.org/posts/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae#activity_comment_0590a394-7178-4d76-bec2-11bc68b0e569 and https://en.wikipedia.org/wiki/Black-bellied_sunbird).

Publicado el 27 de septiembre de 2024 a las 10:49 AM por milewski