The ecological and biogeographical significance of Prodotiscus regulus, an anthropogenic addition to the avifauna of Cape Town, South Africa

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Prodotiscus regulus (https://www.inaturalist.org/taxa/17591-Prodotiscus-regulus) is

• a rather nondescript brown bird,
• a non-passerine (Indicatoridae, https://www.inaturalist.org/observations?place_id=any&taxon_id=17551&view=species) that unusually resembles passerines such as warblers and flycatchers,
• known mainly for laying its eggs in the nests of other birds, and
• brood-parasitic on small passerines (https://birdsoftheworld.org/bow/ornith-notes/JN100021).

The aim of this Post is to explain how P. regulus has come to be the only species of bird in the Cape Floristic Region (https://en.wikipedia.org/wiki/Cape_Floristic_Region) that specialises dietarily on the exudates of sap-sucking hemipteran insects (https://tcimag.tcia.org/training/sap-sucking-insects-how-they-feed-and-the-damage-they-cause/).

Like all members of its family (https://www.sciencedirect.com/science/article/abs/pii/S1095643302001307#:~:text=Birds%20ate%20significantly%20more%20new,transit%20time%20of%20256%20min. and https://pubmed.ncbi.nlm.nih.gov/12160878/), P. regulus can digest wax (https://en.wikipedia.org/wiki/Wax), as a major source of metabolic energy.

This is remarkable, because wax is

• digestible to few other birds (https://www.jstor.org/stable/4162225) or mammals (https://www2.whoi.edu/staff/bvanmooy/projects/host-and-microbial-contributions-to-wax-ester-lipid-digestion-in-arctic-whales/), and
• synthesized by various sap-sucking hemipterans, for protection against predators and fungi (https://www.tandfonline.com/doi/abs/10.1080/002229399300227 and https://academic.oup.com/aesa/article/97/6/1294/55003?login=false and https://www.researchgate.net/publication/233349841_Wax_glands_wax_production_and_the_functional_significance_of_wax_use_in_three_aphid_species_Homoptera_Aphididae).

Before European arrival, there was no niche for P. regulus in the southwestern part of South Africa.

This is mainly because

• the genus Prodotiscus, unlike the confamilial genus Indicator, seldom eats beeswax, and
• sap-sucking hemipterans were oddly scarce in the indigenous entomofauna of this region (https://www.inaturalist.org/journal/milewski/96522-a-puzzling-lack-of-honeydew-producing-hemipteran-insects-in-the-cape-floristic-region-of-south-africa#).

Exudates of sap-sucking hemipterans are mainly of two kinds, viz.

• liquid, sugary exudates called honeydew (https://en.wikipedia.org/wiki/Honeydew_(secretion)), and
• wax, suitable as food for P. regulus (https://www.biodiversityexplorer.info/birds/indicatoridae/prodotiscus_regulus.htm and https://en.wikipedia.org/wiki/Eriosoma_lanigerum and https://www.perplexity.ai/search/do-membracidae-produce-wax-l_Bvw430QxCpWNle8zIMrg and https://en.wikipedia.org/wiki/Treehopper and https://en.wikipedia.org/wiki/Monophlebidae).

Europeans introduced several spp. of Acacia (https://en.wikipedia.org/wiki/Acacia) from Australia to the Cape Floristic Region.

These shrubs and trees have proven to be so ecologically vigorous in their new environment that they are regarded as invasive (https://www.cabidigitallibrary.org/doi/10.1079/9781800622197.0026).

Also introduced - albeit mainly inadvertently - from various parts of the world were several hemipterans capable of sucking the sap of these acacias (https://www.perplexity.ai/search/australian-spp-of-acacia-have-5WdSTOFwQHK06p4MSWQYiw and https://en.wikipedia.org/wiki/Icerya_purchasi).

Now, for the first time in and near Cape Town (https://en.wikipedia.org/wiki/Cape_Town), there was a plentiful source of sap-sucking hemipterans and their exudates, as potential food for arboreal birds indigenous to South Africa.

In the case of honeydew, the main indigenous bird that seems to have benefited is Zosterops, a genus observed elsewhere in Africa to forage side-by-side with P. regulus (Friedmann 1955, https://repository.si.edu/handle/10088/10101 and https://scholar.google.com/citations?user=62DqSSUAAAAJ&hl=en).

Zosterops (https://www.inaturalist.org/observations?place_id=6986&taxon_id=17439&view=species) is a small-bodied passerine with the odd combination of a short, thin beak and a brush-tipped tongue. This allows it to lap up the newly-provided honeydew, in addition to its original staple diet of fleshy fruit-pulp (and -juice) and insects (https://www.researchgate.net/publication/249439178_Summer_and_winter_diet_of_the_Cape_white-eye_Zosterops_pallidus_in_South_African_grassland#:~:text=...-,The%20Cape%20white%2Deye%20is%20described%20as%20a%20generalist%20feeder,their%20diet%20(Kopij%202004)%20.).

However, this hardly changed Zosterops biogeographically, because it had been present in the Cape Floristic Region in the first place.

In the case of the waxy exudates, the only indigenous birds that might benefit were Indicatoridae.

As many as three spp. of Indicator may have been indigenous to the Cape Floristic Region, viz.

• https://www.inaturalist.org/taxa/17570-Indicator-indicator,
• https://www.inaturalist.org/taxa/204526-Indicator-minor, and
• https://www.inaturalist.org/taxa/17556-Indicator-variegatus.

However, this genus is adapted to take wax from the nests of Hymenoptera, not from the exudates of hemipterans. This preference may be explained partly by the fact that Indicator is larger-bodied than Prodotiscus (https://en.wikipedia.org/wiki/Honeyguide).

Instead, what seems to have happened is that a small-bodied species, viz. P. regulus, entered the Cape Floristic Region for the first time during the twentieth century. This spontaneous recruitment filled the newly-provided niche.

There was no competition between P. regulus and Zosterops, because

• the former is hardly able to ingest honeydew, and
• the latter is unable to digest wax.

Honeydew is not utilised by Indicatoridae, despite

• the general association of this family with honey, and
• the common name 'honeybird', sometimes applied to P. regulus (https://www.birdlife.org.za/wp-content/uploads/2020/06/60-63_Honeybird4.pdf).

The inutility of honeydew for Indicatoridae, including P. regulus, seems to be because they

• lack a brush-tipped tongue, and
• thus differ remarkably from other members of Piciformes, viz. woodpeckers, barbets, and toucans, all of which include spp. with brush-tipped tongues (https://www.perplexity.ai/search/do-any-piciformes-nave-a-brush-QQ1oKDaiQjyqIbdsIT9Fig and https://www.proquest.com/openview/acbe0a58fe96bea399b0987a1c1eb827/1?pq-origsite=gscholar&cbl=18750&diss=y).

What has arisen is something biogeographically remarkable, and overlooked by naturalists despite the avifauna of Cape Town, and the Cape Floristic Region, being intensively studied.

This is that

• the recent advent of P. regulus in this vicinity is a categorical addition, in terms of niches occupied,
• South Africa now has the only region of mediterranean-type climate (https://en.wikipedia.org/wiki/Mediterranean_climate), worldwide, that features any bird capable of digesting the waxy exudates of hemipterans, and
• something has fortuitously been achieved by the African avifauna that has not been achieved in Australia, whence Acacia originates, viz. that a bird is capable of not only predating wax-exuding hemipterans, but also benefiting energetically from the wax (https://www.perplexity.ai/search/is-any-australian-bird-known-t-o9NO7Rr7TFyOcz6qKxTgtw).

Publicado el 07 de julio de 2024 por milewski