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

INTRODUCTION

Australia and southern Africa are ecologically comparable.

Both landmasses possess a range of climates, from mediterranean and adjacent arid to summer-rainfall tropical. Furthermore, in both cases the substrates tend to be nutrient-poor, owing to profound weathering on a largely flat topography.

In both Australia and southern Africa, there are many and various plants pollinated mainly by birds. On both landmasses, the flowers/inflorescences in question tend to be bright-hued, bearing copious nectar deep within a structure that makes it adaptive for nectarivorous animals to have long mouthparts, including long, curved beaks in the case of birds.

It is widely believed that certain Meliphagidae in Australia and certain Nectariniidae in southern Africa show evolutionary convergence, as part of an adaptive syndrome of mutualism with ornithophilous plants (https://en.wikipedia.org/wiki/Ornithophily).

However, closer scrutiny undermines this 'textbook' interpretation. The fallacy is evident not only because most meliphagids differ obviously from nectariniids in having relatively large bodies and relatively short beaks. Even when the closest intercontinental counterparts are carefully compared, noteworthy disparities emerge.

Meliphagids, which are broadly associated with ornithophily, have undergone an extreme evolutionary radiation in Australasia. However, my main finding is that it is the differences between meliphagids and nectariniids that are more significant than the similarities.

NON-CONVERGENT ASPECTS

The main intercontinental differences are as follows.

Firstly, the tongues are remarkably different.

In meliphagids, the tongue is fairly simple except for its length and its fimbriated (brush-like) tip (https://www.merriam-webster.com/dictionary/fimbriated and https://www.perplexity.ai/search/does-the-morphology-of-the-ton-dWPr2lh8Qi.prkVX4Uv7Jg).

By contrast, in nectariniids the tongue is extremely specialised, relative to all nectarivorous birds worldwide (https://www.biorxiv.org/content/10.1101/2024.05.14.594085v1.full.pdf). This is because it operates by means of suction, not capillarity or pressure exerted by the closure of the beak.

In other words, the tongue of nectariniids serves as a drinking straw (https://en.wikipedia.org/wiki/Drinking_straw), the main complication being the distal bifurcation of the straw.

Secondly, no meliphagid has iridescent plumage. By contrast, most nectariniids possess iridescent feathers, at least in males in breeding plumage.

Among meliphagids, the genus most closely approaching iridescence is Myzomela. However, the beak in Myzomela is relatively short, reflecting a relatively generalised diet, lacking any ornithophilous specialisation.

Thirdly, no meliphagid builds a 'dome' nest. By contrast, all nectariniids build nests with roofs. The closest convergence is achieved by e.g. Lichmera indistincta, the nest of which is sock-like - but still open at the top.

Fourthly, meliphagids tend to eat honeydew/lerp /manna/extrafloral nectar as well as floral nectar. By contrast, nectariniids have not been recorded eating these alternative sugary exudates.

CLOSEST INTERCONTINENTAL COUNTERPARTS

Please note: in all cases the difference remains that

• the tongue of meliphagids is fimbriate, whereas that of nectariniids is tubular, and
• the nest of meliphagids is open at the top, whereas that of nectariniids is sealed at the top.

The meliphagid Sugomel nigrum (https://www.inaturalist.org/taxa/367631-Sugomel-nigrum, body length 11 cm, body mass 9.5 g) is fairly closely matched with the nectariniid Cinnyris fuscus (https://www.inaturalist.org/taxa/145189-Cinnyris-fuscus and https://thebdi.org/2024/06/04/dusky-sunbird-cinnyris-fuscus/, body length 10 cm, body mass 7-10 g).

In both cases,

• the plumage is dull-hued (dark and pale), with iridescence slight even in the nectariniid,
• the habitat is the semi-arid interior of the landmass, where the vegetation is sparse, and
• the populations tend to be nomadic,

However, the following differences remain. The meliphagid

• is somewhat larger-bodied than the nectariniid,
• is associated mainly with Myoporaceae (https://www.inaturalist.org/observations?subview=map&taxon_id=136365&view=species), whereas the nectariniid is associated mainly with Asphodelaceae (https://www.inaturalist.org/observations?place_id=113055&subview=map&taxon_id=71400&view=species), and
• qualifies, in males, as pied/piebald (https://www.merriam-webster.com/dictionary/piebald), which is not emulated by C. fuscus, let alone any other nectariniid.

