Failure of evolutionary convergence in 'nectarivorous' birds between Australia (Meliphagidae) and southern Africa (Nectariniidae), part 1

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INTRODUCTION

Australia and southern Africa are ecologically comparable.

Both landmasses possess a range of climates, from mediterranean (https://en.wikipedia.org/wiki/Mediterranean_climate) and adjacent arid to summer-rainfall tropical (https://en.wikipedia.org/wiki/Climate_of_Australia#:~:text=The%20largest%20part%20of%20Australia,varying%20between%20grasslands%20and%20desert. and https://en.wikipedia.org/wiki/Climate_of_South_Africa).

Furthermore, in both cases the substrates tend to be nutrient-poor, owing to profound weathering on a largely flat topography (https://pubmed.ncbi.nlm.nih.gov/17624961/ and https://www.researchgate.net/publication/6213166_Ecology_of_Australia_The_effects_of_nutrient-poor_soils_and_intense_fires and https://www.jstor.org/stable/2845371).

In both Australia and southern Africa, there are many and various plants pollinated mainly by birds (https://www.tandfonline.com/doi/pdf/10.1080/0028825X.1979.10432566#:~:text=The%20genera%20most%20frequently%20visited,insects%20as%20well%20as%20birds. and https://www.tandfonline.com/doi/abs/10.1080/0028825X.1979.10432566 and https://mdahlem.net/birds/plant/pollen.php and https://science.uct.ac.za/fitzpatrick/research-understanding-biodiversity-evolutionary-and-behavioural-ecology/bird-pollination-cape-floristic-region and https://naturesvalleytrust.co.za/wp-content/uploads/2023/07/Whitehead-K.-2018.pdf and https://www.sciencedirect.com/science/article/abs/pii/S0367253006000259 and https://www.jstor.org/stable/43234070).

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 passerine birds to have long, curved beaks.

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 variation seems sufficient for close counterparts potentially to have evolved on the two separate landmasses.

AIMS

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

The aim of this Post is to test evolutionary convergence in the case of meliphagids in Australia vs nectariniids in southern Africa.

NON-CONVERGENCES EMERGING FROM MY INTERCONTINENTAL COMPARISON OF MELIPHAGIDS WITH NECTARINIIDS

The following scrutiny undermines the 'textbook' interpretation.

Meliphagids have undergone an extreme evolutionary radiation on the 'island continent', Australia. However, it is the differences between meliphagids and nectariniids that I find to be more significant than the similarities.

This failure of evolutionary convergence is partly in line with the observation that meliphagids generally differ from nectariniids in having relatively large bodies and relatively short beaks. However, the biological disparities are manifold, and most remain even in the closest intercontinental counterparts.

The main intercontinental disparities are as follows.

Firstly, no meliphagid in Australia has a beak as proportionately long as that of certain nectariniids in southern Africa.

In three genera (Nectarinia, Chalcomitra, and Cinnyris) in southern Africa, the length of the beak is at least 18% of the total length of the body (including the beak) in certain spp.

This is illustrated for Cinnyris afer in https://www.youtube.com/watch?app=desktop&v=d7Gg2wa9AJw.

The genus of meliphagids with the proportionately longest beak in Australia is Acanthorhynchus, for which I have found no measurements of beak length. However, this is less than 15% of body length, based on photos.

The meliphagid with the longest beak occurs beyond Australia, in New Guinea (https://en.wikipedia.org/wiki/Long-billed_honeyeater and https://ebird.org/species/lobhon2 and https://www.inaturalist.org/taxa/12359-Melilestes-megarhynchus). Even in this case, the beak seems not to rival that in the most extreme nectariniids in slenderness and proportional length.

Secondly, the tongues are remarkably different.

In meliphagids, the tongue is fairly simple except for its brush-like tip (https://www.perplexity.ai/search/does-the-morphology-of-the-ton-dWPr2lh8Qi.prkVX4Uv7Jg and https://www.perplexity.ai/search/can-the-tongue-of-any-nectarin-M7OaIr9IR86NcZp.ZVFZjA).

By contrast, in nectariniids the tongue is odd even among nectarivorous birds (https://www.biorxiv.org/content/10.1101/2024.05.14.594085v1.full.pdf). This is because it operates by means of suction, as opposed to either 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.

Thirdly, the nostrils are not covered (operculate) in all meliphagids (https://avis.indianbiodiversity.org/books/meliphagidae/40335), whereas they are typically covered in nectariniids (https://biodiversity.org.au/afd/taxa/Nectariniidae and scroll to photo of Nectarinia famosa in https://www.sciencenews.org/article/sunbirds-feathers-iridescence-heat-light).

Fourthly, plumage and colouration differ according to the following syndrome:

• meliphagids tend to be less sexually dimorphic than nectariniids;
• males of meliphagids lack breeding plumage in a seasonal context (https://www.perplexity.ai/search/in-nectariniidae-do-adult-male-rxXZ85P0T62TD9WeS6wEgA);
• meliphagids (except for one species in New Zealand) lack iridescence in their plumage; by contrast, most nectariniids possess iridescent feathers, at least in males;
• meliphagids never possess eclipse plumage (https://thebdi.org/glossary/eclipse-plumage/), which is common in nectariniids; and
• meliphagids lack the pectoral tufts (which are conspicuously bright-hued) possessed by many nectariniids (https://www.perplexity.ai/search/does-any-meliphagid-possess-pe-xLw1BDwASGGbtUQe5_JhhA).

Fifthly, most meliphagids do not build a 'dome' nest. By contrast, all nectariniids build nests protected and hidden by a roof.

Sixthly, the smallest-bodied meliphagids exceed the smallest-bodied nectariniids in body mass. This applies within Australia and southern Africa, where the minima are reached, respectively, by Myzomela sanguinolenta (females 8 g) and Cinnyris neergardi (females 6 g, https://www.inaturalist.org/posts/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae#activity_comment_65e44197-f882-4a14-909c-03d7b2635e5d).

CLOSEST INTERCONTINENTAL COUNTERPARTS

(My values for body length and body mass refer to adult females.)

In all the following cases:

• the tongue of meliphagids is brush-like, whereas that of nectariniids is tubular,
• the nest of meliphagids is open at the top, whereas that of nectariniids is sealed above, and
• there are no records of the consumption of sugary exudates other than floral nectar.

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 ?8 g).

The habitat in both cases is the semi-arid interior of the landmass, where vegetation (including acacias in both cases) is sparse,

In both of these approximate counterparts,

• the plumage of males achieves conspicuousness by means of darkness, not hue or iridescence,
• the colouration differs between the sexes, and
• the populations tend to be nomadic.

The similarity in colouration is greatest when males of the nectariniid are in non-breeding plumage (https://www.inaturalist.org/posts/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae#activity_comment_68c95b95-0846-4b9d-85ab-c2631edd7749).

However, the following differences remain:

• the meliphagid remains larger-bodied than the nectariniid, with a proportionately smaller head and shorter beak,
• the meliphagid qualifies, in males, as pied/piebald (https://www.merriam-webster.com/dictionary/piebald), which is not quite emulated by C. fuscus (https://www.inaturalist.org/observations/229807463), let alone any other nectariniid,
• the meliphagid has frequently been recorded eating ash (https://www.researchgate.net/publication/289010983_Some_notes_on_the_Black_Honeyeater_Sugomel_niger_in_Western_Australia_with_special_reference_to_feeding_on_ash), which as far as I know has never been recorded in any nectariniid,
• the nectariniid remains categorically more iridescent (https://www.inaturalist.org/observations/47327328 and https://www.inaturalist.org/observations/152404812 and https://www.inaturalist.org/observations/122277798 and https://www.inaturalist.org/observations/103055723 and https://www.inaturalist.org/observations/74663407 and https://www.inaturalist.org/observations/47327356 and https://www.inaturalist.org/observations/27908216 and https://www.inaturalist.org/observations/156621773) than any meliphagid in Australia, and
• males of the nectariniid - even in non-breeding plumage - retain bright-hued pectoral tufts (https://www.inaturalist.org/observations/17509372 and https://www.inaturalist.org/observations/16648040 and https://www.inaturalist.org/observations/6591102), absent from meliphagids.

Sugomel nigrum is associated mainly with Myoporaceae (https://www.inaturalist.org/observations?subview=map&taxon_id=136365&view=species). The nectariniid is instead associated mainly with Asphodelaceae (https://www.inaturalist.org/observations?place_id=113055&subview=map&taxon_id=71400&view=species and https://www.naturepl.com/stock-photo-dusky-sunbird-cinnyris-fuscus-feeding-on-the-nectar-of-quiver-tree-nature-image01596050.html and https://www.inaturalist.org/observations/224213747 and https://www.inaturalist.org/observations/55666885).

The meliphagid Acanthorhynchus superciliosus (https://www.inaturalist.org/taxa/12237-Acanthorhynchus-superciliosus, female length 14 cm, body mass 9 g) is somewhat similar to the nectariniid Anthobaphes violacea (https://www.inaturalist.org/taxa/145130-Anthobaphes-violacea and http://stellenboschwriters.com/Stellenbirds/sun1.html, female length 12 cm, body mass 9.5 g, https://sora.unm.edu/sites/default/files/journals/condor/v095n01/p0115-p0126.pdf).