The meliphagid Acanthorhynchus superciliosus (https://www.inaturalist.org/taxa/12237-Acanthorhynchus-superciliosus, female length 14 cm, body mass g) is somewhat similar to the nectariniid Anthobaphes violacea (https://www.inaturalist.org/taxa/145130-Anthobaphes-violacea, female length 12 cm, body mass ? g).

In both cases,

• the beak is proportionately fairly long,
• there is a particular coevolutionary (mutualistic) association with extremely diverse floras (e.g. Proteaceae) in sclerophyllous, fire-prone vegetation (https://en.wikipedia.org/wiki/Kwongan and https://en.wikipedia.org/wiki/Fynbos) on nutrient-poor, sandy substrates under mediterranean climates, and
• clutch-size, normally two in both families of birds, is often only one in both A. supercilious and A. violacea.

However, differences remains in

• body size, with the meliphagid somewhat the larger (additionally verified by the fact that egg sizes differ, viz. 18X13 mm in the meliphagid, vs 16.5X12.4 mm in the nectariniid), and
• colouration, with conspicuously pale-tipped tail (in flight) only in the Australian species, and iridescence only in the southern African species.

Myzomela sanguinolenta (https://www.inaturalist.org/taxa/12443-Myzomela-sanguinolenta, body length 10 cm, body mass 8 g) is fairly similar to Hedydipna collaris (https://www.inaturalist.org/taxa/145122-Hedydipna-collaris, body length 10 cm, body mass 8 g).

Both spp.

• are equally small-bodied, with M. sanguinolenta being the smallest-bodied and brightest-hued (in the case of males) of all meliphagids (other than Ephthianura),
• have short beaks,
• are insectivorous as much as nectarivorous,
• depend on dense forests, and
• reach nearly to the southern extreme of the mainland, where rain falls year-round.

However,

• females of the meliphagid (https://www.graemechapman.com.au/library/viewphotos.php?c=268&pg=2) are duller-hued than those of the nectariniid (which are yellow and green, https://ebird.org/species/colsun2/TZ#:~:text=The%20male%20has%20a%20green,hover%20to%20catch%20insect%20prey.),
• the meliphagid, unlike the nectariniid, is nomadic and associated mainly with flowering Myrtaceae,
• the meliphagid forages conspicuously, whereas the nectariniid forages unobtrusively, and
• the size of the clutch may differ, with only the nectariniid likely to lay as many as three eggs per breeding-attempt.

The intercontinental difference in the nests - in size as well as shape - is illustrated in:

• M. sanguinolenta https://www.birdsinbackyards.net/species/Myzomela-sanguinolenta, vs
• H. collaris https://upload.wikimedia.org/wikipedia/commons/9/94/Collared_Sunbird_%28Hedydipna_collaris%29_female_on_nest_..._%2830429792623%29.jpg.

The meliphagid Lichmera indistincta (https://www.inaturalist.org/taxa/12526-Lichmera-indistincta), widespread in Australia, is a qualified counterpart for the nectarinid Cyanomitra olivacea (https://www.inaturalist.org/taxa/145136-Cyanomitra-olivacea).

For further investigation: which is more similar to C. olivacea, L. indistincta or Myzomela obscura? (https://www.inaturalist.org/taxa/12376-Myzomela-obscura)

Both spp.

• are similar in body size and length of beak,
• occur (inter alia in the Australian case) in wildfire-free forest on the eastern coastal strips of the landmasses,
• are remarkably dull-coloured (with no iridescence in the meliphagid, even in males during the breeding season), and
• have remarkably loud vocalisations (as if in compensation for the visual inconspicuousness).

Furthermore, the nest of L. indistincta is more similar to that of nectariniids than is true for most meliphagids.

Finally:
Cinnyris frenatus (https://www.inaturalist.org/taxa/1504995-Cinnyris-frenatus) of tropical northeasternmost Australia is extremely similar to Cinnyris venustus (https://www.inaturalist.org/taxa/145188-Cinnyris-venustus, length 11 cm) of northeasternmost southern Africa. Both spp. are equatorial to tropical. The colouration is similar, including the sexual difference.

This, the closest matching of all, is not a case of evolutionary convergence. This is because the intercontinental counterparts belong to a single, exceptionally widespread nectariniid genus.

DISCUSSION

In general, the norm in meliphagids is relatively large-bodied and short-beaked, whereas the norm in nectariniids is small-bodied and long-beaked.

However, the intercontinental differences pointed out here cannot be explained by means of phylogenetic constraints and the geographical isolation of Australia.

This is partly because:
Meliphagids have not reached southern Africa, but the converse does not apply. Nectariniids, although originating in Africa/Asia, have in fact reached Australasia - where they have failed to undergo evolutionary radiation.

There are two permanent sea barriers between southeast Asia and first Wallacea (https://en.wikipedia.org/wiki/Wallacea) and then Australasia. The first is called the Wallace Line (https://en.wikipedia.org/wiki/Wallace_Line), and the second is called the Weber Line.

The following nectariniids occur in Wallacea:

• Anthreptes malaccensis (https://www.inaturalist.org/taxa/979598-Anthreptes-malacensis) extends to Sulawesi and the Lesser Sunda islands.
• Aethopyga siparaja (https://www.inaturalist.org/taxa/204458-Aethopyga-siparaja) extends to Sulawesi.
• Aethopyga duyvenbodei (https://www.inaturalist.org/taxa/13420-Aethopyga-duyvenbodei) occurs on Sangihe, an island located immediately north of Sulawesi.
• Cinnyris ornatus (https://www.inaturalist.org/taxa/1505007-Cinnyris-ornatus) extends to the Lesser Sunda islands (https://en.wikipedia.org/wiki/Lesser_Sunda_Islands).
• Cinnyris infrenatus (https://www.inaturalist.org/taxa/1505008-Cinnyris-infrenatus) occurs in southeasternmost Sulawesi (https://en.wikipedia.org/wiki/Wakatobi_Regency).
• Cinnyris teysmanni (https://www.inaturalist.org/taxa/1505012-Cinnyris-teysmanni) occurs on Kalaotoa Island.
• Aethopyga duyvenbodei (https://www.inaturalist.org/taxa/13420-Aethopyga-duyvenbodei) occurs in northeasternmost Sulawesi.
• Cinnyris buettikoferi (https://www.inaturalist.org/taxa/145195-Cinnyris-buettikoferi) occurs on Sumba Island.
• Cinnyris solaris (https://www.inaturalist.org/taxa/145196-Cinnyris-solaris) occurs on the Lesser Sunda islands and Timor island.

The following nectariniids occur in Australasia (https://en.wikipedia.org/wiki/Australasia):

• Cinnyris clementei (https://www.inaturalist.org/taxa/1505011-Cinnyris-clementiae) occurs in Maluku.
• Leptocoma aspasia (https://www.inaturalist.org/taxa/979764-Leptocoma-aspasia) of New Guinea is generalised in diet - with invertebrates and fleshy fruit-pulp apparently being equally important - than its congeners and most other nectariniids.
• Cinnyris fuscatus (https://www.inaturalist.org/taxa/1504995-Cinnyris-frenatus) occurs from Sulawesi through New Guinea (but not the island chain of the Lesser Sundas/Sumba/Timor) to northeastern Australia.

It is noteworthy that C. fuscatus coexists in northeastern Australia with the superficially nectariniid-like meliphagid, Myzomela obscura (https://www.inaturalist.org/taxa/12376-Myzomela-obscura).

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).

Publicado el septiembre 17, 2024 11:18 TARDE por milewski