In both cases,

• the colouration differs between the sexes,
• the beak is proportionately long, and
• 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://heartlandjourneys.com.au/about-us/ancient-wonderland/kwongan/ and https://en.wikipedia.org/wiki/Fynbos) on nutrient-poor, sandy substrates under mediterranean climates.

However, differences remain in

• the length of the beak (https://www.inaturalist.org/posts/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae#activity_comment_90cc4622-bbd7-4503-ab74-f7891d54c419),
• aerial acrobatics, with the meliphagid the more powerful and rapid flier,
• colouration, with conspicuously pale-tipped tail (in flight) only in the Australian species, and iridescence only in the southern African species,
• clutch-size, which is normally two in the relevant meliphagids and nectariniids alike, but is often only one in A. superciliosus,
• egg size, which is 18 X 13 mm in the meliphagid, vs 16.5 X 12.4 mm in the nectariniid, and
• breeding season (https://www.inaturalist.org/posts/98889-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae#activity_comment_12c18a8d-90b2-4d37-9f98-ad42baca9ec5).

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 and https://dewetswild.com/tag/anthreptes-collaris/#:~:text=Collared%20Sunbirds%20breed%20mainly%20during,hives%20of%20bees%20or%20wasps. and https://www.biodiversityexplorer.info/birds/nectariniidae/hedydipna_collaris.htm, body length 9.5 cm, body mass 8 g).

The meliphagid actually exceeds the nectariniid in sexual dimorphism in colouration.

Furthermore, both spp.

• are equally small-bodied, with M. sanguinolenta being the smallest-bodied and brightest-hued (in the case of males, which qualify as glossy albeit not iridescent) 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.),
• not only does iridescence remain in the nectariniid, but it occurs in both females (https://www.inaturalist.org/observations/242214266) and males (https://www.inaturalist.org/observations/229059268 and https://www.inaturalist.org/observations/232094757 and https://www.inaturalist.org/observations/160806904),
• 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 Myzomela obscura (https://www.inaturalist.org/taxa/12376-Myzomela-obscura and https://search.informit.org/doi/abs/10.3316/informit.658559031573707, body length 13 cm, body mass ? g) is a counterpart for the nectarinid Cyanomitra olivacea (https://www.inaturalist.org/taxa/145136-Cyanomitra-olivacea, body length 13 cm, body mass 9 g).

Both forms

• lack sexual dimorphism in colouration,
• lack iridescence, even in males while breeding, and
• are remarkably dull-hued.

Myzomela obscura and C. olivacea are similar. They occur in wildfire-free, dense forests on the northeastern coastal strips of the landmasses.

However,

• the beak of the meliphagid (https://www.inaturalist.org/observations/191065425 and https://www.inaturalist.org/observations/217817820 and https://www.inaturalist.org/observations/215657135 and https://www.inaturalist.org/observations/199062178 and https://www.inaturalist.org/observations/101925662) is shorter than that of the nectariniid (https://www.inaturalist.org/observations/107888671 and https://www.inaturalist.org/observations/65335377 and https://www.inaturalist.org/observations/223149125),
• the diet of the meliphagid is more generalised than that of the nectariniid,
• the tongue of the meliphagid is particularly different from that of the nectariniid, because its tip has a particularly complex brush-like structure,
• the meliphagid lacks the bright-hued pectoral tufts retained by both sexes of the nectariniid (https://www.inaturalist.org/observations/7734503 and https://www.inaturalist.org/observations/91081303 and https://www.inaturalist.org/observations/226685593 and https://www.inaturalist.org/observations/193779256), and
• the nest of the nectariniid is even more different than usual from that of the meliphagid, because it has a 'tail', and the entrance is hooded.

Finally:
Cinnyris frenatus (https://www.inaturalist.org/taxa/1504995-Cinnyris-frenatus, body length 10 cm, body mass ?8 g) of tropical northeasternmost Australia is extremely similar to Cinnyris venustus (https://www.inaturalist.org/taxa/145188-Cinnyris-venustus, body length 10 cm, body mass 7 g) of northeasternmost southern Africa.

Both spp. are equatorial to tropical. The colouration is similar, including the sexual difference. (Is the Australian species the less sexually dimorphic in colouration?)

This, the closest intercontinental matching of all in the present context, does not represent much evolutionary convergence. This is because

• the intercontinental counterparts belong to a single, exceptionally widespread and speciose (total 63 spp.) genus, and
• the main convergence is in body size within the genus (relative to e.g. Cinnyris coccinigastrus, which may have double this body mass, based on body length of 14 cm).

to be continued in https://www.inaturalist.org/journal/milewski/99413-failure-of-evolutionary-convergence-in-nectarivorous-birds-between-australia-meliphagidae-and-southern-africa-nectariniidae-part-2#...

Posted on 17 September, 2024 23:18 by milewski